src/HOL/Analysis/Infinite_Products.thy
 author eberlm Sat Jul 15 14:33:56 2017 +0100 (2017-07-15) changeset 66277 512b0dc09061 child 68064 b249fab48c76 permissions -rw-r--r--
HOL-Analysis: Infinite products
```     1 (*
```
```     2   File:      HOL/Analysis/Infinite_Product.thy
```
```     3   Author:    Manuel Eberl
```
```     4
```
```     5   Basic results about convergence and absolute convergence of infinite products
```
```     6   and their connection to summability.
```
```     7 *)
```
```     8 section \<open>Infinite Products\<close>
```
```     9 theory Infinite_Products
```
```    10   imports Complex_Main
```
```    11 begin
```
```    12
```
```    13 lemma sum_le_prod:
```
```    14   fixes f :: "'a \<Rightarrow> 'b :: linordered_semidom"
```
```    15   assumes "\<And>x. x \<in> A \<Longrightarrow> f x \<ge> 0"
```
```    16   shows   "sum f A \<le> (\<Prod>x\<in>A. 1 + f x)"
```
```    17   using assms
```
```    18 proof (induction A rule: infinite_finite_induct)
```
```    19   case (insert x A)
```
```    20   from insert.hyps have "sum f A + f x * (\<Prod>x\<in>A. 1) \<le> (\<Prod>x\<in>A. 1 + f x) + f x * (\<Prod>x\<in>A. 1 + f x)"
```
```    21     by (intro add_mono insert mult_left_mono prod_mono) (auto intro: insert.prems)
```
```    22   with insert.hyps show ?case by (simp add: algebra_simps)
```
```    23 qed simp_all
```
```    24
```
```    25 lemma prod_le_exp_sum:
```
```    26   fixes f :: "'a \<Rightarrow> real"
```
```    27   assumes "\<And>x. x \<in> A \<Longrightarrow> f x \<ge> 0"
```
```    28   shows   "prod (\<lambda>x. 1 + f x) A \<le> exp (sum f A)"
```
```    29   using assms
```
```    30 proof (induction A rule: infinite_finite_induct)
```
```    31   case (insert x A)
```
```    32   have "(1 + f x) * (\<Prod>x\<in>A. 1 + f x) \<le> exp (f x) * exp (sum f A)"
```
```    33     using insert.prems by (intro mult_mono insert prod_nonneg exp_ge_add_one_self) auto
```
```    34   with insert.hyps show ?case by (simp add: algebra_simps exp_add)
```
```    35 qed simp_all
```
```    36
```
```    37 lemma lim_ln_1_plus_x_over_x_at_0: "(\<lambda>x::real. ln (1 + x) / x) \<midarrow>0\<rightarrow> 1"
```
```    38 proof (rule lhopital)
```
```    39   show "(\<lambda>x::real. ln (1 + x)) \<midarrow>0\<rightarrow> 0"
```
```    40     by (rule tendsto_eq_intros refl | simp)+
```
```    41   have "eventually (\<lambda>x::real. x \<in> {-1/2<..<1/2}) (nhds 0)"
```
```    42     by (rule eventually_nhds_in_open) auto
```
```    43   hence *: "eventually (\<lambda>x::real. x \<in> {-1/2<..<1/2}) (at 0)"
```
```    44     by (rule filter_leD [rotated]) (simp_all add: at_within_def)
```
```    45   show "eventually (\<lambda>x::real. ((\<lambda>x. ln (1 + x)) has_field_derivative inverse (1 + x)) (at x)) (at 0)"
```
```    46     using * by eventually_elim (auto intro!: derivative_eq_intros simp: field_simps)
```
```    47   show "eventually (\<lambda>x::real. ((\<lambda>x. x) has_field_derivative 1) (at x)) (at 0)"
```
```    48     using * by eventually_elim (auto intro!: derivative_eq_intros simp: field_simps)
```
```    49   show "\<forall>\<^sub>F x in at 0. x \<noteq> 0" by (auto simp: at_within_def eventually_inf_principal)
```
```    50   show "(\<lambda>x::real. inverse (1 + x) / 1) \<midarrow>0\<rightarrow> 1"
```
```    51     by (rule tendsto_eq_intros refl | simp)+
```
```    52 qed auto
```
```    53
```
```    54 definition convergent_prod :: "(nat \<Rightarrow> 'a :: {t2_space,comm_semiring_1}) \<Rightarrow> bool" where
```
```    55   "convergent_prod f \<longleftrightarrow> (\<exists>M L. (\<lambda>n. \<Prod>i\<le>n. f (i+M)) \<longlonglongrightarrow> L \<and> L \<noteq> 0)"
```
```    56
```
```    57 lemma convergent_prod_altdef:
```
```    58   fixes f :: "nat \<Rightarrow> 'a :: {t2_space,comm_semiring_1}"
```
```    59   shows "convergent_prod f \<longleftrightarrow> (\<exists>M L. (\<forall>n\<ge>M. f n \<noteq> 0) \<and> (\<lambda>n. \<Prod>i\<le>n. f (i+M)) \<longlonglongrightarrow> L \<and> L \<noteq> 0)"
