Renamed upd_snd_conv to apsnd_conv to be consistent with apfst_conv; Added apsnd_apfst_commute
(* Title : Limits.thy
Author : Brian Huffman
*)
header {* Filters and Limits *}
theory Limits
imports RealVector RComplete
begin
subsection {* Nets *}
text {*
A net is now defined as a filter base.
The definition also allows non-proper filter bases.
*}
typedef (open) 'a net =
"{net :: 'a set set. (\<exists>A. A \<in> net)
\<and> (\<forall>A\<in>net. \<forall>B\<in>net. \<exists>C\<in>net. C \<subseteq> A \<and> C \<subseteq> B)}"
proof
show "UNIV \<in> ?net" by auto
qed
lemma Rep_net_nonempty: "\<exists>A. A \<in> Rep_net net"
using Rep_net [of net] by simp
lemma Rep_net_directed:
"A \<in> Rep_net net \<Longrightarrow> B \<in> Rep_net net \<Longrightarrow> \<exists>C\<in>Rep_net net. C \<subseteq> A \<and> C \<subseteq> B"
using Rep_net [of net] by simp
lemma Abs_net_inverse':
assumes "\<exists>A. A \<in> net"
assumes "\<And>A B. A \<in> net \<Longrightarrow> B \<in> net \<Longrightarrow> \<exists>C\<in>net. C \<subseteq> A \<and> C \<subseteq> B"
shows "Rep_net (Abs_net net) = net"
using assms by (simp add: Abs_net_inverse)
lemma image_nonempty: "\<exists>x. x \<in> A \<Longrightarrow> \<exists>x. x \<in> f ` A"
by auto
subsection {* Eventually *}
definition
eventually :: "('a \<Rightarrow> bool) \<Rightarrow> 'a net \<Rightarrow> bool" where
[code del]: "eventually P net \<longleftrightarrow> (\<exists>A\<in>Rep_net net. \<forall>x\<in>A. P x)"
lemma eventually_True [simp]: "eventually (\<lambda>x. True) net"
unfolding eventually_def using Rep_net_nonempty [of net] by fast
lemma eventually_mono:
"(\<forall>x. P x \<longrightarrow> Q x) \<Longrightarrow> eventually P net \<Longrightarrow> eventually Q net"
unfolding eventually_def by blast
lemma eventually_conj:
assumes P: "eventually (\<lambda>x. P x) net"
assumes Q: "eventually (\<lambda>x. Q x) net"
shows "eventually (\<lambda>x. P x \<and> Q x) net"
proof -
obtain A where A: "A \<in> Rep_net net" "\<forall>x\<in>A. P x"
using P unfolding eventually_def by fast
obtain B where B: "B \<in> Rep_net net" "\<forall>x\<in>B. Q x"
using Q unfolding eventually_def by fast
obtain C where C: "C \<in> Rep_net net" "C \<subseteq> A" "C \<subseteq> B"
using Rep_net_directed [OF A(1) B(1)] by fast
then have "\<forall>x\<in>C. P x \<and> Q x" "C \<in> Rep_net net"
using A(2) B(2) by auto
then show ?thesis unfolding eventually_def ..
