src/HOL/Probability/Radon_Nikodym.thy
author haftmann
Sun Jun 23 21:16:07 2013 +0200 (2013-06-23)
changeset 52435 6646bb548c6b
parent 52141 eff000cab70f
child 53015 a1119cf551e8
permissions -rw-r--r--
migration from code_(const|type|class|instance) to code_printing and from code_module to code_identifier
     1 (*  Title:      HOL/Probability/Radon_Nikodym.thy
     2     Author:     Johannes Hölzl, TU München
     3 *)
     4 
     5 header {*Radon-Nikod{\'y}m derivative*}
     6 
     7 theory Radon_Nikodym
     8 imports Lebesgue_Integration
     9 begin
    10 
    11 definition "diff_measure M N =
    12   measure_of (space M) (sets M) (\<lambda>A. emeasure M A - emeasure N A)"
    13 
    14 lemma 
    15   shows space_diff_measure[simp]: "space (diff_measure M N) = space M"
    16     and sets_diff_measure[simp]: "sets (diff_measure M N) = sets M"
    17   by (auto simp: diff_measure_def)
    18 
    19 lemma emeasure_diff_measure:
    20   assumes fin: "finite_measure M" "finite_measure N" and sets_eq: "sets M = sets N"
    21   assumes pos: "\<And>A. A \<in> sets M \<Longrightarrow> emeasure N A \<le> emeasure M A" and A: "A \<in> sets M"
    22   shows "emeasure (diff_measure M N) A = emeasure M A - emeasure N A" (is "_ = ?\<mu> A")
    23   unfolding diff_measure_def
    24 proof (rule emeasure_measure_of_sigma)
    25   show "sigma_algebra (space M) (sets M)" ..
    26   show "positive (sets M) ?\<mu>"
    27     using pos by (simp add: positive_def ereal_diff_positive)
    28   show "countably_additive (sets M) ?\<mu>"
    29   proof (rule countably_additiveI)
    30     fix A :: "nat \<Rightarrow> _"  assume A: "range A \<subseteq> sets M" and "disjoint_family A"
    31     then have suminf:
    32       "(\<Sum>i. emeasure M (A i)) = emeasure M (\<Union>i. A i)"
    33       "(\<Sum>i. emeasure N (A i)) = emeasure N (\<Union>i. A i)"
    34       by (simp_all add: suminf_emeasure sets_eq)
    35     with A have "(\<Sum>i. emeasure M (A i) - emeasure N (A i)) =
    36       (\<Sum>i. emeasure M (A i)) - (\<Sum>i. emeasure N (A i))"
    37       using fin
    38       by (intro suminf_ereal_minus pos emeasure_nonneg)
    39          (auto simp: sets_eq finite_measure.emeasure_eq_measure suminf_emeasure)
    40     then show "(\<Sum>i. emeasure M (A i) - emeasure N (A i)) =
    41       emeasure M (\<Union>i. A i) - emeasure N (\<Union>i. A i) "
    42       by (simp add: suminf)
    43   qed
    44 qed fact
    45 
    46 lemma (in sigma_finite_measure) Ex_finite_integrable_function:
    47   shows "\<exists>h\<in>borel_measurable M. integral\<^isup>P M h \<noteq> \<infinity> \<and> (\<forall>x\<in>space M. 0 < h x \<and> h x < \<infinity>) \<and> (\<forall>x. 0 \<le> h x)"
    48 proof -
    49   obtain A :: "nat \<Rightarrow> 'a set" where
    50     range[measurable]: "range A \<subseteq> sets M" and
    51     space: "(\<Union>i. A i) = space M" and
    52     measure: "\<And>i. emeasure M (A i) \<noteq> \<infinity>" and
    53     disjoint: "disjoint_family A"
    54     using sigma_finite_disjoint by auto
    55   let ?B = "\<lambda>i. 2^Suc i * emeasure M (A i)"
    56   have "\<forall>i. \<exists>x. 0 < x \<and> x < inverse (?B i)"
    57   proof
    58     fix i show "\<exists>x. 0 < x \<and> x < inverse (?B i)"
    59       using measure[of i] emeasure_nonneg[of M "A i"]
    60       by (auto intro!: dense simp: ereal_0_gt_inverse ereal_zero_le_0_iff)
    61   qed
    62   from choice[OF this] obtain n where n: "\<And>i. 0 < n i"
    63     "\<And>i. n i < inverse (2^Suc i * emeasure M (A i))" by auto
    64   { fix i have "0 \<le> n i" using n(1)[of i] by auto } note pos = this
    65   let ?h = "\<lambda>x. \<Sum>i. n i * indicator (A i) x"
    66   show ?thesis
    67   proof (safe intro!: bexI[of _ ?h] del: notI)
    68     have "\<And>i. A i \<in> sets M"
    69       using range by fastforce+
    70     then have "integral\<^isup>P M ?h = (\<Sum>i. n i * emeasure M (A i))" using pos
    71       by (simp add: positive_integral_suminf positive_integral_cmult_indicator)
    72     also have "\<dots> \<le> (\<Sum>i. (1 / 2)^Suc i)"
    73     proof (rule suminf_le_pos)
    74       fix N
    75       have "n N * emeasure M (A N) \<le> inverse (2^Suc N * emeasure M (A N)) * emeasure M (A N)"
    76         using n[of N]
    77         by (intro ereal_mult_right_mono) auto
    78       also have "\<dots> \<le> (1 / 2) ^ Suc N"
    79         using measure[of N] n[of N]
    80         by (cases rule: ereal2_cases[of "n N" "emeasure M (A N)"])
    81            (simp_all add: inverse_eq_divide power_divide one_ereal_def ereal_power_divide)
    82       finally show "n N * emeasure M (A N) \<le> (1 / 2) ^ Suc N" .
    83       show "0 \<le> n N * emeasure M (A N)" using n[of N] `A N \<in> sets M` by (simp add: emeasure_nonneg)
    84     qed
    85     finally show "integral\<^isup>P M ?h \<noteq> \<infinity>" unfolding suminf_half_series_ereal by auto
    86   next
    87     { fix x assume "x \<in> space M"
    88       then obtain i where "x \<in> A i" using space[symmetric] by auto
    89       with disjoint n have "?h x = n i"
    90         by (auto intro!: suminf_cmult_indicator intro: less_imp_le)
    91       then show "0 < ?h x" and "?h x < \<infinity>" using n[of i] by auto }
    92     note pos = this
    93     fix x show "0 \<le> ?h x"
    94     proof cases
    95       assume "x \<in> space M" then show "0 \<le> ?h x" using pos by (auto intro: less_imp_le)
    96     next
    97       assume "x \<notin> space M" then have "\<And>i. x \<notin> A i" using space by auto
    98       then show "0 \<le> ?h x" by auto
    99     qed
   100   qed measurable
   101 qed
   102 
   103 subsection "Absolutely continuous"
   104 
   105 definition absolutely_continuous :: "'a measure \<Rightarrow> 'a measure \<Rightarrow> bool" where
   106   "absolutely_continuous M N \<longleftrightarrow> null_sets M \<subseteq> null_sets N"
   107 
   108 lemma absolutely_continuousI_count_space: "absolutely_continuous (count_space A) M"
   109   unfolding absolutely_continuous_def by (auto simp: null_sets_count_space)
   110 
   111 lemma absolutely_continuousI_density:
   112   "f \<in> borel_measurable M \<Longrightarrow> absolutely_continuous M (density M f)"
   113   by (force simp add: absolutely_continuous_def null_sets_density_iff dest: AE_not_in)
   114 
   115 lemma absolutely_continuousI_point_measure_finite:
   116   "(\<And>x. \<lbrakk> x \<in> A ; f x \<le> 0 \<rbrakk> \<Longrightarrow> g x \<le> 0) \<Longrightarrow> absolutely_continuous (point_measure A f) (point_measure A g)"
   117   unfolding absolutely_continuous_def by (force simp: null_sets_point_measure_iff)
   118 
   119 lemma absolutely_continuous_AE:
   120   assumes sets_eq: "sets M' = sets M"
   121     and "absolutely_continuous M M'" "AE x in M. P x"
   122    shows "AE x in M'. P x"
   123 proof -
   124   from `AE x in M. P x` obtain N where N: "N \<in> null_sets M" "{x\<in>space M. \<not> P x} \<subseteq> N"
   125     unfolding eventually_ae_filter by auto
   126   show "AE x in M'. P x"
   127   proof (rule AE_I')
   128     show "{x\<in>space M'. \<not> P x} \<subseteq> N" using sets_eq_imp_space_eq[OF sets_eq] N(2) by simp
   129     from `absolutely_continuous M M'` show "N \<in> null_sets M'"
   130       using N unfolding absolutely_continuous_def sets_eq null_sets_def by auto
   131   qed
   132 qed
   133 
   134 subsection "Existence of the Radon-Nikodym derivative"
   135 
   136 lemma (in finite_measure) Radon_Nikodym_aux_epsilon:
   137   fixes e :: real assumes "0 < e"
   138   assumes "finite_measure N" and sets_eq: "sets N = sets M"
   139   shows "\<exists>A\<in>sets M. measure M (space M) - measure N (space M) \<le> measure M A - measure N A \<and>
   140                     (\<forall>B\<in>sets M. B \<subseteq> A \<longrightarrow> - e < measure M B - measure N B)"
   141 proof -
   142   interpret M': finite_measure N by fact
   143   let ?d = "\<lambda>A. measure M A - measure N A"
   144   let ?A = "\<lambda>A. if (\<forall>B\<in>sets M. B \<subseteq> space M - A \<longrightarrow> -e < ?d B)
   145     then {}
   146     else (SOME B. B \<in> sets M \<and> B \<subseteq> space M - A \<and> ?d B \<le> -e)"
   147   def A \<equiv> "\<lambda>n. ((\<lambda>B. B \<union> ?A B) ^^ n) {}"
   148   have A_simps[simp]:
   149     "A 0 = {}"
   150     "\<And>n. A (Suc n) = (A n \<union> ?A (A n))" unfolding A_def by simp_all
   151   { fix A assume "A \<in> sets M"
   152     have "?A A \<in> sets M"
   153       by (auto intro!: someI2[of _ _ "\<lambda>A. A \<in> sets M"] simp: not_less) }
   154   note A'_in_sets = this
   155   { fix n have "A n \<in> sets M"
   156     proof (induct n)
   157       case (Suc n) thus "A (Suc n) \<in> sets M"
   158         using A'_in_sets[of "A n"] by (auto split: split_if_asm)
   159     qed (simp add: A_def) }
   160   note A_in_sets = this
   161   hence "range A \<subseteq> sets M" by auto
   162   { fix n B
   163     assume Ex: "\<exists>B. B \<in> sets M \<and> B \<subseteq> space M - A n \<and> ?d B \<le> -e"
   164     hence False: "\<not> (\<forall>B\<in>sets M. B \<subseteq> space M - A n \<longrightarrow> -e < ?d B)" by (auto simp: not_less)
   165     have "?d (A (Suc n)) \<le> ?d (A n) - e" unfolding A_simps if_not_P[OF False]
   166     proof (rule someI2_ex[OF Ex])
   167       fix B assume "B \<in> sets M \<and> B \<subseteq> space M - A n \<and> ?d B \<le> - e"
   168       hence "A n \<inter> B = {}" "B \<in> sets M" and dB: "?d B \<le> -e" by auto
   169       hence "?d (A n \<union> B) = ?d (A n) + ?d B"
   170         using `A n \<in> sets M` finite_measure_Union M'.finite_measure_Union by (simp add: sets_eq)
   171       also have "\<dots> \<le> ?d (A n) - e" using dB by simp
   172       finally show "?d (A n \<union> B) \<le> ?d (A n) - e" .