```
```    60 proof
```
```    61   assume "convergent_prod f"
```
```    62   then obtain M L where *: "(\<lambda>n. \<Prod>i\<le>n. f (i+M)) \<longlonglongrightarrow> L" "L \<noteq> 0"
```
```    63     by (auto simp: convergent_prod_def)
```
```    64   have "f i \<noteq> 0" if "i \<ge> M" for i
```
```    65   proof
```
```    66     assume "f i = 0"
```
```    67     have **: "eventually (\<lambda>n. (\<Prod>i\<le>n. f (i+M)) = 0) sequentially"
```
```    68       using eventually_ge_at_top[of "i - M"]
```
```    69     proof eventually_elim
```
```    70       case (elim n)
```
```    71       with \<open>f i = 0\<close> and \<open>i \<ge> M\<close> show ?case
```
```    72         by (auto intro!: bexI[of _ "i - M"] prod_zero)
```
```    73     qed
```
```    74     have "(\<lambda>n. (\<Prod>i\<le>n. f (i+M))) \<longlonglongrightarrow> 0"
```
```    75       unfolding filterlim_iff
```
```    76       by (auto dest!: eventually_nhds_x_imp_x intro!: eventually_mono[OF **])
```
```    77     from tendsto_unique[OF _ this *(1)] and *(2)
```
```    78       show False by simp
```
```    79   qed
```
```    80   with * show "(\<exists>M L. (\<forall>n\<ge>M. f n \<noteq> 0) \<and> (\<lambda>n. \<Prod>i\<le>n. f (i+M)) \<longlonglongrightarrow> L \<and> L \<noteq> 0)"
```
```    81     by blast
```
```    82 qed (auto simp: convergent_prod_def)
```
```    83
```
```    84 definition abs_convergent_prod :: "(nat \<Rightarrow> _) \<Rightarrow> bool" where
```
```    85   "abs_convergent_prod f \<longleftrightarrow> convergent_prod (\<lambda>i. 1 + norm (f i - 1))"
```
```    86
```
```    87 lemma abs_convergent_prodI:
```
```    88   assumes "convergent (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```    89   shows   "abs_convergent_prod f"
```
```    90 proof -
```
```    91   from assms obtain L where L: "(\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1)) \<longlonglongrightarrow> L"
```
```    92     by (auto simp: convergent_def)
```
```    93   have "L \<ge> 1"
```
```    94   proof (rule tendsto_le)
```
```    95     show "eventually (\<lambda>n. (\<Prod>i\<le>n. 1 + norm (f i - 1)) \<ge> 1) sequentially"
```
```    96     proof (intro always_eventually allI)
```
```    97       fix n
```
```    98       have "(\<Prod>i\<le>n. 1 + norm (f i - 1)) \<ge> (\<Prod>i\<le>n. 1)"
```
```    99         by (intro prod_mono) auto
```
```   100       thus "(\<Prod>i\<le>n. 1 + norm (f i - 1)) \<ge> 1" by simp
```
```   101     qed
```
```   102   qed (use L in simp_all)
```
```   103   hence "L \<noteq> 0" by auto
```
```   104   with L show ?thesis unfolding abs_convergent_prod_def convergent_prod_def
```
```   105     by (intro exI[of _ "0::nat"] exI[of _ L]) auto
```
```   106 qed
```
```   107
```
```   108 lemma
```
```   109   fixes f :: "nat \<Rightarrow> 'a :: {real_normed_div_algebra,idom}"
```
```   110   assumes "convergent_prod f"
```
```   111   shows   convergent_prod_imp_convergent: "convergent (\<lambda>n. \<Prod>i\<le>n. f i)"
```
```   112     and   convergent_prod_to_zero_iff:    "(\<lambda>n. \<Prod>i\<le>n. f i) \<longlonglongrightarrow> 0 \<longleftrightarrow> (\<exists>i. f i = 0)"
```
```   113 proof -
```
```   114   from assms obtain M L
```
```   115     where M: "\<And>n. n \<ge> M \<Longrightarrow> f n \<noteq> 0" and "(\<lambda>n. \<Prod>i\<le>n. f (i + M)) \<longlonglongrightarrow> L" and "L \<noteq> 0"
```
```   116     by (auto simp: convergent_prod_altdef)
```
```   117   note this(2)
```
```   118   also have "(\<lambda>n. \<Prod>i\<le>n. f (i + M)) = (\<lambda>n. \<Prod>i=M..M+n. f i)"
```
```   119     by (intro ext prod.reindex_bij_witness[of _ "\<lambda>n. n - M" "\<lambda>n. n + M"]) auto
```
```   120   finally have "(\<lambda>n. (\<Prod>i<M. f i) * (\<Prod>i=M..M+n. f i)) \<longlonglongrightarrow> (\<Prod>i<M. f i) * L"