qed
lemma eventually_mp:
assumes "eventually (\<lambda>x. P x \<longrightarrow> Q x) net"
assumes "eventually (\<lambda>x. P x) net"
shows "eventually (\<lambda>x. Q x) net"
proof (rule eventually_mono)
show "\<forall>x. (P x \<longrightarrow> Q x) \<and> P x \<longrightarrow> Q x" by simp
show "eventually (\<lambda>x. (P x \<longrightarrow> Q x) \<and> P x) net"
using assms by (rule eventually_conj)
qed
lemma eventually_rev_mp:
assumes "eventually (\<lambda>x. P x) net"
assumes "eventually (\<lambda>x. P x \<longrightarrow> Q x) net"
shows "eventually (\<lambda>x. Q x) net"
using assms(2) assms(1) by (rule eventually_mp)
lemma eventually_conj_iff:
"eventually (\<lambda>x. P x \<and> Q x) net \<longleftrightarrow> eventually P net \<and> eventually Q net"
by (auto intro: eventually_conj elim: eventually_rev_mp)
lemma eventually_elim1:
assumes "eventually (\<lambda>i. P i) net"
assumes "\<And>i. P i \<Longrightarrow> Q i"
shows "eventually (\<lambda>i. Q i) net"
using assms by (auto elim!: eventually_rev_mp)
lemma eventually_elim2:
assumes "eventually (\<lambda>i. P i) net"
assumes "eventually (\<lambda>i. Q i) net"
assumes "\<And>i. P i \<Longrightarrow> Q i \<Longrightarrow> R i"
shows "eventually (\<lambda>i. R i) net"
using assms by (auto elim!: eventually_rev_mp)
subsection {* Standard Nets *}
definition
sequentially :: "nat net" where
[code del]: "sequentially = Abs_net (range (\<lambda>n. {n..}))"
definition
within :: "'a net \<Rightarrow> 'a set \<Rightarrow> 'a net" (infixr "within" 70) where
[code del]: "net within S = Abs_net ((\<lambda>A. A \<inter> S) ` Rep_net net)"
definition
at :: "'a::topological_space \<Rightarrow> 'a net" where
[code del]: "at a = Abs_net ((\<lambda>S. S - {a}) ` {S. open S \<and> a \<in> S})"
lemma Rep_net_sequentially:
"Rep_net sequentially = range (\<lambda>n. {n..})"
unfolding sequentially_def
apply (rule Abs_net_inverse')
apply (rule image_nonempty, simp)
apply (clarsimp, rename_tac m n)
apply (rule_tac x="max m n" in exI, auto)
done
lemma Rep_net_within:
"Rep_net (net within S) = (\<lambda>A. A \<inter> S) ` Rep_net net"
unfolding within_def
apply (rule Abs_net_inverse')
apply (rule image_nonempty, rule Rep_net_nonempty)
apply (clarsimp, rename_tac A B)
apply (drule (1) Rep_net_directed)
apply (clarify, rule_tac x=C in bexI, auto)
done
lemma Rep_net_at:
"Rep_net (at a) = ((\<lambda>S. S - {a}) ` {S. open S \<and> a \<in> S})"
unfolding at_def
apply (rule Abs_net_inverse')
apply (rule image_nonempty)
apply (rule_tac x="UNIV" in exI, simp)
apply (clarsimp, rename_tac S T)
apply (rule_tac x="S \<inter> T" in exI, auto simp add: open_Int)
done
lemma eventually_sequentially:
"eventually P sequentially \<longleftrightarrow> (\<exists>N. \<forall>n\<ge>N. P n)"
unfolding eventually_def Rep_net_sequentially by auto
lemma eventually_within:
"eventually P (net within S) = eventually (\<lambda>x. x \<in> S \<longrightarrow> P x) net"
unfolding eventually_def Rep_net_within by auto
lemma eventually_at_topological:
"eventually P (at a) \<longleftrightarrow> (\<exists>S. open S \<and> a \<in> S \<and> (\<forall>x\<in>S. x \<noteq> a \<longrightarrow> P x))"
unfolding eventually_def Rep_net_at by auto
lemma eventually_at:
fixes a :: "'a::metric_space"
shows "eventually P (at a) \<longleftrightarrow> (\<exists>d>0. \<forall>x. x \<noteq> a \<and> dist x a < d \<longrightarrow> P x)"
