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author | hoelzl |

Wed, 23 Feb 2011 11:33:45 +0100 | |

changeset 41830 | 719b0a517c33 |

parent 41829 | 455cbcbba8c2 |

child 41831 | 91a2b435dd7a |

log is borel measurable

--- a/src/HOL/Probability/Borel_Space.thy Tue Feb 22 16:07:23 2011 +0100 +++ b/src/HOL/Probability/Borel_Space.thy Wed Feb 23 11:33:45 2011 +0100 @@ -48,6 +48,9 @@ thus ?thesis by simp qed +lemma borel_comp[intro,simp]: "A \<in> sets borel \<Longrightarrow> - A \<in> sets borel" + unfolding Compl_eq_Diff_UNIV by (intro borel.Diff) auto + lemma (in sigma_algebra) borel_measurable_vimage: fixes f :: "'a \<Rightarrow> 'x::t2_space" assumes borel: "f \<in> borel_measurable M" @@ -1118,6 +1121,73 @@ using * ** by auto qed +lemma borel_measurable_continuous_on1: + fixes f :: "'a::topological_space \<Rightarrow> 'b::t1_space" + assumes "continuous_on UNIV f" + shows "f \<in> borel_measurable borel" + apply(rule borel.borel_measurableI) + using continuous_open_preimage[OF assms] unfolding vimage_def by auto + +lemma borel_measurable_continuous_on: + fixes f :: "'a::topological_space \<Rightarrow> 'b::t1_space" + assumes cont: "continuous_on A f" "open A" and f: "f -` {c} \<inter> A \<in> sets borel" + shows "(\<lambda>x. if x \<in> A then f x else c) \<in> borel_measurable borel" (is "?f \<in> _") +proof (rule borel.borel_measurableI) + fix S :: "'b set" assume "open S" + then have "open {x\<in>A. f x \<in> S - {c}}" + by (intro continuous_open_preimage[OF cont]) auto + then have *: "{x\<in>A. f x \<in> S - {c}} \<in> sets borel" by auto + show "?f -` S \<inter> space borel \<in> sets borel" + proof cases + assume "c \<in> S" + then have "?f -` S = {x\<in>A. f x \<in> S - {c}} \<union> (f -` {c} \<inter> A) \<union> -A" + by auto + with * show "?f -` S \<inter> space borel \<in> sets borel" + using `open A` f by (auto intro!: borel.Un) + next + assume "c \<notin> S" + then have "?f -` S = {x\<in>A. f x \<in> S - {c}}" by (auto split: split_if_asm) + with * show "?f -` S \<inter> space borel \<in> sets borel" by auto + qed +qed + +lemma borel_measurable_borel_log: assumes "1 < b" shows "log b \<in> borel_measurable borel" +proof - + { fix x :: real assume x: "x \<le> 0" + { fix x::real assume "x \<le> 0" then have "\<And>u. exp u = x \<longleftrightarrow> False" by auto } + from this[of x] x this[of 0] have "log b 0 = log b x" + by (auto simp: ln_def log_def) } + note log_imp = this + have "(\<lambda>x. if x \<in> {0<..} then log b x else log b 0) \<in> borel_measurable borel" + proof (rule borel_measurable_continuous_on) + show "continuous_on {0<..} (log b)" + by (auto intro!: continuous_at_imp_continuous_on DERIV_log DERIV_isCont + simp: continuous_isCont[symmetric]) + show "open ({0<..}::real set)" by auto + show "log b -` {log b 0} \<inter> {0<..} \<in> sets borel" + proof cases + assume "log b -` {log b 0} \<inter> {0<..} = {}" + then show ?thesis by simp + next + assume "log b -` {log b 0} \<inter> {0<..} \<noteq> {}" + then obtain x where "0 < x" "log b x = log b 0" by auto + with log_inj[OF `1 < b`] have "log b -` {log b 0} \<inter> {0<..} = {x}" + by (auto simp: inj_on_def) + then show ?thesis by simp + qed + qed + also have "(\<lambda>x. if x \<in> {0<..} then log b x else log b 0) = log b" + by (simp add: fun_eq_iff not_less log_imp) + finally show ?thesis . +qed + +lemma (in sigma_algebra) borel_measurable_log[simp,intro]: + assumes f: "f \<in> borel_measurable M" and "1 < b" + shows "(\<lambda>x. log b (f x)) \<in> borel_measurable M" + using measurable_comp[OF f borel_measurable_borel_log[OF `1 < b`]] + by (simp add: comp_def) + + lemma (in sigma_algebra) less_eq_ge_measurable: fixes f :: "'a \<Rightarrow> 'c::linorder" shows "{x\<in>space M. a < f x} \<in> sets M \<longleftrightarrow> {x\<in>space M. f x \<le> a} \<in> sets M"