```
```   121     by (intro tendsto_mult tendsto_const)
```
```   122   also have "(\<lambda>n. (\<Prod>i<M. f i) * (\<Prod>i=M..M+n. f i)) = (\<lambda>n. (\<Prod>i\<in>{..<M}\<union>{M..M+n}. f i))"
```
```   123     by (subst prod.union_disjoint) auto
```
```   124   also have "(\<lambda>n. {..<M} \<union> {M..M+n}) = (\<lambda>n. {..n+M})" by auto
```
```   125   finally have lim: "(\<lambda>n. prod f {..n}) \<longlonglongrightarrow> prod f {..<M} * L"
```
```   126     by (rule LIMSEQ_offset)
```
```   127   thus "convergent (\<lambda>n. \<Prod>i\<le>n. f i)"
```
```   128     by (auto simp: convergent_def)
```
```   129
```
```   130   show "(\<lambda>n. \<Prod>i\<le>n. f i) \<longlonglongrightarrow> 0 \<longleftrightarrow> (\<exists>i. f i = 0)"
```
```   131   proof
```
```   132     assume "\<exists>i. f i = 0"
```
```   133     then obtain i where "f i = 0" by auto
```
```   134     moreover with M have "i < M" by (cases "i < M") auto
```
```   135     ultimately have "(\<Prod>i<M. f i) = 0" by auto
```
```   136     with lim show "(\<lambda>n. \<Prod>i\<le>n. f i) \<longlonglongrightarrow> 0" by simp
```
```   137   next
```
```   138     assume "(\<lambda>n. \<Prod>i\<le>n. f i) \<longlonglongrightarrow> 0"
```
```   139     from tendsto_unique[OF _ this lim] and \<open>L \<noteq> 0\<close>
```
```   140     show "\<exists>i. f i = 0" by auto
```
```   141   qed
```
```   142 qed
```
```   143
```
```   144 lemma abs_convergent_prod_altdef:
```
```   145   "abs_convergent_prod f \<longleftrightarrow> convergent (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```   146 proof
```
```   147   assume "abs_convergent_prod f"
```
```   148   thus "convergent (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```   149     by (auto simp: abs_convergent_prod_def intro!: convergent_prod_imp_convergent)
```
```   150 qed (auto intro: abs_convergent_prodI)
```
```   151
```
```   152 lemma weierstrass_prod_ineq:
```
```   153   fixes f :: "'a \<Rightarrow> real"
```
```   154   assumes "\<And>x. x \<in> A \<Longrightarrow> f x \<in> {0..1}"
```
```   155   shows   "1 - sum f A \<le> (\<Prod>x\<in>A. 1 - f x)"
```
```   156   using assms
```
```   157 proof (induction A rule: infinite_finite_induct)
```
```   158   case (insert x A)
```
```   159   from insert.hyps and insert.prems
```
```   160     have "1 - sum f A + f x * (\<Prod>x\<in>A. 1 - f x) \<le> (\<Prod>x\<in>A. 1 - f x) + f x * (\<Prod>x\<in>A. 1)"
```
```   161     by (intro insert.IH add_mono mult_left_mono prod_mono) auto
```
```   162   with insert.hyps show ?case by (simp add: algebra_simps)
```
```   163 qed simp_all
```
```   164
```
```   165 lemma norm_prod_minus1_le_prod_minus1:
```
```   166   fixes f :: "nat \<Rightarrow> 'a :: {real_normed_div_algebra,comm_ring_1}"
```
```   167   shows "norm (prod (\<lambda>n. 1 + f n) A - 1) \<le> prod (\<lambda>n. 1 + norm (f n)) A - 1"
```
```   168 proof (induction A rule: infinite_finite_induct)
```
```   169   case (insert x A)
```
```   170   from insert.hyps have
```
```   171     "norm ((\<Prod>n\<in>insert x A. 1 + f n) - 1) =
```
```   172        norm ((\<Prod>n\<in>A. 1 + f n) - 1 + f x * (\<Prod>n\<in>A. 1 + f n))"
```
```   173     by (simp add: algebra_simps)
```
```   174   also have "\<dots> \<le> norm ((\<Prod>n\<in>A. 1 + f n) - 1) + norm (f x * (\<Prod>n\<in>A. 1 + f n))"
```
```   175     by (rule norm_triangle_ineq)
```
```   176   also have "norm (f x * (\<Prod>n\<in>A. 1 + f n)) = norm (f x) * (\<Prod>x\<in>A. norm (1 + f x))"
```
```   177     by (simp add: prod_norm norm_mult)
```
```   178   also have "(\<Prod>x\<in>A. norm (1 + f x)) \<le> (\<Prod>x\<in>A. norm (1::'a) + norm (f x))"
```
```   179     by (intro prod_mono norm_triangle_ineq ballI conjI) auto
```
```   180   also have "norm (1::'a) = 1" by simp
```
```   181   also note insert.IH
```
```   182   also have "(\<Prod>n\<in>A. 1 + norm (f n)) - 1 + norm (f x) * (\<Prod>x\<in>A. 1 + norm (f x)) =