unfolding eventually_at_topological open_dist
apply safe
apply fast
apply (rule_tac x="{x. dist x a < d}" in exI, simp)
apply clarsimp
apply (rule_tac x="d - dist x a" in exI, clarsimp)
apply (simp only: less_diff_eq)
apply (erule le_less_trans [OF dist_triangle])
done
subsection {* Boundedness *}
definition
Bfun :: "('a \<Rightarrow> 'b::real_normed_vector) \<Rightarrow> 'a net \<Rightarrow> bool" where
[code del]: "Bfun f net = (\<exists>K>0. eventually (\<lambda>x. norm (f x) \<le> K) net)"
lemma BfunI:
assumes K: "eventually (\<lambda>x. norm (f x) \<le> K) net" shows "Bfun f net"
unfolding Bfun_def
proof (intro exI conjI allI)
show "0 < max K 1" by simp
next
show "eventually (\<lambda>x. norm (f x) \<le> max K 1) net"
using K by (rule eventually_elim1, simp)
qed
lemma BfunE:
assumes "Bfun f net"
obtains B where "0 < B" and "eventually (\<lambda>x. norm (f x) \<le> B) net"
using assms unfolding Bfun_def by fast
subsection {* Convergence to Zero *}
definition
Zfun :: "('a \<Rightarrow> 'b::real_normed_vector) \<Rightarrow> 'a net \<Rightarrow> bool" where
[code del]: "Zfun f net = (\<forall>r>0. eventually (\<lambda>x. norm (f x) < r) net)"
lemma ZfunI:
"(\<And>r. 0 < r \<Longrightarrow> eventually (\<lambda>x. norm (f x) < r) net) \<Longrightarrow> Zfun f net"
unfolding Zfun_def by simp
lemma ZfunD:
"\<lbrakk>Zfun f net; 0 < r\<rbrakk> \<Longrightarrow> eventually (\<lambda>x. norm (f x) < r) net"
unfolding Zfun_def by simp
lemma Zfun_ssubst:
"eventually (\<lambda>x. f x = g x) net \<Longrightarrow> Zfun g net \<Longrightarrow> Zfun f net"
unfolding Zfun_def by (auto elim!: eventually_rev_mp)
lemma Zfun_zero: "Zfun (\<lambda>x. 0) net"
unfolding Zfun_def by simp
lemma Zfun_norm_iff: "Zfun (\<lambda>x. norm (f x)) net = Zfun (\<lambda>x. f x) net"
unfolding Zfun_def by simp
lemma Zfun_imp_Zfun:
assumes f: "Zfun f net"
assumes g: "eventually (\<lambda>x. norm (g x) \<le> norm (f x) * K) net"
shows "Zfun (\<lambda>x. g x) net"
proof (cases)
assume K: "0 < K"
show ?thesis
proof (rule ZfunI)
fix r::real assume "0 < r"
hence "0 < r / K"
using K by (rule divide_pos_pos)
then have "eventually (\<lambda>x. norm (f x) < r / K) net"
using ZfunD [OF f] by fast
with g show "eventually (\<lambda>x. norm (g x) < r) net"
proof (rule eventually_elim2)
fix x
assume *: "norm (g x) \<le> norm (f x) * K"
assume "norm (f x) < r / K"
hence "norm (f x) * K < r"
by (simp add: pos_less_divide_eq K)
thus "norm (g x) < r"
by (simp add: order_le_less_trans [OF *])
qed
qed
next
assume "\<not> 0 < K"
hence K: "K \<le> 0" by (simp only: not_less)
show ?thesis
proof (rule ZfunI)
fix r :: real
assume "0 < r"
from g show "eventually (\<lambda>x. norm (g x) < r) net"
proof (rule eventually_elim1)
fix x
assume "norm (g x) \<le> norm (f x) * K"
also have "\<dots> \<le> norm (f x) * 0"
using K norm_ge_zero by (rule mult_left_mono)
finally show "norm (g x) < r"
using `0 < r` by simp
qed
qed
qed
lemma Zfun_le: "\<lbrakk>Zfun g net; \<forall>x. norm (f x) \<le> norm (g x)\<rbrakk> \<Longrightarrow> Zfun f net"
by (erule_tac K="1" in Zfun_imp_Zfun, simp)
lemma Zfun_add:
assumes f: "Zfun f net" and g: "Zfun g net"
shows "Zfun (\<lambda>x. f x + g x) net"
proof (rule ZfunI)
fix r::real assume "0 < r"
hence r: "0 < r / 2" by simp