   173     qed }
   174   note dA_epsilon = this
   175   { fix n have "?d (A (Suc n)) \<le> ?d (A n)"
   176     proof (cases "\<exists>B. B\<in>sets M \<and> B \<subseteq> space M - A n \<and> ?d B \<le> - e")
   177       case True from dA_epsilon[OF this] show ?thesis using `0 < e` by simp
   178     next
   179       case False
   180       hence "\<forall>B\<in>sets M. B \<subseteq> space M - A n \<longrightarrow> -e < ?d B" by (auto simp: not_le)
   181       thus ?thesis by simp
   182     qed }
   183   note dA_mono = this
   184   show ?thesis
   185   proof (cases "\<exists>n. \<forall>B\<in>sets M. B \<subseteq> space M - A n \<longrightarrow> -e < ?d B")
   186     case True then obtain n where B: "\<And>B. \<lbrakk> B \<in> sets M; B \<subseteq> space M - A n\<rbrakk> \<Longrightarrow> -e < ?d B" by blast
   187     show ?thesis
   188     proof (safe intro!: bexI[of _ "space M - A n"])
   189       fix B assume "B \<in> sets M" "B \<subseteq> space M - A n"
   190       from B[OF this] show "-e < ?d B" .
   191     next
   192       show "space M - A n \<in> sets M" by (rule sets.compl_sets) fact
   193     next
   194       show "?d (space M) \<le> ?d (space M - A n)"
   195       proof (induct n)
   196         fix n assume "?d (space M) \<le> ?d (space M - A n)"
   197         also have "\<dots> \<le> ?d (space M - A (Suc n))"
   198           using A_in_sets sets.sets_into_space dA_mono[of n] finite_measure_compl M'.finite_measure_compl
   199           by (simp del: A_simps add: sets_eq sets_eq_imp_space_eq[OF sets_eq])
   200         finally show "?d (space M) \<le> ?d (space M - A (Suc n))" .
   201       qed simp
   202     qed
   203   next
   204     case False hence B: "\<And>n. \<exists>B. B\<in>sets M \<and> B \<subseteq> space M - A n \<and> ?d B \<le> - e"
   205       by (auto simp add: not_less)
   206     { fix n have "?d (A n) \<le> - real n * e"
   207       proof (induct n)
   208         case (Suc n) with dA_epsilon[of n, OF B] show ?case by (simp del: A_simps add: real_of_nat_Suc field_simps)
   209       next
   210         case 0 with measure_empty show ?case by (simp add: zero_ereal_def)
   211       qed } note dA_less = this
   212     have decseq: "decseq (\<lambda>n. ?d (A n))" unfolding decseq_eq_incseq
   213     proof (rule incseq_SucI)
   214       fix n show "- ?d (A n) \<le> - ?d (A (Suc n))" using dA_mono[of n] by auto
   215     qed
   216     have A: "incseq A" by (auto intro!: incseq_SucI)
   217     from finite_Lim_measure_incseq[OF _ A] `range A \<subseteq> sets M`
   218       M'.finite_Lim_measure_incseq[OF _ A]
   219     have convergent: "(\<lambda>i. ?d (A i)) ----> ?d (\<Union>i. A i)"
   220       by (auto intro!: tendsto_diff simp: sets_eq)
   221     obtain n :: nat where "- ?d (\<Union>i. A i) / e < real n" using reals_Archimedean2 by auto
   222     moreover from order_trans[OF decseq_le[OF decseq convergent] dA_less]
   223     have "real n \<le> - ?d (\<Union>i. A i) / e" using `0<e` by (simp add: field_simps)
   224     ultimately show ?thesis by auto
   225   qed
   226 qed
   227 
   228 lemma (in finite_measure) Radon_Nikodym_aux:
   229   assumes "finite_measure N" and sets_eq: "sets N = sets M"
   230   shows "\<exists>A\<in>sets M. measure M (space M) - measure N (space M) \<le>
   231                     measure M A - measure N A \<and>
   232                     (\<forall>B\<in>sets M. B \<subseteq> A \<longrightarrow> 0 \<le> measure M B - measure N B)"
   233 proof -
   234   interpret N: finite_measure N by fact
   235   let ?d = "\<lambda>A. measure M A - measure N A"
   236   let ?P = "\<lambda>A B n. A \<in> sets M \<and> A \<subseteq> B \<and> ?d B \<le> ?d A \<and> (\<forall>C\<in>sets M. C \<subseteq> A \<longrightarrow> - 1 / real (Suc n) < ?d C)"
   237   let ?r = "\<lambda>S. restricted_space S"
   238   { fix S n assume S: "S \<in> sets M"
   239     then have "finite_measure (density M (indicator S))" "0 < 1 / real (Suc n)"
   240          "finite_measure (density N (indicator S))" "sets (density N (indicator S)) = sets (density M (indicator S))"
   241       by (auto simp: finite_measure_restricted N.finite_measure_restricted sets_eq)
   242     from finite_measure.Radon_Nikodym_aux_epsilon[OF this] guess X .. note X = this
   243     with S have "?P (S \<inter> X) S n"
   244       by (simp add: measure_restricted sets_eq sets.Int) (metis inf_absorb2)
   245     hence "\<exists>A. ?P A S n" .. }
   246   note Ex_P = this
   247   def A \<equiv> "nat_rec (space M) (\<lambda>n A. SOME B. ?P B A n)"
   248   have A_Suc: "\<And>n. A (Suc n) = (SOME B. ?P B (A n) n)" by (simp add: A_def)
   249   have A_0[simp]: "A 0 = space M" unfolding A_def by simp
   250   { fix i have "A i \<in> sets M" unfolding A_def
   251     proof (induct i)
   252       case (Suc i)
   253       from Ex_P[OF this, of i] show ?case unfolding nat_rec_Suc
   254         by (rule someI2_ex) simp
   255     qed simp }
   256   note A_in_sets = this
   257   { fix n have "?P (A (Suc n)) (A n) n"
   258       using Ex_P[OF A_in_sets] unfolding A_Suc
   259       by (rule someI2_ex) simp }
   260   note P_A = this
   261   have "range A \<subseteq> sets M" using A_in_sets by auto
   262   have A_mono: "\<And>i. A (Suc i) \<subseteq> A i" using P_A by simp
   263   have mono_dA: "mono (\<lambda>i. ?d (A i))" using P_A by (simp add: mono_iff_le_Suc)
   264   have epsilon: "\<And>C i. \<lbrakk> C \<in> sets M; C \<subseteq> A (Suc i) \<rbrakk> \<Longrightarrow> - 1 / real (Suc i) < ?d C"
   265       using P_A by auto
   266   show ?thesis
   267   proof (safe intro!: bexI[of _ "\<Inter>i. A i"])
   268     show "(\<Inter>i. A i) \<in> sets M" using A_in_sets by auto
   269     have A: "decseq A" using A_mono by (auto intro!: decseq_SucI)
   270     from `range A \<subseteq> sets M`
   271       finite_Lim_measure_decseq[OF _ A] N.finite_Lim_measure_decseq[OF _ A]
   272     have "(\<lambda>i. ?d (A i)) ----> ?d (\<Inter>i. A i)" by (auto intro!: tendsto_diff simp: sets_eq)
   273     thus "?d (space M) \<le> ?d (\<Inter>i. A i)" using mono_dA[THEN monoD, of 0 _]
   274       by (rule_tac LIMSEQ_le_const) auto
   275   next
   276     fix B assume B: "B \<in> sets M" "B \<subseteq> (\<Inter>i. A i)"
   277     show "0 \<le> ?d B"
   278     proof (rule ccontr)
   279       assume "\<not> 0 \<le> ?d B"
   280       hence "0 < - ?d B" by auto
   281       from ex_inverse_of_nat_Suc_less[OF this]
   282       obtain n where *: "?d B < - 1 / real (Suc n)"
   283         by (auto simp: real_eq_of_nat inverse_eq_divide field_simps)
   284       have "B \<subseteq> A (Suc n)" using B by (auto simp del: nat_rec_Suc)
   285       from epsilon[OF B(1) this] *
   286       show False by auto
   287     qed
   288   qed
   289 qed
   290 
   291 lemma (in finite_measure) Radon_Nikodym_finite_measure:
   292   assumes "finite_measure N" and sets_eq: "sets N = sets M"
   293   assumes "absolutely_continuous M N"
   294   shows "\<exists>f \<in> borel_measurable M. (\<forall>x. 0 \<le> f x) \<and> density M f = N"
   295 proof -
   296   interpret N: finite_measure N by fact
   297   def G \<equiv> "{g \<in> borel_measurable M. (\<forall>x. 0 \<le> g x) \<and> (\<forall>A\<in>sets M. (\<integral>\<^isup>+x. g x * indicator A x \<partial>M) \<le> N A)}"
   298   { fix f have "f \<in> G \<Longrightarrow> f \<in> borel_measurable M" by (auto simp: G_def) }
   299   note this[measurable_dest]
   300   have "(\<lambda>x. 0) \<in> G" unfolding G_def by auto
   301   hence "G \<noteq> {}" by auto
   302   { fix f g assume f: "f \<in> G" and g: "g \<in> G"
   303     have "(\<lambda>x. max (g x) (f x)) \<in> G" (is "?max \<in> G") unfolding G_def
   304     proof safe
   305       show "?max \<in> borel_measurable M" using f g unfolding G_def by auto
   306       let ?A = "{x \<in> space M. f x \<le> g x}"
   307       have "?A \<in> sets M" using f g unfolding G_def by auto
   308       fix A assume "A \<in> sets M"
   309       hence sets: "?A \<inter> A \<in> sets M" "(space M - ?A) \<inter> A \<in> sets M" using `?A \<in> sets M` by auto
   310       hence sets': "?A \<inter> A \<in> sets N" "(space M - ?A) \<inter> A \<in> sets N" by (auto simp: sets_eq)
   311       have union: "((?A \<inter> A) \<union> ((space M - ?A) \<inter> A)) = A"
   312         using sets.sets_into_space[OF `A \<in> sets M`] by auto
   313       have "\<And>x. x \<in> space M \<Longrightarrow> max (g x) (f x) * indicator A x =
   314         g x * indicator (?A \<inter> A) x + f x * indicator ((space M - ?A) \<inter> A) x"
   315         by (auto simp: indicator_def max_def)
   316       hence "(\<integral>\<^isup>+x. max (g x) (f x) * indicator A x \<partial>M) =
   317         (\<integral>\<^isup>+x. g x * indicator (?A \<inter> A) x \<partial>M) +
   318         (\<integral>\<^isup>+x. f x * indicator ((space M - ?A) \<inter> A) x \<partial>M)"
   319         using f g sets unfolding G_def
   320         by (auto cong: positive_integral_cong intro!: positive_integral_add)
   321       also have "\<dots> \<le> N (?A \<inter> A) + N ((space M - ?A) \<inter> A)"
   322         using f g sets unfolding G_def by (auto intro!: add_mono)
   323       also have "\<dots> = N A"
   324         using plus_emeasure[OF sets'] union by auto
   325       finally show "(\<integral>\<^isup>+x. max (g x) (f x) * indicator A x \<partial>M) \<le> N A" .