```
```   183                (\<Prod>n\<in>insert x A. 1 + norm (f n)) - 1"
```
```   184     using insert.hyps by (simp add: algebra_simps)
```
```   185   finally show ?case by - (simp_all add: mult_left_mono)
```
```   186 qed simp_all
```
```   187
```
```   188 lemma convergent_prod_imp_ev_nonzero:
```
```   189   fixes f :: "nat \<Rightarrow> 'a :: {t2_space,comm_semiring_1}"
```
```   190   assumes "convergent_prod f"
```
```   191   shows   "eventually (\<lambda>n. f n \<noteq> 0) sequentially"
```
```   192   using assms by (auto simp: eventually_at_top_linorder convergent_prod_altdef)
```
```   193
```
```   194 lemma convergent_prod_imp_LIMSEQ:
```
```   195   fixes f :: "nat \<Rightarrow> 'a :: {real_normed_field}"
```
```   196   assumes "convergent_prod f"
```
```   197   shows   "f \<longlonglongrightarrow> 1"
```
```   198 proof -
```
```   199   from assms obtain M L where L: "(\<lambda>n. \<Prod>i\<le>n. f (i+M)) \<longlonglongrightarrow> L" "\<And>n. n \<ge> M \<Longrightarrow> f n \<noteq> 0" "L \<noteq> 0"
```
```   200     by (auto simp: convergent_prod_altdef)
```
```   201   hence L': "(\<lambda>n. \<Prod>i\<le>Suc n. f (i+M)) \<longlonglongrightarrow> L" by (subst filterlim_sequentially_Suc)
```
```   202   have "(\<lambda>n. (\<Prod>i\<le>Suc n. f (i+M)) / (\<Prod>i\<le>n. f (i+M))) \<longlonglongrightarrow> L / L"
```
```   203     using L L' by (intro tendsto_divide) simp_all
```
```   204   also from L have "L / L = 1" by simp
```
```   205   also have "(\<lambda>n. (\<Prod>i\<le>Suc n. f (i+M)) / (\<Prod>i\<le>n. f (i+M))) = (\<lambda>n. f (n + Suc M))"
```
```   206     using assms L by (auto simp: fun_eq_iff atMost_Suc)
```
```   207   finally show ?thesis by (rule LIMSEQ_offset)
```
```   208 qed
```
```   209
```
```   210 lemma abs_convergent_prod_imp_summable:
```
```   211   fixes f :: "nat \<Rightarrow> 'a :: real_normed_div_algebra"
```
```   212   assumes "abs_convergent_prod f"
```
```   213   shows "summable (\<lambda>i. norm (f i - 1))"
```
```   214 proof -
```
```   215   from assms have "convergent (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```   216     unfolding abs_convergent_prod_def by (rule convergent_prod_imp_convergent)
```
```   217   then obtain L where L: "(\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1)) \<longlonglongrightarrow> L"
```
```   218     unfolding convergent_def by blast
```
```   219   have "convergent (\<lambda>n. \<Sum>i\<le>n. norm (f i - 1))"
```
```   220   proof (rule Bseq_monoseq_convergent)
```
```   221     have "eventually (\<lambda>n. (\<Prod>i\<le>n. 1 + norm (f i - 1)) < L + 1) sequentially"
```
```   222       using L(1) by (rule order_tendstoD) simp_all
```
```   223     hence "\<forall>\<^sub>F x in sequentially. norm (\<Sum>i\<le>x. norm (f i - 1)) \<le> L + 1"
```
```   224     proof eventually_elim
```
```   225       case (elim n)
```
```   226       have "norm (\<Sum>i\<le>n. norm (f i - 1)) = (\<Sum>i\<le>n. norm (f i - 1))"
```
```   227         unfolding real_norm_def by (intro abs_of_nonneg sum_nonneg) simp_all
```
```   228       also have "\<dots> \<le> (\<Prod>i\<le>n. 1 + norm (f i - 1))" by (rule sum_le_prod) auto
```
```   229       also have "\<dots> < L + 1" by (rule elim)
```
```   230       finally show ?case by simp
```
```   231     qed
```
```   232     thus "Bseq (\<lambda>n. \<Sum>i\<le>n. norm (f i - 1))" by (rule BfunI)
```
```   233   next
```
```   234     show "monoseq (\<lambda>n. \<Sum>i\<le>n. norm (f i - 1))"
```
```   235       by (rule mono_SucI1) auto
```
```   236   qed
```
```   237   thus "summable (\<lambda>i. norm (f i - 1))" by (simp add: summable_iff_convergent')
```
```   238 qed
```
```   239
```
```   240 lemma summable_imp_abs_convergent_prod:
```
```   241   fixes f :: "nat \<Rightarrow> 'a :: real_normed_div_algebra"
```
```   242   assumes "summable (\<lambda>i. norm (f i - 1))"
```