have "eventually (\<lambda>x. norm (f x) < r/2) net"
using f r by (rule ZfunD)
moreover
have "eventually (\<lambda>x. norm (g x) < r/2) net"
using g r by (rule ZfunD)
ultimately
show "eventually (\<lambda>x. norm (f x + g x) < r) net"
proof (rule eventually_elim2)
fix x
assume *: "norm (f x) < r/2" "norm (g x) < r/2"
have "norm (f x + g x) \<le> norm (f x) + norm (g x)"
by (rule norm_triangle_ineq)
also have "\<dots> < r/2 + r/2"
using * by (rule add_strict_mono)
finally show "norm (f x + g x) < r"
by simp
qed
qed
lemma Zfun_minus: "Zfun f net \<Longrightarrow> Zfun (\<lambda>x. - f x) net"
unfolding Zfun_def by simp
lemma Zfun_diff: "\<lbrakk>Zfun f net; Zfun g net\<rbrakk> \<Longrightarrow> Zfun (\<lambda>x. f x - g x) net"
by (simp only: diff_minus Zfun_add Zfun_minus)
lemma (in bounded_linear) Zfun:
assumes g: "Zfun g net"
shows "Zfun (\<lambda>x. f (g x)) net"
proof -
obtain K where "\<And>x. norm (f x) \<le> norm x * K"
using bounded by fast
then have "eventually (\<lambda>x. norm (f (g x)) \<le> norm (g x) * K) net"
by simp
with g show ?thesis
by (rule Zfun_imp_Zfun)
qed
lemma (in bounded_bilinear) Zfun:
assumes f: "Zfun f net"
assumes g: "Zfun g net"
shows "Zfun (\<lambda>x. f x ** g x) net"
proof (rule ZfunI)
fix r::real assume r: "0 < r"
obtain K where K: "0 < K"
and norm_le: "\<And>x y. norm (x ** y) \<le> norm x * norm y * K"
using pos_bounded by fast
from K have K': "0 < inverse K"
by (rule positive_imp_inverse_positive)
have "eventually (\<lambda>x. norm (f x) < r) net"
using f r by (rule ZfunD)
moreover
have "eventually (\<lambda>x. norm (g x) < inverse K) net"
using g K' by (rule ZfunD)
ultimately
show "eventually (\<lambda>x. norm (f x ** g x) < r) net"
proof (rule eventually_elim2)
fix x
assume *: "norm (f x) < r" "norm (g x) < inverse K"
have "norm (f x ** g x) \<le> norm (f x) * norm (g x) * K"
by (rule norm_le)
also have "norm (f x) * norm (g x) * K < r * inverse K * K"
by (intro mult_strict_right_mono mult_strict_mono' norm_ge_zero * K)
also from K have "r * inverse K * K = r"
by simp
finally show "norm (f x ** g x) < r" .
qed
qed
lemma (in bounded_bilinear) Zfun_left:
"Zfun f net \<Longrightarrow> Zfun (\<lambda>x. f x ** a) net"
by (rule bounded_linear_left [THEN bounded_linear.Zfun])
lemma (in bounded_bilinear) Zfun_right:
"Zfun f net \<Longrightarrow> Zfun (\<lambda>x. a ** f x) net"
by (rule bounded_linear_right [THEN bounded_linear.Zfun])
lemmas Zfun_mult = mult.Zfun
lemmas Zfun_mult_right = mult.Zfun_right
lemmas Zfun_mult_left = mult.Zfun_left
subsection {* Limits *}
definition
tendsto :: "('a \<Rightarrow> 'b::topological_space) \<Rightarrow> 'b \<Rightarrow> 'a net \<Rightarrow> bool"
(infixr "--->" 55)
where [code del]:
"(f ---> l) net \<longleftrightarrow> (\<forall>S. open S \<longrightarrow> l \<in> S \<longrightarrow> eventually (\<lambda>x. f x \<in> S) net)"
ML {*
structure Tendsto_Intros = Named_Thms
(
val name = "tendsto_intros"
val description = "introduction rules for tendsto"
)
*}
setup Tendsto_Intros.setup
lemma topological_tendstoI:
"(\<And>S. open S \<Longrightarrow> l \<in> S \<Longrightarrow> eventually (\<lambda>x. f x \<in> S) net)
\<Longrightarrow> (f ---> l) net"
unfolding tendsto_def by auto
lemma topological_tendstoD:
"(f ---> l) net \<Longrightarrow> open S \<Longrightarrow> l \<in> S \<Longrightarrow> eventually (\<lambda>x. f x \<in> S) net"