   326     next
   327       fix x show "0 \<le> max (g x) (f x)" using f g by (auto simp: G_def split: split_max)
   328     qed }
   329   note max_in_G = this
   330   { fix f assume  "incseq f" and f: "\<And>i. f i \<in> G"
   331     then have [measurable]: "\<And>i. f i \<in> borel_measurable M" by (auto simp: G_def)
   332     have "(\<lambda>x. SUP i. f i x) \<in> G" unfolding G_def
   333     proof safe
   334       show "(\<lambda>x. SUP i. f i x) \<in> borel_measurable M" by measurable
   335       { fix x show "0 \<le> (SUP i. f i x)"
   336           using f by (auto simp: G_def intro: SUP_upper2) }
   337     next
   338       fix A assume "A \<in> sets M"
   339       have "(\<integral>\<^isup>+x. (SUP i. f i x) * indicator A x \<partial>M) =
   340         (\<integral>\<^isup>+x. (SUP i. f i x * indicator A x) \<partial>M)"
   341         by (intro positive_integral_cong) (simp split: split_indicator)
   342       also have "\<dots> = (SUP i. (\<integral>\<^isup>+x. f i x * indicator A x \<partial>M))"
   343         using `incseq f` f `A \<in> sets M`
   344         by (intro positive_integral_monotone_convergence_SUP)
   345            (auto simp: G_def incseq_Suc_iff le_fun_def split: split_indicator)
   346       finally show "(\<integral>\<^isup>+x. (SUP i. f i x) * indicator A x \<partial>M) \<le> N A"
   347         using f `A \<in> sets M` by (auto intro!: SUP_least simp: G_def)
   348     qed }
   349   note SUP_in_G = this
   350   let ?y = "SUP g : G. integral\<^isup>P M g"
   351   have y_le: "?y \<le> N (space M)" unfolding G_def
   352   proof (safe intro!: SUP_least)
   353     fix g assume "\<forall>A\<in>sets M. (\<integral>\<^isup>+x. g x * indicator A x \<partial>M) \<le> N A"
   354     from this[THEN bspec, OF sets.top] show "integral\<^isup>P M g \<le> N (space M)"
   355       by (simp cong: positive_integral_cong)
   356   qed
   357   from SUPR_countable_SUPR[OF `G \<noteq> {}`, of "integral\<^isup>P M"] guess ys .. note ys = this
   358   then have "\<forall>n. \<exists>g. g\<in>G \<and> integral\<^isup>P M g = ys n"
   359   proof safe
   360     fix n assume "range ys \<subseteq> integral\<^isup>P M ` G"
   361     hence "ys n \<in> integral\<^isup>P M ` G" by auto
   362     thus "\<exists>g. g\<in>G \<and> integral\<^isup>P M g = ys n" by auto
   363   qed
   364   from choice[OF this] obtain gs where "\<And>i. gs i \<in> G" "\<And>n. integral\<^isup>P M (gs n) = ys n" by auto
   365   hence y_eq: "?y = (SUP i. integral\<^isup>P M (gs i))" using ys by auto
   366   let ?g = "\<lambda>i x. Max ((\<lambda>n. gs n x) ` {..i})"
   367   def f \<equiv> "\<lambda>x. SUP i. ?g i x"
   368   let ?F = "\<lambda>A x. f x * indicator A x"
   369   have gs_not_empty: "\<And>i x. (\<lambda>n. gs n x) ` {..i} \<noteq> {}" by auto
   370   { fix i have "?g i \<in> G"
   371     proof (induct i)
   372       case 0 thus ?case by simp fact
   373     next
   374       case (Suc i)
   375       with Suc gs_not_empty `gs (Suc i) \<in> G` show ?case
   376         by (auto simp add: atMost_Suc intro!: max_in_G)
   377     qed }
   378   note g_in_G = this
   379   have "incseq ?g" using gs_not_empty
   380     by (auto intro!: incseq_SucI le_funI simp add: atMost_Suc)
   381   from SUP_in_G[OF this g_in_G] have [measurable]: "f \<in> G" unfolding f_def .
   382   then have [simp, intro]: "f \<in> borel_measurable M" unfolding G_def by auto
   383   have "integral\<^isup>P M f = (SUP i. integral\<^isup>P M (?g i))" unfolding f_def
   384     using g_in_G `incseq ?g`
   385     by (auto intro!: positive_integral_monotone_convergence_SUP simp: G_def)
   386   also have "\<dots> = ?y"
   387   proof (rule antisym)
   388     show "(SUP i. integral\<^isup>P M (?g i)) \<le> ?y"
   389       using g_in_G by (auto intro: Sup_mono simp: SUP_def)
   390     show "?y \<le> (SUP i. integral\<^isup>P M (?g i))" unfolding y_eq
   391       by (auto intro!: SUP_mono positive_integral_mono Max_ge)
   392   qed
   393   finally have int_f_eq_y: "integral\<^isup>P M f = ?y" .
   394   have "\<And>x. 0 \<le> f x"
   395     unfolding f_def using `\<And>i. gs i \<in> G`
   396     by (auto intro!: SUP_upper2 Max_ge_iff[THEN iffD2] simp: G_def)
   397   let ?t = "\<lambda>A. N A - (\<integral>\<^isup>+x. ?F A x \<partial>M)"
   398   let ?M = "diff_measure N (density M f)"
   399   have f_le_N: "\<And>A. A \<in> sets M \<Longrightarrow> (\<integral>\<^isup>+x. ?F A x \<partial>M) \<le> N A"
   400     using `f \<in> G` unfolding G_def by auto
   401   have emeasure_M: "\<And>A. A \<in> sets M \<Longrightarrow> emeasure ?M A = ?t A"
   402   proof (subst emeasure_diff_measure)
   403     from f_le_N[of "space M"] show "finite_measure N" "finite_measure (density M f)"
   404       by (auto intro!: finite_measureI simp: emeasure_density cong: positive_integral_cong)
   405   next
   406     fix B assume "B \<in> sets N" with f_le_N[of B] show "emeasure (density M f) B \<le> emeasure N B"
   407       by (auto simp: sets_eq emeasure_density cong: positive_integral_cong)
   408   qed (auto simp: sets_eq emeasure_density)
   409   from emeasure_M[of "space M"] N.finite_emeasure_space positive_integral_positive[of M "?F (space M)"]
   410   interpret M': finite_measure ?M
   411     by (auto intro!: finite_measureI simp: sets_eq_imp_space_eq[OF sets_eq] N.emeasure_eq_measure ereal_minus_eq_PInfty_iff)
   412 
   413   have ac: "absolutely_continuous M ?M" unfolding absolutely_continuous_def
   414   proof
   415     fix A assume A: "A \<in> null_sets M"
   416     with `absolutely_continuous M N` have "A \<in> null_sets N"
   417       unfolding absolutely_continuous_def by auto
   418     moreover with A have "(\<integral>\<^isup>+ x. ?F A x \<partial>M) \<le> N A" using `f \<in> G` by (auto simp: G_def)
   419     ultimately have "N A - (\<integral>\<^isup>+ x. ?F A x \<partial>M) = 0"
   420       using positive_integral_positive[of M] by (auto intro!: antisym)
   421     then show "A \<in> null_sets ?M"
   422       using A by (simp add: emeasure_M null_sets_def sets_eq)
   423   qed
   424   have upper_bound: "\<forall>A\<in>sets M. ?M A \<le> 0"
   425   proof (rule ccontr)
   426     assume "\<not> ?thesis"
   427     then obtain A where A: "A \<in> sets M" and pos: "0 < ?M A"
   428       by (auto simp: not_le)
   429     note pos
   430     also have "?M A \<le> ?M (space M)"
   431       using emeasure_space[of ?M A] by (simp add: sets_eq[THEN sets_eq_imp_space_eq])
   432     finally have pos_t: "0 < ?M (space M)" by simp
   433     moreover
   434     then have "emeasure M (space M) \<noteq> 0"
   435       using ac unfolding absolutely_continuous_def by (auto simp: null_sets_def)
   436     then have pos_M: "0 < emeasure M (space M)"
   437       using emeasure_nonneg[of M "space M"] by (simp add: le_less)
   438     moreover
   439     have "(\<integral>\<^isup>+x. f x * indicator (space M) x \<partial>M) \<le> N (space M)"
   440       using `f \<in> G` unfolding G_def by auto
   441     hence "(\<integral>\<^isup>+x. f x * indicator (space M) x \<partial>M) \<noteq> \<infinity>"
   442       using M'.finite_emeasure_space by auto
   443     moreover
   444     def b \<equiv> "?M (space M) / emeasure M (space M) / 2"
   445     ultimately have b: "b \<noteq> 0 \<and> 0 \<le> b \<and> b \<noteq> \<infinity>"
   446       by (auto simp: ereal_divide_eq)
   447     then have b: "b \<noteq> 0" "0 \<le> b" "0 < b"  "b \<noteq> \<infinity>" by auto
   448     let ?Mb = "density M (\<lambda>_. b)"
   449     have Mb: "finite_measure ?Mb" "sets ?Mb = sets ?M"
   450         using b by (auto simp: emeasure_density_const sets_eq intro!: finite_measureI)
   451     from M'.Radon_Nikodym_aux[OF this] guess A0 ..
   452     then have "A0 \<in> sets M"
   453       and space_less_A0: "measure ?M (space M) - real b * measure M (space M) \<le> measure ?M A0 - real b * measure M A0"
   454       and *: "\<And>B. B \<in> sets M \<Longrightarrow> B \<subseteq> A0 \<Longrightarrow> 0 \<le> measure ?M B - real b * measure M B"
   455       using b by (simp_all add: measure_density_const sets_eq_imp_space_eq[OF sets_eq] sets_eq)
   456     { fix B assume B: "B \<in> sets M" "B \<subseteq> A0"
   457       with *[OF this] have "b * emeasure M B \<le> ?M B"
   458         using b unfolding M'.emeasure_eq_measure emeasure_eq_measure by (cases b) auto }
   459     note bM_le_t = this
   460     let ?f0 = "\<lambda>x. f x + b * indicator A0 x"
   461     { fix A assume A: "A \<in> sets M"
   462       hence "A \<inter> A0 \<in> sets M" using `A0 \<in> sets M` by auto
   463       have "(\<integral>\<^isup>+x. ?f0 x  * indicator A x \<partial>M) =
   464         (\<integral>\<^isup>+x. f x * indicator A x + b * indicator (A \<inter> A0) x \<partial>M)"
   465         by (auto intro!: positive_integral_cong split: split_indicator)
   466       hence "(\<integral>\<^isup>+x. ?f0 x * indicator A x \<partial>M) =
   467           (\<integral>\<^isup>+x. f x * indicator A x \<partial>M) + b * emeasure M (A \<inter> A0)"
   468         using `A0 \<in> sets M` `A \<inter> A0 \<in> sets M` A b `f \<in> G`
   469         by (simp add: positive_integral_add positive_integral_cmult_indicator G_def) }
   470     note f0_eq = this
   471     { fix A assume A: "A \<in> sets M"
   472       hence "A \<inter> A0 \<in> sets M" using `A0 \<in> sets M` by auto
   473       have f_le_v: "(\<integral>\<^isup>+x. ?F A x \<partial>M) \<le> N A" using `f \<in> G` A unfolding G_def by auto
   474       note f0_eq[OF A]
   475       also have "(\<integral>\<^isup>+x. ?F A x \<partial>M) + b * emeasure M (A \<inter> A0) \<le> (\<integral>\<^isup>+x. ?F A x \<partial>M) + ?M (A \<inter> A0)"
   476         using bM_le_t[OF `A \<inter> A0 \<in> sets M`] `A \<in> sets M` `A0 \<in> sets M`
   477         by (auto intro!: add_left_mono)
   478       also have "\<dots> \<le> (\<integral>\<^isup>+x. f x * indicator A x \<partial>M) + ?M A"
   479         using emeasure_mono[of "A \<inter> A0" A ?M] `A \<in> sets M` `A0 \<in> sets M`
   480         by (auto intro!: add_left_mono simp: sets_eq)
   481       also have "\<dots> \<le> N A"
   482         unfolding emeasure_M[OF `A \<in> sets M`]
   483         using f_le_v N.emeasure_eq_measure[of A] positive_integral_positive[of M "?F A"]