```   243   shows   "abs_convergent_prod f"
```
```   244 proof (intro abs_convergent_prodI Bseq_monoseq_convergent)
```
```   245   show "monoseq (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```   246     by (intro mono_SucI1)
```
```   247        (auto simp: atMost_Suc algebra_simps intro!: mult_nonneg_nonneg prod_nonneg)
```
```   248 next
```
```   249   show "Bseq (\<lambda>n. \<Prod>i\<le>n. 1 + norm (f i - 1))"
```
```   250   proof (rule Bseq_eventually_mono)
```
```   251     show "eventually (\<lambda>n. norm (\<Prod>i\<le>n. 1 + norm (f i - 1)) \<le>
```
```   252             norm (exp (\<Sum>i\<le>n. norm (f i - 1)))) sequentially"
```
```   253       by (intro always_eventually allI) (auto simp: abs_prod exp_sum intro!: prod_mono)
```
```   254   next
```
```   255     from assms have "(\<lambda>n. \<Sum>i\<le>n. norm (f i - 1)) \<longlonglongrightarrow> (\<Sum>i. norm (f i - 1))"
```
```   256       using sums_def_le by blast
```
```   257     hence "(\<lambda>n. exp (\<Sum>i\<le>n. norm (f i - 1))) \<longlonglongrightarrow> exp (\<Sum>i. norm (f i - 1))"
```
```   258       by (rule tendsto_exp)
```
```   259     hence "convergent (\<lambda>n. exp (\<Sum>i\<le>n. norm (f i - 1)))"
```
```   260       by (rule convergentI)
```
```   261     thus "Bseq (\<lambda>n. exp (\<Sum>i\<le>n. norm (f i - 1)))"
```
```   262       by (rule convergent_imp_Bseq)
```
```   263   qed
```
```   264 qed
```
```   265
```
```   266 lemma abs_convergent_prod_conv_summable:
```
```   267   fixes f :: "nat \<Rightarrow> 'a :: real_normed_div_algebra"
```
```   268   shows "abs_convergent_prod f \<longleftrightarrow> summable (\<lambda>i. norm (f i - 1))"
```
```   269   by (blast intro: abs_convergent_prod_imp_summable summable_imp_abs_convergent_prod)
```
```   270
```
```   271 lemma abs_convergent_prod_imp_LIMSEQ:
```
```   272   fixes f :: "nat \<Rightarrow> 'a :: {comm_ring_1,real_normed_div_algebra}"
```
```   273   assumes "abs_convergent_prod f"
```
```   274   shows   "f \<longlonglongrightarrow> 1"
```
```   275 proof -
```
```   276   from assms have "summable (\<lambda>n. norm (f n - 1))"
```
```   277     by (rule abs_convergent_prod_imp_summable)
```
```   278   from summable_LIMSEQ_zero[OF this] have "(\<lambda>n. f n - 1) \<longlonglongrightarrow> 0"
```
```   279     by (simp add: tendsto_norm_zero_iff)
```
```   280   from tendsto_add[OF this tendsto_const[of 1]] show ?thesis by simp
```
```   281 qed
```
```   282
```
```   283 lemma abs_convergent_prod_imp_ev_nonzero:
```
```   284   fixes f :: "nat \<Rightarrow> 'a :: {comm_ring_1,real_normed_div_algebra}"
```
```   285   assumes "abs_convergent_prod f"
```
```   286   shows   "eventually (\<lambda>n. f n \<noteq> 0) sequentially"
```
```   287 proof -
```
```   288   from assms have "f \<longlonglongrightarrow> 1"
```
```   289     by (rule abs_convergent_prod_imp_LIMSEQ)
```
```   290   hence "eventually (\<lambda>n. dist (f n) 1 < 1) at_top"
```
```   291     by (auto simp: tendsto_iff)
```
```   292   thus ?thesis by eventually_elim auto
```
```   293 qed
```
```   294
```
```   295 lemma convergent_prod_offset:
```
```   296   assumes "convergent_prod (\<lambda>n. f (n + m))"
```
```   297   shows   "convergent_prod f"
```
```   298 proof -
```
```   299   from assms obtain M L where "(\<lambda>n. \<Prod>k\<le>n. f (k + (M + m))) \<longlonglongrightarrow> L" "L \<noteq> 0"
```
```   300     by (auto simp: convergent_prod_def add.assoc)
```
```   301   thus "convergent_prod f" unfolding convergent_prod_def by blast
```
```   302 qed
```
```   303
```
```   304 lemma abs_convergent_prod_offset:
```
```   305   assumes "abs_convergent_prod (\<lambda>n. f (n + m))"
```
```   306   shows   "abs_convergent_prod f"
```
```   307   using assms unfolding abs_convergent_prod_def by (rule convergent_prod_offset)
```
```   308
```
```   309 lemma convergent_prod_ignore_initial_segment:
```