   484         by (cases "\<integral>\<^isup>+x. ?F A x \<partial>M", cases "N A") auto
   485       finally have "(\<integral>\<^isup>+x. ?f0 x * indicator A x \<partial>M) \<le> N A" . }
   486     hence "?f0 \<in> G" using `A0 \<in> sets M` b `f \<in> G`
   487       by (auto intro!: ereal_add_nonneg_nonneg simp: G_def)
   488     have int_f_finite: "integral\<^isup>P M f \<noteq> \<infinity>"
   489       by (metis N.emeasure_finite ereal_infty_less_eq2(1) int_f_eq_y y_le)
   490     have  "0 < ?M (space M) - emeasure ?Mb (space M)"
   491       using pos_t
   492       by (simp add: b emeasure_density_const)
   493          (simp add: M'.emeasure_eq_measure emeasure_eq_measure pos_M b_def)
   494     also have "\<dots> \<le> ?M A0 - b * emeasure M A0"
   495       using space_less_A0 `A0 \<in> sets M` b
   496       by (cases b) (auto simp add: b emeasure_density_const sets_eq M'.emeasure_eq_measure emeasure_eq_measure)
   497     finally have 1: "b * emeasure M A0 < ?M A0"
   498       by (metis M'.emeasure_real `A0 \<in> sets M` bM_le_t diff_self ereal_less(1) ereal_minus(1)
   499                 less_eq_ereal_def mult_zero_left not_square_less_zero subset_refl zero_ereal_def)
   500     with b have "0 < ?M A0"
   501       by (metis M'.emeasure_real MInfty_neq_PInfty(1) emeasure_real ereal_less_eq(5) ereal_zero_times
   502                ereal_mult_eq_MInfty ereal_mult_eq_PInfty ereal_zero_less_0_iff less_eq_ereal_def)
   503     then have "emeasure M A0 \<noteq> 0" using ac `A0 \<in> sets M`
   504       by (auto simp: absolutely_continuous_def null_sets_def)
   505     then have "0 < emeasure M A0" using emeasure_nonneg[of M A0] by auto
   506     hence "0 < b * emeasure M A0" using b by (auto simp: ereal_zero_less_0_iff)
   507     with int_f_finite have "?y + 0 < integral\<^isup>P M f + b * emeasure M A0" unfolding int_f_eq_y
   508       using `f \<in> G`
   509       by (intro ereal_add_strict_mono) (auto intro!: SUP_upper2 positive_integral_positive)
   510     also have "\<dots> = integral\<^isup>P M ?f0" using f0_eq[OF sets.top] `A0 \<in> sets M` sets.sets_into_space
   511       by (simp cong: positive_integral_cong)
   512     finally have "?y < integral\<^isup>P M ?f0" by simp
   513     moreover from `?f0 \<in> G` have "integral\<^isup>P M ?f0 \<le> ?y" by (auto intro!: SUP_upper)
   514     ultimately show False by auto
   515   qed
   516   let ?f = "\<lambda>x. max 0 (f x)"
   517   show ?thesis
   518   proof (intro bexI[of _ ?f] measure_eqI conjI)
   519     show "sets (density M ?f) = sets N"
   520       by (simp add: sets_eq)
   521     fix A assume A: "A\<in>sets (density M ?f)"
   522     then show "emeasure (density M ?f) A = emeasure N A"
   523       using `f \<in> G` A upper_bound[THEN bspec, of A] N.emeasure_eq_measure[of A]
   524       by (cases "integral\<^isup>P M (?F A)")
   525          (auto intro!: antisym simp add: emeasure_density G_def emeasure_M density_max_0[symmetric])
   526   qed auto
   527 qed
   528 
   529 lemma (in finite_measure) split_space_into_finite_sets_and_rest:
   530   assumes ac: "absolutely_continuous M N" and sets_eq: "sets N = sets M"
   531   shows "\<exists>A0\<in>sets M. \<exists>B::nat\<Rightarrow>'a set. disjoint_family B \<and> range B \<subseteq> sets M \<and> A0 = space M - (\<Union>i. B i) \<and>
   532     (\<forall>A\<in>sets M. A \<subseteq> A0 \<longrightarrow> (emeasure M A = 0 \<and> N A = 0) \<or> (emeasure M A > 0 \<and> N A = \<infinity>)) \<and>
   533     (\<forall>i. N (B i) \<noteq> \<infinity>)"
   534 proof -
   535   let ?Q = "{Q\<in>sets M. N Q \<noteq> \<infinity>}"
   536   let ?a = "SUP Q:?Q. emeasure M Q"
   537   have "{} \<in> ?Q" by auto
   538   then have Q_not_empty: "?Q \<noteq> {}" by blast
   539   have "?a \<le> emeasure M (space M)" using sets.sets_into_space
   540     by (auto intro!: SUP_least emeasure_mono)
   541   then have "?a \<noteq> \<infinity>" using finite_emeasure_space
   542     by auto
   543   from SUPR_countable_SUPR[OF Q_not_empty, of "emeasure M"]
   544   obtain Q'' where "range Q'' \<subseteq> emeasure M ` ?Q" and a: "?a = (SUP i::nat. Q'' i)"
   545     by auto
   546   then have "\<forall>i. \<exists>Q'. Q'' i = emeasure M Q' \<and> Q' \<in> ?Q" by auto
   547   from choice[OF this] obtain Q' where Q': "\<And>i. Q'' i = emeasure M (Q' i)" "\<And>i. Q' i \<in> ?Q"
   548     by auto
   549   then have a_Lim: "?a = (SUP i::nat. emeasure M (Q' i))" using a by simp
   550   let ?O = "\<lambda>n. \<Union>i\<le>n. Q' i"
   551   have Union: "(SUP i. emeasure M (?O i)) = emeasure M (\<Union>i. ?O i)"
   552   proof (rule SUP_emeasure_incseq[of ?O])
   553     show "range ?O \<subseteq> sets M" using Q' by auto
   554     show "incseq ?O" by (fastforce intro!: incseq_SucI)
   555   qed
   556   have Q'_sets: "\<And>i. Q' i \<in> sets M" using Q' by auto
   557   have O_sets: "\<And>i. ?O i \<in> sets M" using Q' by auto
   558   then have O_in_G: "\<And>i. ?O i \<in> ?Q"
   559   proof (safe del: notI)
   560     fix i have "Q' ` {..i} \<subseteq> sets M" using Q' by auto
   561     then have "N (?O i) \<le> (\<Sum>i\<le>i. N (Q' i))"
   562       by (simp add: sets_eq emeasure_subadditive_finite)
   563     also have "\<dots> < \<infinity>" using Q' by (simp add: setsum_Pinfty)
   564     finally show "N (?O i) \<noteq> \<infinity>" by simp
   565   qed auto
   566   have O_mono: "\<And>n. ?O n \<subseteq> ?O (Suc n)" by fastforce
   567   have a_eq: "?a = emeasure M (\<Union>i. ?O i)" unfolding Union[symmetric]
   568   proof (rule antisym)
   569     show "?a \<le> (SUP i. emeasure M (?O i))" unfolding a_Lim
   570       using Q' by (auto intro!: SUP_mono emeasure_mono)
   571     show "(SUP i. emeasure M (?O i)) \<le> ?a" unfolding SUP_def
   572     proof (safe intro!: Sup_mono, unfold bex_simps)
   573       fix i
   574       have *: "(\<Union>(Q' ` {..i})) = ?O i" by auto
   575       then show "\<exists>x. (x \<in> sets M \<and> N x \<noteq> \<infinity>) \<and>
   576         emeasure M (\<Union>(Q' ` {..i})) \<le> emeasure M x"
   577         using O_in_G[of i] by (auto intro!: exI[of _ "?O i"])
   578     qed
   579   qed
   580   let ?O_0 = "(\<Union>i. ?O i)"
   581   have "?O_0 \<in> sets M" using Q' by auto
   582   def Q \<equiv> "\<lambda>i. case i of 0 \<Rightarrow> Q' 0 | Suc n \<Rightarrow> ?O (Suc n) - ?O n"
   583   { fix i have "Q i \<in> sets M" unfolding Q_def using Q'[of 0] by (cases i) (auto intro: O_sets) }
   584   note Q_sets = this
   585   show ?thesis
   586   proof (intro bexI exI conjI ballI impI allI)
   587     show "disjoint_family Q"
   588       by (fastforce simp: disjoint_family_on_def Q_def
   589         split: nat.split_asm)
   590     show "range Q \<subseteq> sets M"
   591       using Q_sets by auto
   592     { fix A assume A: "A \<in> sets M" "A \<subseteq> space M - ?O_0"
   593       show "emeasure M A = 0 \<and> N A = 0 \<or> 0 < emeasure M A \<and> N A = \<infinity>"
   594       proof (rule disjCI, simp)
   595         assume *: "0 < emeasure M A \<longrightarrow> N A \<noteq> \<infinity>"
   596         show "emeasure M A = 0 \<and> N A = 0"
   597         proof cases
   598           assume "emeasure M A = 0" moreover with ac A have "N A = 0"
   599             unfolding absolutely_continuous_def by auto
   600           ultimately show ?thesis by simp
   601         next
   602           assume "emeasure M A \<noteq> 0" with * have "N A \<noteq> \<infinity>" using emeasure_nonneg[of M A] by auto
   603           with A have "emeasure M ?O_0 + emeasure M A = emeasure M (?O_0 \<union> A)"
   604             using Q' by (auto intro!: plus_emeasure sets.countable_UN)
   605           also have "\<dots> = (SUP i. emeasure M (?O i \<union> A))"
   606           proof (rule SUP_emeasure_incseq[of "\<lambda>i. ?O i \<union> A", symmetric, simplified])
   607             show "range (\<lambda>i. ?O i \<union> A) \<subseteq> sets M"
   608               using `N A \<noteq> \<infinity>` O_sets A by auto
   609           qed (fastforce intro!: incseq_SucI)
   610           also have "\<dots> \<le> ?a"
   611           proof (safe intro!: SUP_least)
   612             fix i have "?O i \<union> A \<in> ?Q"
   613             proof (safe del: notI)
   614               show "?O i \<union> A \<in> sets M" using O_sets A by auto
   615               from O_in_G[of i] have "N (?O i \<union> A) \<le> N (?O i) + N A"
   616                 using emeasure_subadditive[of "?O i" N A] A O_sets by (auto simp: sets_eq)
   617               with O_in_G[of i] show "N (?O i \<union> A) \<noteq> \<infinity>"
   618                 using `N A \<noteq> \<infinity>` by auto
   619             qed
   620             then show "emeasure M (?O i \<union> A) \<le> ?a" by (rule SUP_upper)
   621           qed
   622           finally have "emeasure M A = 0"
   623             unfolding a_eq using measure_nonneg[of M A] by (simp add: emeasure_eq_measure)
   624           with `emeasure M A \<noteq> 0` show ?thesis by auto
   625         qed
   626       qed }
   627     { fix i show "N (Q i) \<noteq> \<infinity>"
   628       proof (cases i)
   629         case 0 then show ?thesis
   630           unfolding Q_def using Q'[of 0] by simp
   631       next
   632         case (Suc n)
   633         with `?O n \<in> ?Q` `?O (Suc n) \<in> ?Q`
   634             emeasure_Diff[OF _ _ _ O_mono, of N n] emeasure_nonneg[of N "(\<Union> x\<le>n. Q' x)"]
   635         show ?thesis
   636           by (auto simp: sets_eq ereal_minus_eq_PInfty_iff Q_def)
   637       qed }
   638     show "space M - ?O_0 \<in> sets M" using Q'_sets by auto
   639     { fix j have "(\<Union>i\<le>j. ?O i) = (\<Union>i\<le>j. Q i)"
   640       proof (induct j)
   641         case 0 then show ?case by (simp add: Q_def)
   642       next
   643         case (Suc j)
   644         have eq: "\<And>j. (\<Union>i\<le>j. ?O i) = (\<Union>i\<le>j. Q' i)" by fastforce
   645         have "{..j} \<union> {..Suc j} = {..Suc j}" by auto
   646         then have "(\<Union>i\<le>Suc j. Q' i) = (\<Union>i\<le>j. Q' i) \<union> Q (Suc j)"
   647           by (simp add: UN_Un[symmetric] Q_def del: UN_Un)
   648         then show ?case using Suc by (auto simp add: eq atMost_Suc)