```   310   fixes f :: "nat \<Rightarrow> 'a :: {real_normed_field}"
```
```   311   assumes "convergent_prod f"
```
```   312   shows   "convergent_prod (\<lambda>n. f (n + m))"
```
```   313 proof -
```
```   314   from assms obtain M L
```
```   315     where L: "(\<lambda>n. \<Prod>k\<le>n. f (k + M)) \<longlonglongrightarrow> L" "L \<noteq> 0" and nz: "\<And>n. n \<ge> M \<Longrightarrow> f n \<noteq> 0"
```
```   316     by (auto simp: convergent_prod_altdef)
```
```   317   define C where "C = (\<Prod>k<m. f (k + M))"
```
```   318   from nz have [simp]: "C \<noteq> 0"
```
```   319     by (auto simp: C_def)
```
```   320
```
```   321   from L(1) have "(\<lambda>n. \<Prod>k\<le>n+m. f (k + M)) \<longlonglongrightarrow> L"
```
```   322     by (rule LIMSEQ_ignore_initial_segment)
```
```   323   also have "(\<lambda>n. \<Prod>k\<le>n+m. f (k + M)) = (\<lambda>n. C * (\<Prod>k\<le>n. f (k + M + m)))"
```
```   324   proof (rule ext, goal_cases)
```
```   325     case (1 n)
```
```   326     have "{..n+m} = {..<m} \<union> {m..n+m}" by auto
```
```   327     also have "(\<Prod>k\<in>\<dots>. f (k + M)) = C * (\<Prod>k=m..n+m. f (k + M))"
```
```   328       unfolding C_def by (rule prod.union_disjoint) auto
```
```   329     also have "(\<Prod>k=m..n+m. f (k + M)) = (\<Prod>k\<le>n. f (k + m + M))"
```
```   330       by (intro ext prod.reindex_bij_witness[of _ "\<lambda>k. k + m" "\<lambda>k. k - m"]) auto
```
```   331     finally show ?case by (simp add: add_ac)
```
```   332   qed
```
```   333   finally have "(\<lambda>n. C * (\<Prod>k\<le>n. f (k + M + m)) / C) \<longlonglongrightarrow> L / C"
```
```   334     by (intro tendsto_divide tendsto_const) auto
```
```   335   hence "(\<lambda>n. \<Prod>k\<le>n. f (k + M + m)) \<longlonglongrightarrow> L / C" by simp
```
```   336   moreover from \<open>L \<noteq> 0\<close> have "L / C \<noteq> 0" by simp
```
```   337   ultimately show ?thesis unfolding convergent_prod_def by blast
```
```   338 qed
```
```   339
```
```   340 lemma abs_convergent_prod_ignore_initial_segment:
```
```   341   assumes "abs_convergent_prod f"
```
```   342   shows   "abs_convergent_prod (\<lambda>n. f (n + m))"
```
```   343   using assms unfolding abs_convergent_prod_def
```
```   344   by (rule convergent_prod_ignore_initial_segment)
```
```   345
```
```   346 lemma summable_LIMSEQ':
```
```   347   assumes "summable (f::nat\<Rightarrow>'a::{t2_space,comm_monoid_add})"
```
```   348   shows   "(\<lambda>n. \<Sum>i\<le>n. f i) \<longlonglongrightarrow> suminf f"
```
```   349   using assms sums_def_le by blast
```
```   350
```
```   351 lemma abs_convergent_prod_imp_convergent_prod:
```
```   352   fixes f :: "nat \<Rightarrow> 'a :: {real_normed_div_algebra,complete_space,comm_ring_1}"
```
```   353   assumes "abs_convergent_prod f"
```
```   354   shows   "convergent_prod f"
```
```   355 proof -
```
```   356   from assms have "eventually (\<lambda>n. f n \<noteq> 0) sequentially"
```
```   357     by (rule abs_convergent_prod_imp_ev_nonzero)
```
```   358   then obtain N where N: "f n \<noteq> 0" if "n \<ge> N" for n
```
```   359     by (auto simp: eventually_at_top_linorder)
```
```   360   let ?P = "\<lambda>n. \<Prod>i\<le>n. f (i + N)" and ?Q = "\<lambda>n. \<Prod>i\<le>n. 1 + norm (f (i + N) - 1)"
```
```   361
```
```   362   have "Cauchy ?P"
```
```   363   proof (rule CauchyI', goal_cases)
```
```   364     case (1 \<epsilon>)
```
```   365     from assms have "abs_convergent_prod (\<lambda>n. f (n + N))"
```
```   366       by (rule abs_convergent_prod_ignore_initial_segment)
```
```   367     hence "Cauchy ?Q"
```
```   368       unfolding abs_convergent_prod_def
```
```   369       by (intro convergent_Cauchy convergent_prod_imp_convergent)
```
```   370     from CauchyD[OF this 1] obtain M where M: "norm (?Q m - ?Q n) < \<epsilon>" if "m \<ge> M" "n \<ge> M" for m n
```
```   371       by blast
```
```   372     show ?case
```
```   373     proof (rule exI[of _ M], safe, goal_cases)
```
```   374       case (1 m n)
```
```   375       have "dist (?P m) (?P n) = norm (?P n - ?P m)"
```
```   376         by (simp add: dist_norm norm_minus_commute)
```
```   377       also from 1 have "{..n} = {..m} \<union> {m<..n}" by auto
```
```   378       hence "norm (?P n - ?P m) = norm (?P m * (\<Prod>k\<in>{m<..n}. f (k + N)) - ?P m)"
```
```   379         by (subst prod.union_disjoint [symmetric]) (auto simp: algebra_simps)
```
```   380       also have "\<dots> = norm (?P m * ((\<Prod>k\<in>{m<..n}. f (k + N)) - 1))"
```
```   381         by (simp add: algebra_simps)
```
```   382       also have "\<dots> = (\<Prod>k\<le>m. norm (f (k + N))) * norm ((\<Prod>k\<in>{m<..n}. f (k + N)) - 1)"
```
```   383         by (simp add: norm_mult prod_norm)
```
```   384       also have "\<dots> \<le> ?Q m * ((\<Prod>k\<in>{m<..n}. 1 + norm (f (k + N) - 1)) - 1)"
```
```   385         using norm_prod_minus1_le_prod_minus1[of "\<lambda>k. f (k + N) - 1" "{m<..n}"]
```
```   386               norm_triangle_ineq[of 1 "f k - 1" for k]
```
```   387         by (intro mult_mono prod_mono ballI conjI norm_prod_minus1_le_prod_minus1 prod_nonneg) auto
```
```   388       also have "\<dots> = ?Q m * (\<Prod>k\<in>{m<..n}. 1 + norm (f (k + N) - 1)) - ?Q m"
```
```   389         by (simp add: algebra_simps)
```
```   390       also have "?Q m * (\<Prod>k\<in>{m<..n}. 1 + norm (f (k + N) - 1)) =
```
```   391                    (\<Prod>k\<in>{..m}\<union>{m<..n}. 1 + norm (f (k + N) - 1))"
```
```   392         by (rule prod.union_disjoint [symmetric]) auto
```
```   393       also from 1 have "{..m}\<union>{m<..n} = {..n}" by auto
```
```   394       also have "?Q n - ?Q m \<le> norm (?Q n - ?Q m)" by simp
```
```   395       also from 1 have "\<dots> < \<epsilon>" by (intro M) auto
```
```   396       finally show ?case .
```
```   397     qed
```
```   398   qed
```
```   399   hence conv: "convergent ?P" by (rule Cauchy_convergent)
```
```   400   then obtain L where L: "?P \<longlonglongrightarrow> L"
```
```   401     by (auto simp: convergent_def)
```
```   402
```
```   403   have "L \<noteq> 0"
```
```   404   proof
```
```   405     assume [simp]: "L = 0"
```
```   406     from tendsto_norm[OF L] have limit: "(\<lambda>n. \<Prod>k\<le>n. norm (f (k + N))) \<longlonglongrightarrow> 0"
```
```   407       by (simp add: prod_norm)
```
```   408
```
```   409     from assms have "(\<lambda>n. f (n + N)) \<longlonglongrightarrow> 1"
```
```   410       by (intro abs_convergent_prod_imp_LIMSEQ abs_convergent_prod_ignore_initial_segment)
```
```   411     hence "eventually (\<lambda>n. norm (f (n + N) - 1) < 1) sequentially"
```
```   412       by (auto simp: tendsto_iff dist_norm)
```
```   413     then obtain M0 where M0: "norm (f (n + N) - 1) < 1" if "n \<ge> M0" for n
```
```   414       by (auto simp: eventually_at_top_linorder)
```
```   415
```
```   416     {
```
```   417       fix M assume M: "M \<ge> M0"
```
```   418       with M0 have M: "norm (f (n + N) - 1) < 1" if "n \<ge> M" for n using that by simp
```
```   419
```
```   420       have "(\<lambda>n. \<Prod>k\<le>n. 1 - norm (f (k+M+N) - 1)) \<longlonglongrightarrow> 0"
```
```   421       proof (rule tendsto_sandwich)
```
```   422         show "eventually (\<lambda>n. (\<Prod>k\<le>n. 1 - norm (f (k+M+N) - 1)) \<ge> 0) sequentially"
```
```   423           using M by (intro always_eventually prod_nonneg allI ballI) (auto intro: less_imp_le)
```
```   424         have "norm (1::'a) - norm (f (i + M + N) - 1) \<le> norm (f (i + M + N))" for i
```
```   425           using norm_triangle_ineq3[of "f (i + M + N)" 1] by simp
```
```   426         thus "eventually (\<lambda>n. (\<Prod>k\<le>n. 1 - norm (f (k+M+N) - 1)) \<le> (\<Prod>k\<le>n. norm (f (k+M+N)))) at_top"
```
```   427           using M by (intro always_eventually allI prod_mono ballI conjI) (auto intro: less_imp_le)