   649       qed }
   650     then have "(\<Union>j. (\<Union>i\<le>j. ?O i)) = (\<Union>j. (\<Union>i\<le>j. Q i))" by simp
   651     then show "space M - ?O_0 = space M - (\<Union>i. Q i)" by fastforce
   652   qed
   653 qed
   654 
   655 lemma (in finite_measure) Radon_Nikodym_finite_measure_infinite:
   656   assumes "absolutely_continuous M N" and sets_eq: "sets N = sets M"
   657   shows "\<exists>f\<in>borel_measurable M. (\<forall>x. 0 \<le> f x) \<and> density M f = N"
   658 proof -
   659   from split_space_into_finite_sets_and_rest[OF assms]
   660   obtain Q0 and Q :: "nat \<Rightarrow> 'a set"
   661     where Q: "disjoint_family Q" "range Q \<subseteq> sets M"
   662     and Q0: "Q0 \<in> sets M" "Q0 = space M - (\<Union>i. Q i)"
   663     and in_Q0: "\<And>A. A \<in> sets M \<Longrightarrow> A \<subseteq> Q0 \<Longrightarrow> emeasure M A = 0 \<and> N A = 0 \<or> 0 < emeasure M A \<and> N A = \<infinity>"
   664     and Q_fin: "\<And>i. N (Q i) \<noteq> \<infinity>" by force
   665   from Q have Q_sets: "\<And>i. Q i \<in> sets M" by auto
   666   let ?N = "\<lambda>i. density N (indicator (Q i))" and ?M = "\<lambda>i. density M (indicator (Q i))"
   667   have "\<forall>i. \<exists>f\<in>borel_measurable (?M i). (\<forall>x. 0 \<le> f x) \<and> density (?M i) f = ?N i"
   668   proof (intro allI finite_measure.Radon_Nikodym_finite_measure)
   669     fix i
   670     from Q show "finite_measure (?M i)"
   671       by (auto intro!: finite_measureI cong: positive_integral_cong
   672                simp add: emeasure_density subset_eq sets_eq)
   673     from Q have "emeasure (?N i) (space N) = emeasure N (Q i)"
   674       by (simp add: sets_eq[symmetric] emeasure_density subset_eq cong: positive_integral_cong)
   675     with Q_fin show "finite_measure (?N i)"
   676       by (auto intro!: finite_measureI)
   677     show "sets (?N i) = sets (?M i)" by (simp add: sets_eq)
   678     have [measurable]: "\<And>A. A \<in> sets M \<Longrightarrow> A \<in> sets N" by (simp add: sets_eq)
   679     show "absolutely_continuous (?M i) (?N i)"
   680       using `absolutely_continuous M N` `Q i \<in> sets M`
   681       by (auto simp: absolutely_continuous_def null_sets_density_iff sets_eq
   682                intro!: absolutely_continuous_AE[OF sets_eq])
   683   qed
   684   from choice[OF this[unfolded Bex_def]]
   685   obtain f where borel: "\<And>i. f i \<in> borel_measurable M" "\<And>i x. 0 \<le> f i x"
   686     and f_density: "\<And>i. density (?M i) (f i) = ?N i"
   687     by auto
   688   { fix A i assume A: "A \<in> sets M"
   689     with Q borel have "(\<integral>\<^isup>+x. f i x * indicator (Q i \<inter> A) x \<partial>M) = emeasure (density (?M i) (f i)) A"
   690       by (auto simp add: emeasure_density positive_integral_density subset_eq
   691                intro!: positive_integral_cong split: split_indicator)
   692     also have "\<dots> = emeasure N (Q i \<inter> A)"
   693       using A Q by (simp add: f_density emeasure_restricted subset_eq sets_eq)
   694     finally have "emeasure N (Q i \<inter> A) = (\<integral>\<^isup>+x. f i x * indicator (Q i \<inter> A) x \<partial>M)" .. }
   695   note integral_eq = this
   696   let ?f = "\<lambda>x. (\<Sum>i. f i x * indicator (Q i) x) + \<infinity> * indicator Q0 x"
   697   show ?thesis
   698   proof (safe intro!: bexI[of _ ?f])
   699     show "?f \<in> borel_measurable M" using Q0 borel Q_sets
   700       by (auto intro!: measurable_If)
   701     show "\<And>x. 0 \<le> ?f x" using borel by (auto intro!: suminf_0_le simp: indicator_def)
   702     show "density M ?f = N"
   703     proof (rule measure_eqI)
   704       fix A assume "A \<in> sets (density M ?f)"
   705       then have "A \<in> sets M" by simp
   706       have Qi: "\<And>i. Q i \<in> sets M" using Q by auto
   707       have [intro,simp]: "\<And>i. (\<lambda>x. f i x * indicator (Q i \<inter> A) x) \<in> borel_measurable M"
   708         "\<And>i. AE x in M. 0 \<le> f i x * indicator (Q i \<inter> A) x"
   709         using borel Qi Q0(1) `A \<in> sets M` by (auto intro!: borel_measurable_ereal_times)
   710       have "(\<integral>\<^isup>+x. ?f x * indicator A x \<partial>M) = (\<integral>\<^isup>+x. (\<Sum>i. f i x * indicator (Q i \<inter> A) x) + \<infinity> * indicator (Q0 \<inter> A) x \<partial>M)"
   711         using borel by (intro positive_integral_cong) (auto simp: indicator_def)
   712       also have "\<dots> = (\<integral>\<^isup>+x. (\<Sum>i. f i x * indicator (Q i \<inter> A) x) \<partial>M) + \<infinity> * emeasure M (Q0 \<inter> A)"
   713         using borel Qi Q0(1) `A \<in> sets M`
   714         by (subst positive_integral_add) (auto simp del: ereal_infty_mult
   715             simp add: positive_integral_cmult_indicator sets.Int intro!: suminf_0_le)
   716       also have "\<dots> = (\<Sum>i. N (Q i \<inter> A)) + \<infinity> * emeasure M (Q0 \<inter> A)"
   717         by (subst integral_eq[OF `A \<in> sets M`], subst positive_integral_suminf) auto
   718       finally have "(\<integral>\<^isup>+x. ?f x * indicator A x \<partial>M) = (\<Sum>i. N (Q i \<inter> A)) + \<infinity> * emeasure M (Q0 \<inter> A)" .
   719       moreover have "(\<Sum>i. N (Q i \<inter> A)) = N ((\<Union>i. Q i) \<inter> A)"
   720         using Q Q_sets `A \<in> sets M`
   721         by (subst suminf_emeasure) (auto simp: disjoint_family_on_def sets_eq)
   722       moreover have "\<infinity> * emeasure M (Q0 \<inter> A) = N (Q0 \<inter> A)"
   723       proof -
   724         have "Q0 \<inter> A \<in> sets M" using Q0(1) `A \<in> sets M` by blast
   725         from in_Q0[OF this] show ?thesis by auto
   726       qed
   727       moreover have "Q0 \<inter> A \<in> sets M" "((\<Union>i. Q i) \<inter> A) \<in> sets M"
   728         using Q_sets `A \<in> sets M` Q0(1) by auto
   729       moreover have "((\<Union>i. Q i) \<inter> A) \<union> (Q0 \<inter> A) = A" "((\<Union>i. Q i) \<inter> A) \<inter> (Q0 \<inter> A) = {}"
   730         using `A \<in> sets M` sets.sets_into_space Q0 by auto
   731       ultimately have "N A = (\<integral>\<^isup>+x. ?f x * indicator A x \<partial>M)"
   732         using plus_emeasure[of "(\<Union>i. Q i) \<inter> A" N "Q0 \<inter> A"] by (simp add: sets_eq)
   733       with `A \<in> sets M` borel Q Q0(1) show "emeasure (density M ?f) A = N A"
   734         by (auto simp: subset_eq emeasure_density)
   735     qed (simp add: sets_eq)
   736   qed
   737 qed
   738 
   739 lemma (in sigma_finite_measure) Radon_Nikodym:
   740   assumes ac: "absolutely_continuous M N" assumes sets_eq: "sets N = sets M"
   741   shows "\<exists>f \<in> borel_measurable M. (\<forall>x. 0 \<le> f x) \<and> density M f = N"
   742 proof -
   743   from Ex_finite_integrable_function
   744   obtain h where finite: "integral\<^isup>P M h \<noteq> \<infinity>" and
   745     borel: "h \<in> borel_measurable M" and
   746     nn: "\<And>x. 0 \<le> h x" and
   747     pos: "\<And>x. x \<in> space M \<Longrightarrow> 0 < h x" and
   748     "\<And>x. x \<in> space M \<Longrightarrow> h x < \<infinity>" by auto
   749   let ?T = "\<lambda>A. (\<integral>\<^isup>+x. h x * indicator A x \<partial>M)"
   750   let ?MT = "density M h"
   751   from borel finite nn interpret T: finite_measure ?MT
   752     by (auto intro!: finite_measureI cong: positive_integral_cong simp: emeasure_density)
   753   have "absolutely_continuous ?MT N" "sets N = sets ?MT"
   754   proof (unfold absolutely_continuous_def, safe)
   755     fix A assume "A \<in> null_sets ?MT"
   756     with borel have "A \<in> sets M" "AE x in M. x \<in> A \<longrightarrow> h x \<le> 0"
   757       by (auto simp add: null_sets_density_iff)
   758     with pos sets.sets_into_space have "AE x in M. x \<notin> A"
   759       by (elim eventually_elim1) (auto simp: not_le[symmetric])
   760     then have "A \<in> null_sets M"
   761       using `A \<in> sets M` by (simp add: AE_iff_null_sets)
   762     with ac show "A \<in> null_sets N"
   763       by (auto simp: absolutely_continuous_def)
   764   qed (auto simp add: sets_eq)
   765   from T.Radon_Nikodym_finite_measure_infinite[OF this]
   766   obtain f where f_borel: "f \<in> borel_measurable M" "\<And>x. 0 \<le> f x" "density ?MT f = N" by auto
   767   with nn borel show ?thesis
   768     by (auto intro!: bexI[of _ "\<lambda>x. h x * f x"] simp: density_density_eq)
   769 qed
   770 
   771 section "Uniqueness of densities"
   772 
   773 lemma finite_density_unique:
   774   assumes borel: "f \<in> borel_measurable M" "g \<in> borel_measurable M"
   775   assumes pos: "AE x in M. 0 \<le> f x" "AE x in M. 0 \<le> g x"
   776   and fin: "integral\<^isup>P M f \<noteq> \<infinity>"
   777   shows "density M f = density M g \<longleftrightarrow> (AE x in M. f x = g x)"
   778 proof (intro iffI ballI)
   779   fix A assume eq: "AE x in M. f x = g x"
   780   with borel show "density M f = density M g"
   781     by (auto intro: density_cong)
   782 next
   783   let ?P = "\<lambda>f A. \<integral>\<^isup>+ x. f x * indicator A x \<partial>M"
   784   assume "density M f = density M g"
   785   with borel have eq: "\<forall>A\<in>sets M. ?P f A = ?P g A"
   786     by (simp add: emeasure_density[symmetric])
   787   from this[THEN bspec, OF sets.top] fin
   788   have g_fin: "integral\<^isup>P M g \<noteq> \<infinity>" by (simp cong: positive_integral_cong)
   789   { fix f g assume borel: "f \<in> borel_measurable M" "g \<in> borel_measurable M"
   790       and pos: "AE x in M. 0 \<le> f x" "AE x in M. 0 \<le> g x"
   791       and g_fin: "integral\<^isup>P M g \<noteq> \<infinity>" and eq: "\<forall>A\<in>sets M. ?P f A = ?P g A"
   792     let ?N = "{x\<in>space M. g x < f x}"
   793     have N: "?N \<in> sets M" using borel by simp
   794     have "?P g ?N \<le> integral\<^isup>P M g" using pos
   795       by (intro positive_integral_mono_AE) (auto split: split_indicator)
   796     then have Pg_fin: "?P g ?N \<noteq> \<infinity>" using g_fin by auto
   797     have "?P (\<lambda>x. (f x - g x)) ?N = (\<integral>\<^isup>+x. f x * indicator ?N x - g x * indicator ?N x \<partial>M)"
   798       by (auto intro!: positive_integral_cong simp: indicator_def)
   799     also have "\<dots> = ?P f ?N - ?P g ?N"
   800     proof (rule positive_integral_diff)
   801       show "(\<lambda>x. f x * indicator ?N x) \<in> borel_measurable M" "(\<lambda>x. g x * indicator ?N x) \<in> borel_measurable M"
   802         using borel N by auto
   803       show "AE x in M. g x * indicator ?N x \<le> f x * indicator ?N x"
   804            "AE x in M. 0 \<le> g x * indicator ?N x"
   805         using pos by (auto split: split_indicator)
   806     qed fact
   807     also have "\<dots> = 0"
   808       unfolding eq[THEN bspec, OF N] using positive_integral_positive[of M] Pg_fin by auto
   809     finally have "AE x in M. f x \<le> g x"
   810       using pos borel positive_integral_PInf_AE[OF borel(2) g_fin]
   811       by (subst (asm) positive_integral_0_iff_AE)
   812          (auto split: split_indicator simp: not_less ereal_minus_le_iff) }
   813   from this[OF borel pos g_fin eq] this[OF borel(2,1) pos(2,1) fin] eq
   814   show "AE x in M. f x = g x" by auto
   815 qed
   816 
   817 lemma (in finite_measure) density_unique_finite_measure:
   818   assumes borel: "f \<in> borel_measurable M" "f' \<in> borel_measurable M"
   819   assumes pos: "AE x in M. 0 \<le> f x" "AE x in M. 0 \<le> f' x"
   820   assumes f: "\<And>A. A \<in> sets M \<Longrightarrow> (\<integral>\<^isup>+x. f x * indicator A x \<partial>M) = (\<integral>\<^isup>+x. f' x * indicator A x \<partial>M)"
   821     (is "\<And>A. A \<in> sets M \<Longrightarrow> ?P f A = ?P f' A")
   822   shows "AE x in M. f x = f' x"
   823 proof -
   824   let ?D = "\<lambda>f. density M f"
   825   let ?N = "\<lambda>A. ?P f A" and ?N' = "\<lambda>A. ?P f' A"
   826   let ?f = "\<lambda>A x. f x * indicator A x" and ?f' = "\<lambda>A x. f' x * indicator A x"
   827 
   828   have ac: "absolutely_continuous M (density M f)" "sets (density M f) = sets M"
   829     using borel by (auto intro!: absolutely_continuousI_density) 
   830   from split_space_into_finite_sets_and_rest[OF this]
   831   obtain Q0 and Q :: "nat \<Rightarrow> 'a set"
   832     where Q: "disjoint_family Q" "range Q \<subseteq> sets M"
   833     and Q0: "Q0 \<in> sets M" "Q0 = space M - (\<Union>i. Q i)"
   834     and in_Q0: "\<And>A. A \<in> sets M \<Longrightarrow> A \<subseteq> Q0 \<Longrightarrow> emeasure M A = 0 \<and> ?D f A = 0 \<or> 0 < emeasure M A \<and> ?D f A = \<infinity>"
   835     and Q_fin: "\<And>i. ?D f (Q i) \<noteq> \<infinity>" by force
   836   with borel pos have in_Q0: "\<And>A. A \<in> sets M \<Longrightarrow> A \<subseteq> Q0 \<Longrightarrow> emeasure M A = 0 \<and> ?N A = 0 \<or> 0 < emeasure M A \<and> ?N A = \<infinity>"
   837     and Q_fin: "\<And>i. ?N (Q i) \<noteq> \<infinity>" by (auto simp: emeasure_density subset_eq)
   838 
   839   from Q have Q_sets: "\<And>i. Q i \<in> sets M" by auto
   840   let ?D = "{x\<in>space M. f x \<noteq> f' x}"
   841   have "?D \<in> sets M" using borel by auto
   842   have *: "\<And>i x A. \<And>y::ereal. y * indicator (Q i) x * indicator A x = y * indicator (Q i \<inter> A) x"
   843     unfolding indicator_def by auto
   844   have "\<forall>i. AE x in M. ?f (Q i) x = ?f' (Q i) x" using borel Q_fin Q pos
   845     by (intro finite_density_unique[THEN iffD1] allI)
   846        (auto intro!: f measure_eqI simp: emeasure_density * subset_eq)
   847   moreover have "AE x in M. ?f Q0 x = ?f' Q0 x"
   848   proof (rule AE_I')
   849     { fix f :: "'a \<Rightarrow> ereal" assume borel: "f \<in> borel_measurable M"
   850         and eq: "\<And>A. A \<in> sets M \<Longrightarrow> ?N A = (\<integral>\<^isup>+x. f x * indicator A x \<partial>M)"
   851       let ?A = "\<lambda>i. Q0 \<inter> {x \<in> space M. f x < (i::nat)}"
   852       have "(\<Union>i. ?A i) \<in> null_sets M"
   853       proof (rule null_sets_UN)
   854         fix i ::nat have "?A i \<in> sets M"
   855           using borel Q0(1) by auto
   856         have "?N (?A i) \<le> (\<integral>\<^isup>+x. (i::ereal) * indicator (?A i) x \<partial>M)"
   857           unfolding eq[OF `?A i \<in> sets M`]
   858           by (auto intro!: positive_integral_mono simp: indicator_def)
   859         also have "\<dots> = i * emeasure M (?A i)"
   860           using `?A i \<in> sets M` by (auto intro!: positive_integral_cmult_indicator)
   861         also have "\<dots> < \<infinity>" using emeasure_real[of "?A i"] by simp
   862         finally have "?N (?A i) \<noteq> \<infinity>" by simp
   863         then show "?A i \<in> null_sets M" using in_Q0[OF `?A i \<in> sets M`] `?A i \<in> sets M` by auto
   864       qed
   865       also have "(\<Union>i. ?A i) = Q0 \<inter> {x\<in>space M. f x \<noteq> \<infinity>}"
   866         by (auto simp: less_PInf_Ex_of_nat real_eq_of_nat)
   867       finally have "Q0 \<inter> {x\<in>space M. f x \<noteq> \<infinity>} \<in> null_sets M" by simp }
   868     from this[OF borel(1) refl] this[OF borel(2) f]
   869     have "Q0 \<inter> {x\<in>space M. f x \<noteq> \<infinity>} \<in> null_sets M" "Q0 \<inter> {x\<in>space M. f' x \<noteq> \<infinity>} \<in> null_sets M" by simp_all
   870     then show "(Q0 \<inter> {x\<in>space M. f x \<noteq> \<infinity>}) \<union> (Q0 \<inter> {x\<in>space M. f' x \<noteq> \<infinity>}) \<in> null_sets M" by (rule null_sets.Un)
   871     show "{x \<in> space M. ?f Q0 x \<noteq> ?f' Q0 x} \<subseteq>
   872       (Q0 \<inter> {x\<in>space M. f x \<noteq> \<infinity>}) \<union> (Q0 \<inter> {x\<in>space M. f' x \<noteq> \<infinity>})" by (auto simp: indicator_def)
   873   qed
   874   moreover have "AE x in M. (?f Q0 x = ?f' Q0 x) \<longrightarrow> (\<forall>i. ?f (Q i) x = ?f' (Q i) x) \<longrightarrow>
   875     ?f (space M) x = ?f' (space M) x"
   876     by (auto simp: indicator_def Q0)
   877   ultimately have "AE x in M. ?f (space M) x = ?f' (space M) x"
   878     unfolding AE_all_countable[symmetric]
   879     by eventually_elim (auto intro!: AE_I2 split: split_if_asm simp: indicator_def)
   880   then show "AE x in M. f x = f' x" by auto
   881 qed
   882 
   883 lemma (in sigma_finite_measure) density_unique:
   884   assumes f: "f \<in> borel_measurable M" "AE x in M. 0 \<le> f x"
   885   assumes f': "f' \<in> borel_measurable M" "AE x in M. 0 \<le> f' x"
   886   assumes density_eq: "density M f = density M f'"
   887   shows "AE x in M. f x = f' x"
   888 proof -
   889   obtain h where h_borel: "h \<in> borel_measurable M"
   890     and fin: "integral\<^isup>P M h \<noteq> \<infinity>" and pos: "\<And>x. x \<in> space M \<Longrightarrow> 0 < h x \<and> h x < \<infinity>" "\<And>x. 0 \<le> h x"
   891     using Ex_finite_integrable_function by auto
   892   then have h_nn: "AE x in M. 0 \<le> h x" by auto
   893   let ?H = "density M h"
   894   interpret h: finite_measure ?H
   895     using fin h_borel pos
   896     by (intro finite_measureI) (simp cong: positive_integral_cong emeasure_density add: fin)
   897   let ?fM = "density M f"
   898   let ?f'M = "density M f'"
   899   { fix A assume "A \<in> sets M"
   900     then have "{x \<in> space M. h x * indicator A x \<noteq> 0} = A"
   901       using pos(1) sets.sets_into_space by (force simp: indicator_def)
   902     then have "(\<integral>\<^isup>+x. h x * indicator A x \<partial>M) = 0 \<longleftrightarrow> A \<in> null_sets M"
   903       using h_borel `A \<in> sets M` h_nn by (subst positive_integral_0_iff) auto }
   904   note h_null_sets = this
   905   { fix A assume "A \<in> sets M"
   906     have "(\<integral>\<^isup>+x. f x * (h x * indicator A x) \<partial>M) = (\<integral>\<^isup>+x. h x * indicator A x \<partial>?fM)"
   907       using `A \<in> sets M` h_borel h_nn f f'
   908       by (intro positive_integral_density[symmetric]) auto
   909     also have "\<dots> = (\<integral>\<^isup>+x. h x * indicator A x \<partial>?f'M)"
   910       by (simp_all add: density_eq)
   911     also have "\<dots> = (\<integral>\<^isup>+x. f' x * (h x * indicator A x) \<partial>M)"
   912       using `A \<in> sets M` h_borel h_nn f f'
   913       by (intro positive_integral_density) auto
   914     finally have "(\<integral>\<^isup>+x. h x * (f x * indicator A x) \<partial>M) = (\<integral>\<^isup>+x. h x * (f' x * indicator A x) \<partial>M)"
   915       by (simp add: ac_simps)
   916     then have "(\<integral>\<^isup>+x. (f x * indicator A x) \<partial>?H) = (\<integral>\<^isup>+x. (f' x * indicator A x) \<partial>?H)"
   917       using `A \<in> sets M` h_borel h_nn f f'
   918       by (subst (asm) (1 2) positive_integral_density[symmetric]) auto }
   919   then have "AE x in ?H. f x = f' x" using h_borel h_nn f f'
   920     by (intro h.density_unique_finite_measure absolutely_continuous_AE[of M])
   921        (auto simp add: AE_density)
   922   then show "AE x in M. f x = f' x"
   923     unfolding eventually_ae_filter using h_borel pos
   924     by (auto simp add: h_null_sets null_sets_density_iff not_less[symmetric]
   925                           AE_iff_null_sets[symmetric]) blast
   926 qed
   927 
   928 lemma (in sigma_finite_measure) density_unique_iff:
   929   assumes f: "f \<in> borel_measurable M" and "AE x in M. 0 \<le> f x"
   930   assumes f': "f' \<in> borel_measurable M" and "AE x in M. 0 \<le> f' x"
   931   shows "density M f = density M f' \<longleftrightarrow> (AE x in M. f x = f' x)"
   932   using density_unique[OF assms] density_cong[OF f f'] by auto
   933 
   934 lemma sigma_finite_density_unique:
   935   assumes borel: "f \<in> borel_measurable M" "g \<in> borel_measurable M"
   936   assumes pos: "AE x in M. 0 \<le> f x" "AE x in M. 0 \<le> g x"
   937   and fin: "sigma_finite_measure (density M f)"
   938   shows "density M f = density M g \<longleftrightarrow> (AE x in M. f x = g x)"
   939 proof
   940   assume "AE x in M. f x = g x" with borel show "density M f = density M g" 
   941     by (auto intro: density_cong)
   942 next
   943   assume eq: "density M f = density M g"
   944   interpret f!: sigma_finite_measure "density M f" by fact
   945   from f.sigma_finite_incseq guess A . note cover = this
   946 
   947   have "AE x in M. \<forall>i. x \<in> A i \<longrightarrow> f x = g x"
   948     unfolding AE_all_countable
   949   proof
   950     fix i
   951     have "density (density M f) (indicator (A i)) = density (density M g) (indicator (A i))"
   952       unfolding eq ..