```
```   428
```
```   429         define C where "C = (\<Prod>k<M. norm (f (k + N)))"
```
```   430         from N have [simp]: "C \<noteq> 0" by (auto simp: C_def)
```
```   431         from L have "(\<lambda>n. norm (\<Prod>k\<le>n+M. f (k + N))) \<longlonglongrightarrow> 0"
```
```   432           by (intro LIMSEQ_ignore_initial_segment) (simp add: tendsto_norm_zero_iff)
```
```   433         also have "(\<lambda>n. norm (\<Prod>k\<le>n+M. f (k + N))) = (\<lambda>n. C * (\<Prod>k\<le>n. norm (f (k + M + N))))"
```
```   434         proof (rule ext, goal_cases)
```
```   435           case (1 n)
```
```   436           have "{..n+M} = {..<M} \<union> {M..n+M}" by auto
```
```   437           also have "norm (\<Prod>k\<in>\<dots>. f (k + N)) = C * norm (\<Prod>k=M..n+M. f (k + N))"
```
```   438             unfolding C_def by (subst prod.union_disjoint) (auto simp: norm_mult prod_norm)
```
```   439           also have "(\<Prod>k=M..n+M. f (k + N)) = (\<Prod>k\<le>n. f (k + N + M))"
```
```   440             by (intro prod.reindex_bij_witness[of _ "\<lambda>i. i + M" "\<lambda>i. i - M"]) auto
```
```   441           finally show ?case by (simp add: add_ac prod_norm)
```
```   442         qed
```
```   443         finally have "(\<lambda>n. C * (\<Prod>k\<le>n. norm (f (k + M + N))) / C) \<longlonglongrightarrow> 0 / C"
```
```   444           by (intro tendsto_divide tendsto_const) auto
```
```   445         thus "(\<lambda>n. \<Prod>k\<le>n. norm (f (k + M + N))) \<longlonglongrightarrow> 0" by simp
```
```   446       qed simp_all
```
```   447
```
```   448       have "1 - (\<Sum>i. norm (f (i + M + N) - 1)) \<le> 0"
```
```   449       proof (rule tendsto_le)
```
```   450         show "eventually (\<lambda>n. 1 - (\<Sum>k\<le>n. norm (f (k+M+N) - 1)) \<le>
```
```   451                                 (\<Prod>k\<le>n. 1 - norm (f (k+M+N) - 1))) at_top"
```
```   452           using M by (intro always_eventually allI weierstrass_prod_ineq) (auto intro: less_imp_le)
```
```   453         show "(\<lambda>n. \<Prod>k\<le>n. 1 - norm (f (k+M+N) - 1)) \<longlonglongrightarrow> 0" by fact
```
```   454         show "(\<lambda>n. 1 - (\<Sum>k\<le>n. norm (f (k + M + N) - 1)))
```
```   455                   \<longlonglongrightarrow> 1 - (\<Sum>i. norm (f (i + M + N) - 1))"
```
```   456           by (intro tendsto_intros summable_LIMSEQ' summable_ignore_initial_segment
```
```   457                 abs_convergent_prod_imp_summable assms)
```
```   458       qed simp_all
```
```   459       hence "(\<Sum>i. norm (f (i + M + N) - 1)) \<ge> 1" by simp
```
```   460       also have "\<dots> + (\<Sum>i<M. norm (f (i + N) - 1)) = (\<Sum>i. norm (f (i + N) - 1))"
```
```   461         by (intro suminf_split_initial_segment [symmetric] summable_ignore_initial_segment
```
```   462               abs_convergent_prod_imp_summable assms)
```
```   463       finally have "1 + (\<Sum>i<M. norm (f (i + N) - 1)) \<le> (\<Sum>i. norm (f (i + N) - 1))" by simp
```
```   464     } note * = this
```
```   465
```
```   466     have "1 + (\<Sum>i. norm (f (i + N) - 1)) \<le> (\<Sum>i. norm (f (i + N) - 1))"
```
```   467     proof (rule tendsto_le)
```
```   468       show "(\<lambda>M. 1 + (\<Sum>i<M. norm (f (i + N) - 1))) \<longlonglongrightarrow> 1 + (\<Sum>i. norm (f (i + N) - 1))"
```
```   469         by (intro tendsto_intros summable_LIMSEQ summable_ignore_initial_segment
```
```   470                 abs_convergent_prod_imp_summable assms)
```
```   471       show "eventually (\<lambda>M. 1 + (\<Sum>i<M. norm (f (i + N) - 1)) \<le> (\<Sum>i. norm (f (i + N) - 1))) at_top"
```
```   472         using eventually_ge_at_top[of M0] by eventually_elim (use * in auto)
```
```   473     qed simp_all
```
```   474     thus False by simp
```
```   475   qed
```
```   476   with L show ?thesis by (auto simp: convergent_prod_def)
```
```   477 qed
```
```   478
```
```   479 end
```