   953     moreover have "(\<integral>\<^isup>+x. f x * indicator (A i) x \<partial>M) \<noteq> \<infinity>"
   954       using cover(1) cover(3)[of i] borel by (auto simp: emeasure_density subset_eq)
   955     ultimately have "AE x in M. f x * indicator (A i) x = g x * indicator (A i) x"
   956       using borel pos cover(1) pos
   957       by (intro finite_density_unique[THEN iffD1])
   958          (auto simp: density_density_eq subset_eq)
   959     then show "AE x in M. x \<in> A i \<longrightarrow> f x = g x"
   960       by auto
   961   qed
   962   with AE_space show "AE x in M. f x = g x"
   963     apply eventually_elim
   964     using cover(2)[symmetric]
   965     apply auto
   966     done
   967 qed
   968 
   969 lemma (in sigma_finite_measure) sigma_finite_iff_density_finite':
   970   assumes f: "f \<in> borel_measurable M" "AE x in M. 0 \<le> f x"
   971   shows "sigma_finite_measure (density M f) \<longleftrightarrow> (AE x in M. f x \<noteq> \<infinity>)"
   972     (is "sigma_finite_measure ?N \<longleftrightarrow> _")
   973 proof
   974   assume "sigma_finite_measure ?N"
   975   then interpret N: sigma_finite_measure ?N .
   976   from N.Ex_finite_integrable_function obtain h where
   977     h: "h \<in> borel_measurable M" "integral\<^isup>P ?N h \<noteq> \<infinity>" and
   978     h_nn: "\<And>x. 0 \<le> h x" and
   979     fin: "\<forall>x\<in>space M. 0 < h x \<and> h x < \<infinity>" by auto
   980   have "AE x in M. f x * h x \<noteq> \<infinity>"
   981   proof (rule AE_I')
   982     have "integral\<^isup>P ?N h = (\<integral>\<^isup>+x. f x * h x \<partial>M)" using f h h_nn
   983       by (auto intro!: positive_integral_density)
   984     then have "(\<integral>\<^isup>+x. f x * h x \<partial>M) \<noteq> \<infinity>"
   985       using h(2) by simp
   986     then show "(\<lambda>x. f x * h x) -` {\<infinity>} \<inter> space M \<in> null_sets M"
   987       using f h(1) by (auto intro!: positive_integral_PInf borel_measurable_vimage)
   988   qed auto
   989   then show "AE x in M. f x \<noteq> \<infinity>"
   990     using fin by (auto elim!: AE_Ball_mp)
   991 next
   992   assume AE: "AE x in M. f x \<noteq> \<infinity>"
   993   from sigma_finite guess Q .. note Q = this
   994   def A \<equiv> "\<lambda>i. f -` (case i of 0 \<Rightarrow> {\<infinity>} | Suc n \<Rightarrow> {.. ereal(of_nat (Suc n))}) \<inter> space M"
   995   { fix i j have "A i \<inter> Q j \<in> sets M"
   996     unfolding A_def using f Q
   997     apply (rule_tac sets.Int)
   998     by (cases i) (auto intro: measurable_sets[OF f(1)]) }
   999   note A_in_sets = this
  1000   let ?A = "\<lambda>n. case prod_decode n of (i,j) \<Rightarrow> A i \<inter> Q j"
  1001   show "sigma_finite_measure ?N"
  1002   proof (default, intro exI conjI subsetI allI)
  1003     fix x assume "x \<in> range ?A"
  1004     then obtain n where n: "x = ?A n" by auto
  1005     then show "x \<in> sets ?N" using A_in_sets by (cases "prod_decode n") auto
  1006   next
  1007     have "(\<Union>i. ?A i) = (\<Union>i j. A i \<inter> Q j)"
  1008     proof safe
  1009       fix x i j assume "x \<in> A i" "x \<in> Q j"
  1010       then show "x \<in> (\<Union>i. case prod_decode i of (i, j) \<Rightarrow> A i \<inter> Q j)"
  1011         by (intro UN_I[of "prod_encode (i,j)"]) auto
  1012     qed auto
  1013     also have "\<dots> = (\<Union>i. A i) \<inter> space M" using Q by auto
  1014     also have "(\<Union>i. A i) = space M"
  1015     proof safe
  1016       fix x assume x: "x \<in> space M"
  1017       show "x \<in> (\<Union>i. A i)"
  1018       proof (cases "f x")
  1019         case PInf with x show ?thesis unfolding A_def by (auto intro: exI[of _ 0])
  1020       next
  1021         case (real r)
  1022         with less_PInf_Ex_of_nat[of "f x"] obtain n :: nat where "f x < n" by (auto simp: real_eq_of_nat)
  1023         then show ?thesis using x real unfolding A_def by (auto intro!: exI[of _ "Suc n"] simp: real_eq_of_nat)
  1024       next
  1025         case MInf with x show ?thesis
  1026           unfolding A_def by (auto intro!: exI[of _ "Suc 0"])
  1027       qed
  1028     qed (auto simp: A_def)
  1029     finally show "(\<Union>i. ?A i) = space ?N" by simp
  1030   next
  1031     fix n obtain i j where
  1032       [simp]: "prod_decode n = (i, j)" by (cases "prod_decode n") auto
  1033     have "(\<integral>\<^isup>+x. f x * indicator (A i \<inter> Q j) x \<partial>M) \<noteq> \<infinity>"
  1034     proof (cases i)
  1035       case 0
  1036       have "AE x in M. f x * indicator (A i \<inter> Q j) x = 0"
  1037         using AE by (auto simp: A_def `i = 0`)
  1038       from positive_integral_cong_AE[OF this] show ?thesis by simp
  1039     next
  1040       case (Suc n)
  1041       then have "(\<integral>\<^isup>+x. f x * indicator (A i \<inter> Q j) x \<partial>M) \<le>
  1042         (\<integral>\<^isup>+x. (Suc n :: ereal) * indicator (Q j) x \<partial>M)"
  1043         by (auto intro!: positive_integral_mono simp: indicator_def A_def real_eq_of_nat)
  1044       also have "\<dots> = Suc n * emeasure M (Q j)"
  1045         using Q by (auto intro!: positive_integral_cmult_indicator)
  1046       also have "\<dots> < \<infinity>"
  1047         using Q by (auto simp: real_eq_of_nat[symmetric])
  1048       finally show ?thesis by simp
  1049     qed
  1050     then show "emeasure ?N (?A n) \<noteq> \<infinity>"
  1051       using A_in_sets Q f by (auto simp: emeasure_density)
  1052   qed
  1053 qed
  1054 
  1055 lemma (in sigma_finite_measure) sigma_finite_iff_density_finite:
  1056   "f \<in> borel_measurable M \<Longrightarrow> sigma_finite_measure (density M f) \<longleftrightarrow> (AE x in M. f x \<noteq> \<infinity>)"
  1057   apply (subst density_max_0)
  1058   apply (subst sigma_finite_iff_density_finite')
  1059   apply (auto simp: max_def intro!: measurable_If)
  1060   done
  1061 
  1062 section "Radon-Nikodym derivative"
  1063 
  1064 definition
  1065   "RN_deriv M N \<equiv> SOME f. f \<in> borel_measurable M \<and> (\<forall>x. 0 \<le> f x) \<and> density M f = N"
  1066 
  1067 lemma
  1068   assumes f: "f \<in> borel_measurable M" "AE x in M. 0 \<le> f x"
  1069   shows borel_measurable_RN_deriv_density: "RN_deriv M (density M f) \<in> borel_measurable M" (is ?borel)
  1070     and density_RN_deriv_density: "density M (RN_deriv M (density M f)) = density M f" (is ?density)
  1071     and RN_deriv_density_nonneg: "0 \<le> RN_deriv M (density M f) x" (is ?pos)
  1072 proof -
  1073   let ?f = "\<lambda>x. max 0 (f x)"
  1074   let ?P = "\<lambda>g. g \<in> borel_measurable M \<and> (\<forall>x. 0 \<le> g x) \<and> density M g = density M f"
  1075   from f have "?P ?f" using f by (auto intro!: density_cong simp: split: split_max)
  1076   then have "?P (RN_deriv M (density M f))"
  1077     unfolding RN_deriv_def by (rule someI[where P="?P"])
  1078   then show ?borel ?density ?pos by auto
  1079 qed
  1080 
  1081 lemma (in sigma_finite_measure) RN_deriv:
  1082   assumes "absolutely_continuous M N" "sets N = sets M"
  1083   shows borel_measurable_RN_deriv[measurable]: "RN_deriv M N \<in> borel_measurable M" (is ?borel)
  1084     and density_RN_deriv: "density M (RN_deriv M N) = N" (is ?density)
  1085     and RN_deriv_nonneg: "0 \<le> RN_deriv M N x" (is ?pos)
  1086 proof -
  1087   note Ex = Radon_Nikodym[OF assms, unfolded Bex_def]
  1088   from Ex show ?borel unfolding RN_deriv_def by (rule someI2_ex) simp
  1089   from Ex show ?density unfolding RN_deriv_def by (rule someI2_ex) simp
  1090   from Ex show ?pos unfolding RN_deriv_def by (rule someI2_ex) simp
  1091 qed
  1092 
  1093 lemma (in sigma_finite_measure) RN_deriv_positive_integral:
  1094   assumes N: "absolutely_continuous M N" "sets N = sets M"
  1095     and f: "f \<in> borel_measurable M"
  1096   shows "integral\<^isup>P N f = (\<integral>\<^isup>+x. RN_deriv M N x * f x \<partial>M)"
  1097 proof -
  1098   have "integral\<^isup>P N f = integral\<^isup>P (density M (RN_deriv M N)) f"
  1099     using N by (simp add: density_RN_deriv)
  1100   also have "\<dots> = (\<integral>\<^isup>+x. RN_deriv M N x * f x \<partial>M)"
  1101     using RN_deriv(1,3)[OF N] f by (simp add: positive_integral_density)
  1102   finally show ?thesis by simp
  1103 qed
  1104 
  1105 lemma null_setsD_AE: "N \<in> null_sets M \<Longrightarrow> AE x in M. x \<notin> N"
  1106   using AE_iff_null_sets[of N M] by auto
  1107 
  1108 lemma (in sigma_finite_measure) RN_deriv_unique:
  1109   assumes f: "f \<in> borel_measurable M" "AE x in M. 0 \<le> f x"
  1110   and eq: "density M f = N"
  1111   shows "AE x in M. f x = RN_deriv M N x"
  1112   unfolding eq[symmetric]
  1113   by (intro density_unique_iff[THEN iffD1] f borel_measurable_RN_deriv_density
  1114             RN_deriv_density_nonneg[THEN AE_I2] density_RN_deriv_density[symmetric])
  1115 
  1116 lemma RN_deriv_unique_sigma_finite:
  1117   assumes f: "f \<in> borel_measurable M" "AE x in M. 0 \<le> f x"
  1118   and eq: "density M f = N" and fin: "sigma_finite_measure N"
  1119   shows "AE x in M. f x = RN_deriv M N x"
  1120   using fin unfolding eq[symmetric]
  1121   by (intro sigma_finite_density_unique[THEN iffD1] f borel_measurable_RN_deriv_density
  1122             RN_deriv_density_nonneg[THEN AE_I2] density_RN_deriv_density[symmetric])
  1123 
  1124 lemma (in sigma_finite_measure) RN_deriv_distr:
  1125   fixes T :: "'a \<Rightarrow> 'b"
  1126   assumes T: "T \<in> measurable M M'" and T': "T' \<in> measurable M' M"
  1127     and inv: "\<forall>x\<in>space M. T' (T x) = x"
  1128   and ac[simp]: "absolutely_continuous (distr M M' T) (distr N M' T)"
  1129   and N: "sets N = sets M"
  1130   shows "AE x in M. RN_deriv (distr M M' T) (distr N M' T) (T x) = RN_deriv M N x"
  1131 proof (rule RN_deriv_unique)
  1132   have [simp]: "sets N = sets M" by fact
  1133   note sets_eq_imp_space_eq[OF N, simp]
  1134   have measurable_N[simp]: "\<And>M'. measurable N M' = measurable M M'" by (auto simp: measurable_def)
  1135   { fix A assume "A \<in> sets M"
  1136     with inv T T' sets.sets_into_space[OF this]
  1137     have "T -` T' -` A \<inter> T -` space M' \<inter> space M = A"
  1138       by (auto simp: measurable_def) }
  1139   note eq = this[simp]
  1140   { fix A assume "A \<in> sets M"
  1141     with inv T T' sets.sets_into_space[OF this]
  1142     have "(T' \<circ> T) -` A \<inter> space M = A"
  1143       by (auto simp: measurable_def) }
  1144   note eq2 = this[simp]
  1145   let ?M' = "distr M M' T" and ?N' = "distr N M' T"
  1146   interpret M': sigma_finite_measure ?M'
  1147   proof
  1148     from sigma_finite guess F .. note F = this
  1149     show "\<exists>A::nat \<Rightarrow> 'b set. range A \<subseteq> sets ?M' \<and> (\<Union>i. A i) = space ?M' \<and> (\<forall>i. emeasure ?M' (A i) \<noteq> \<infinity>)"
  1150     proof (intro exI conjI allI)
  1151       show *: "range (\<lambda>i. T' -` F i \<inter> space ?M') \<subseteq> sets ?M'"
  1152         using F T' by (auto simp: measurable_def)
  1153       show "(\<Union>i. T' -` F i \<inter> space ?M') = space ?M'"
  1154         using F T' by (force simp: measurable_def)
  1155       fix i
  1156       have "T' -` F i \<inter> space M' \<in> sets M'" using * by auto
  1157       moreover
  1158       have Fi: "F i \<in> sets M" using F by auto
  1159       ultimately show "emeasure ?M' (T' -` F i \<inter> space ?M') \<noteq> \<infinity>"
  1160         using F T T' by (simp add: emeasure_distr)
  1161     qed
  1162   qed
  1163   have "(RN_deriv ?M' ?N') \<circ> T \<in> borel_measurable M"
  1164     using T ac by measurable
  1165   then show "(\<lambda>x. RN_deriv ?M' ?N' (T x)) \<in> borel_measurable M"
  1166     by (simp add: comp_def)
  1167   show "AE x in M. 0 \<le> RN_deriv ?M' ?N' (T x)" using M'.RN_deriv_nonneg[OF ac] by auto
  1168 
  1169   have "N = distr N M (T' \<circ> T)"
  1170     by (subst measure_of_of_measure[of N, symmetric])
  1171        (auto simp add: distr_def sets.sigma_sets_eq intro!: measure_of_eq sets.space_closed)
  1172   also have "\<dots> = distr (distr N M' T) M T'"
  1173     using T T' by (simp add: distr_distr)
  1174   also have "\<dots> = distr (density (distr M M' T) (RN_deriv (distr M M' T) (distr N M' T))) M T'"
  1175     using ac by (simp add: M'.density_RN_deriv)
  1176   also have "\<dots> = density M (RN_deriv (distr M M' T) (distr N M' T) \<circ> T)"
  1177     using M'.borel_measurable_RN_deriv[OF ac] by (simp add: distr_density_distr[OF T T', OF inv])
  1178   finally show "density M (\<lambda>x. RN_deriv (distr M M' T) (distr N M' T) (T x)) = N"
  1179     by (simp add: comp_def)
  1180 qed
  1181 
  1182 lemma (in sigma_finite_measure) RN_deriv_finite:
  1183   assumes N: "sigma_finite_measure N" and ac: "absolutely_continuous M N" "sets N = sets M"
  1184   shows "AE x in M. RN_deriv M N x \<noteq> \<infinity>"
  1185 proof -
  1186   interpret N: sigma_finite_measure N by fact
  1187   from N show ?thesis
  1188     using sigma_finite_iff_density_finite[OF RN_deriv(1)[OF ac]] RN_deriv(2,3)[OF ac] by simp
  1189 qed
  1190 
  1191 lemma (in sigma_finite_measure)
  1192   assumes N: "sigma_finite_measure N" and ac: "absolutely_continuous M N" "sets N = sets M"
  1193     and f: "f \<in> borel_measurable M"
  1194   shows RN_deriv_integrable: "integrable N f \<longleftrightarrow>
  1195       integrable M (\<lambda>x. real (RN_deriv M N x) * f x)" (is ?integrable)
  1196     and RN_deriv_integral: "integral\<^isup>L N f =
  1197       (\<integral>x. real (RN_deriv M N x) * f x \<partial>M)" (is ?integral)
  1198 proof -
  1199   note ac(2)[simp] and sets_eq_imp_space_eq[OF ac(2), simp]
  1200   interpret N: sigma_finite_measure N by fact
  1201   have minus_cong: "\<And>A B A' B'::ereal. A = A' \<Longrightarrow> B = B' \<Longrightarrow> real A - real B = real A' - real B'" by simp
  1202   have f': "(\<lambda>x. - f x) \<in> borel_measurable M" using f by auto
  1203   have Nf: "f \<in> borel_measurable N" using f by (simp add: measurable_def)
  1204   { fix f :: "'a \<Rightarrow> real"
  1205     { fix x assume *: "RN_deriv M N x \<noteq> \<infinity>"
  1206       have "ereal (real (RN_deriv M N x)) * ereal (f x) = ereal (real (RN_deriv M N x) * f x)"
  1207         by (simp add: mult_le_0_iff)
  1208       then have "RN_deriv M N x * ereal (f x) = ereal (real (RN_deriv M N x) * f x)"
  1209         using RN_deriv(3)[OF ac] * by (auto simp add: ereal_real split: split_if_asm) }
  1210     then have "(\<integral>\<^isup>+x. ereal (real (RN_deriv M N x) * f x) \<partial>M) = (\<integral>\<^isup>+x. RN_deriv M N x * ereal (f x) \<partial>M)"
  1211               "(\<integral>\<^isup>+x. ereal (- (real (RN_deriv M N x) * f x)) \<partial>M) = (\<integral>\<^isup>+x. RN_deriv M N x * ereal (- f x) \<partial>M)"
  1212       using RN_deriv_finite[OF N ac] unfolding ereal_mult_minus_right uminus_ereal.simps(1)[symmetric]
  1213       by (auto intro!: positive_integral_cong_AE) }
  1214   note * = this
  1215   show ?integral ?integrable
  1216     unfolding lebesgue_integral_def integrable_def *
  1217     using Nf f RN_deriv(1)[OF ac]
  1218     by (auto simp: RN_deriv_positive_integral[OF ac])
  1219 qed
  1220 
  1221 lemma (in sigma_finite_measure) real_RN_deriv:
  1222   assumes "finite_measure N"
  1223   assumes ac: "absolutely_continuous M N" "sets N = sets M"
  1224   obtains D where "D \<in> borel_measurable M"
  1225     and "AE x in M. RN_deriv M N x = ereal (D x)"
  1226     and "AE x in N. 0 < D x"
  1227     and "\<And>x. 0 \<le> D x"
  1228 proof
  1229   interpret N: finite_measure N by fact
  1230   
  1231   note RN = RN_deriv[OF ac]
  1232 
  1233   let ?RN = "\<lambda>t. {x \<in> space M. RN_deriv M N x = t}"
  1234 
  1235   show "(\<lambda>x. real (RN_deriv M N x)) \<in> borel_measurable M"
  1236     using RN by auto
  1237 
  1238   have "N (?RN \<infinity>) = (\<integral>\<^isup>+ x. RN_deriv M N x * indicator (?RN \<infinity>) x \<partial>M)"
  1239     using RN(1,3) by (subst RN(2)[symmetric]) (auto simp: emeasure_density)
  1240   also have "\<dots> = (\<integral>\<^isup>+ x. \<infinity> * indicator (?RN \<infinity>) x \<partial>M)"
  1241     by (intro positive_integral_cong) (auto simp: indicator_def)
  1242   also have "\<dots> = \<infinity> * emeasure M (?RN \<infinity>)"
  1243     using RN by (intro positive_integral_cmult_indicator) auto
  1244   finally have eq: "N (?RN \<infinity>) = \<infinity> * emeasure M (?RN \<infinity>)" .
  1245   moreover
  1246   have "emeasure M (?RN \<infinity>) = 0"
  1247   proof (rule ccontr)
  1248     assume "emeasure M {x \<in> space M. RN_deriv M N x = \<infinity>} \<noteq> 0"
  1249     moreover from RN have "0 \<le> emeasure M {x \<in> space M. RN_deriv M N x = \<infinity>}" by auto
  1250     ultimately have "0 < emeasure M {x \<in> space M. RN_deriv M N x = \<infinity>}" by auto
  1251     with eq have "N (?RN \<infinity>) = \<infinity>" by simp
  1252     with N.emeasure_finite[of "?RN \<infinity>"] RN show False by auto
  1253   qed
  1254   ultimately have "AE x in M. RN_deriv M N x < \<infinity>"
  1255     using RN by (intro AE_iff_measurable[THEN iffD2]) auto
  1256   then show "AE x in M. RN_deriv M N x = ereal (real (RN_deriv M N x))"
  1257     using RN(3) by (auto simp: ereal_real)
  1258   then have eq: "AE x in N. RN_deriv M N x = ereal (real (RN_deriv M N x))"
  1259     using ac absolutely_continuous_AE by auto
  1260 
  1261   show "\<And>x. 0 \<le> real (RN_deriv M N x)"
  1262     using RN by (auto intro: real_of_ereal_pos)
  1263 
  1264   have "N (?RN 0) = (\<integral>\<^isup>+ x. RN_deriv M N x * indicator (?RN 0) x \<partial>M)"
  1265     using RN(1,3) by (subst RN(2)[symmetric]) (auto simp: emeasure_density)
  1266   also have "\<dots> = (\<integral>\<^isup>+ x. 0 \<partial>M)"
  1267     by (intro positive_integral_cong) (auto simp: indicator_def)
  1268   finally have "AE x in N. RN_deriv M N x \<noteq> 0"
  1269     using RN by (subst AE_iff_measurable[OF _ refl]) (auto simp: ac cong: sets_eq_imp_space_eq)
  1270   with RN(3) eq show "AE x in N. 0 < real (RN_deriv M N x)"
  1271     by (auto simp: zero_less_real_of_ereal le_less)
  1272 qed
  1273 
  1274 lemma (in sigma_finite_measure) RN_deriv_singleton:
  1275   assumes ac: "absolutely_continuous M N" "sets N = sets M"
  1276   and x: "{x} \<in> sets M"
  1277   shows "N {x} = RN_deriv M N x * emeasure M {x}"
  1278 proof -
  1279   note deriv = RN_deriv[OF ac]
  1280   from deriv(1,3) `{x} \<in> sets M`
  1281   have "density M (RN_deriv M N) {x} = (\<integral>\<^isup>+w. RN_deriv M N x * indicator {x} w \<partial>M)"
  1282     by (auto simp: indicator_def emeasure_density intro!: positive_integral_cong)
  1283   with x deriv show ?thesis
  1284     by (auto simp: positive_integral_cmult_indicator)
  1285 qed
  1286 
  1287 end