unfolding tendsto_def by auto
lemma tendstoI:
assumes "\<And>e. 0 < e \<Longrightarrow> eventually (\<lambda>x. dist (f x) l < e) net"
shows "(f ---> l) net"
apply (rule topological_tendstoI)
apply (simp add: open_dist)
apply (drule (1) bspec, clarify)
apply (drule assms)
apply (erule eventually_elim1, simp)
done
lemma tendstoD:
"(f ---> l) net \<Longrightarrow> 0 < e \<Longrightarrow> eventually (\<lambda>x. dist (f x) l < e) net"
apply (drule_tac S="{x. dist x l < e}" in topological_tendstoD)
apply (clarsimp simp add: open_dist)
apply (rule_tac x="e - dist x l" in exI, clarsimp)
apply (simp only: less_diff_eq)
apply (erule le_less_trans [OF dist_triangle])
apply simp
apply simp
done
lemma tendsto_iff:
"(f ---> l) net \<longleftrightarrow> (\<forall>e>0. eventually (\<lambda>x. dist (f x) l < e) net)"
using tendstoI tendstoD by fast
lemma tendsto_Zfun_iff: "(f ---> a) net = Zfun (\<lambda>x. f x - a) net"
by (simp only: tendsto_iff Zfun_def dist_norm)
lemma tendsto_ident_at [tendsto_intros]: "((\<lambda>x. x) ---> a) (at a)"
unfolding tendsto_def eventually_at_topological by auto
lemma tendsto_ident_at_within [tendsto_intros]:
"a \<in> S \<Longrightarrow> ((\<lambda>x. x) ---> a) (at a within S)"
unfolding tendsto_def eventually_within eventually_at_topological by auto
lemma tendsto_const [tendsto_intros]: "((\<lambda>x. k) ---> k) net"
by (simp add: tendsto_def)
lemma tendsto_dist [tendsto_intros]:
assumes f: "(f ---> l) net" and g: "(g ---> m) net"
shows "((\<lambda>x. dist (f x) (g x)) ---> dist l m) net"
proof (rule tendstoI)
fix e :: real assume "0 < e"
hence e2: "0 < e/2" by simp
from tendstoD [OF f e2] tendstoD [OF g e2]
show "eventually (\<lambda>x. dist (dist (f x) (g x)) (dist l m) < e) net"
proof (rule eventually_elim2)
fix x assume "dist (f x) l < e/2" "dist (g x) m < e/2"
then show "dist (dist (f x) (g x)) (dist l m) < e"
unfolding dist_real_def
using dist_triangle2 [of "f x" "g x" "l"]
using dist_triangle2 [of "g x" "l" "m"]
using dist_triangle3 [of "l" "m" "f x"]
using dist_triangle [of "f x" "m" "g x"]
by arith
qed
qed
lemma tendsto_norm [tendsto_intros]:
"(f ---> a) net \<Longrightarrow> ((\<lambda>x. norm (f x)) ---> norm a) net"
apply (simp add: tendsto_iff dist_norm, safe)
apply (drule_tac x="e" in spec, safe)
apply (erule eventually_elim1)
apply (erule order_le_less_trans [OF norm_triangle_ineq3])
done
lemma add_diff_add:
fixes a b c d :: "'a::ab_group_add"
shows "(a + c) - (b + d) = (a - b) + (c - d)"
by simp
lemma minus_diff_minus:
fixes a b :: "'a::ab_group_add"
shows "(- a) - (- b) = - (a - b)"
by simp
lemma tendsto_add [tendsto_intros]:
fixes a b :: "'a::real_normed_vector"
shows "\<lbrakk>(f ---> a) net; (g ---> b) net\<rbrakk> \<Longrightarrow> ((\<lambda>x. f x + g x) ---> a + b) net"
by (simp only: tendsto_Zfun_iff add_diff_add Zfun_add)
lemma tendsto_minus [tendsto_intros]:
fixes a :: "'a::real_normed_vector"
shows "(f ---> a) net \<Longrightarrow> ((\<lambda>x. - f x) ---> - a) net"
by (simp only: tendsto_Zfun_iff minus_diff_minus Zfun_minus)
lemma tendsto_minus_cancel:
fixes a :: "'a::real_normed_vector"
shows "((\<lambda>x. - f x) ---> - a) net \<Longrightarrow> (f ---> a) net"
by (drule tendsto_minus, simp)
lemma tendsto_diff [tendsto_intros]:
fixes a b :: "'a::real_normed_vector"
shows "\<lbrakk>(f ---> a) net; (g ---> b) net\<rbrakk> \<Longrightarrow> ((\<lambda>x. f x - g x) ---> a - b) net"
by (simp add: diff_minus tendsto_add tendsto_minus)
lemma tendsto_setsum [tendsto_intros]:
fixes f :: "'a \<Rightarrow> 'b \<Rightarrow> 'c::real_normed_vector"
assumes "\<And>i. i \<in> S \<Longrightarrow> (f i ---> a i) net"
shows "((\<lambda>x. \<Sum>i\<in>S. f i x) ---> (\<Sum>i\<in>S. a i)) net"
proof (cases "finite S")
assume "finite S" thus ?thesis using assms
proof (induct set: finite)
case empty show ?case
by (simp add: tendsto_const)
next
case (insert i F) thus ?case
by (simp add: tendsto_add)
qed
next
assume "\<not> finite S" thus ?thesis
by (simp add: tendsto_const)
qed
lemma (in bounded_linear) tendsto [tendsto_intros]:
"(g ---> a) net \<Longrightarrow> ((\<lambda>x. f (g x)) ---> f a) net"
by (simp only: tendsto_Zfun_iff diff [symmetric] Zfun)
lemma (in bounded_bilinear) tendsto [tendsto_intros]:
"\<lbrakk>(f ---> a) net; (g ---> b) net\<rbrakk> \<Longrightarrow> ((\<lambda>x. f x ** g x) ---> a ** b) net"
by (simp only: tendsto_Zfun_iff prod_diff_prod
Zfun_add Zfun Zfun_left Zfun_right)
subsection {* Continuity of Inverse *}
lemma (in bounded_bilinear) Zfun_prod_Bfun:
assumes f: "Zfun f net"
assumes g: "Bfun g net"
shows "Zfun (\<lambda>x. f x ** g x) net"
proof -
obtain K where K: "0 \<le> K"
and norm_le: "\<And>x y. norm (x ** y) \<le> norm x * norm y * K"
using nonneg_bounded by fast
obtain B where B: "0 < B"
and norm_g: "eventually (\<lambda>x. norm (g x) \<le> B) net"
using g by (rule BfunE)
have "eventually (\<lambda>x. norm (f x ** g x) \<le> norm (f x) * (B * K)) net"
using norm_g proof (rule eventually_elim1)
fix x
assume *: "norm (g x) \<le> B"
have "norm (f x ** g x) \<le> norm (f x) * norm (g x) * K"
by (rule norm_le)
also have "\<dots> \<le> norm (f x) * B * K"
by (intro mult_mono' order_refl norm_g norm_ge_zero
mult_nonneg_nonneg K *)
also have "\<dots> = norm (f x) * (B * K)"
by (rule mult_assoc)
finally show "norm (f x ** g x) \<le> norm (f x) * (B * K)" .
qed
with f show ?thesis
by (rule Zfun_imp_Zfun)
qed
lemma (in bounded_bilinear) flip:
"bounded_bilinear (\<lambda>x y. y ** x)"
apply default
apply (rule add_right)
apply (rule add_left)
apply (rule scaleR_right)
apply (rule scaleR_left)
apply (subst mult_commute)
using bounded by fast
lemma (in bounded_bilinear) Bfun_prod_Zfun:
assumes f: "Bfun f net"
assumes g: "Zfun g net"
shows "Zfun (\<lambda>x. f x ** g x) net"
using flip g f by (rule bounded_bilinear.Zfun_prod_Bfun)
lemma inverse_diff_inverse:
"\<lbrakk>(a::'a::division_ring) \<noteq> 0; b \<noteq> 0\<rbrakk>
\<Longrightarrow> inverse a - inverse b = - (inverse a * (a - b) * inverse b)"
by (simp add: algebra_simps)
lemma Bfun_inverse_lemma:
fixes x :: "'a::real_normed_div_algebra"
shows "\<lbrakk>r \<le> norm x; 0 < r\<rbrakk> \<Longrightarrow> norm (inverse x) \<le> inverse r"
apply (subst nonzero_norm_inverse, clarsimp)
apply (erule (1) le_imp_inverse_le)
done
lemma Bfun_inverse:
fixes a :: "'a::real_normed_div_algebra"
assumes f: "(f ---> a) net"
assumes a: "a \<noteq> 0"
shows "Bfun (\<lambda>x. inverse (f x)) net"
proof -
from a have "0 < norm a" by simp
hence "\<exists>r>0. r < norm a" by (rule dense)
then obtain r where r1: "0 < r" and r2: "r < norm a" by fast
have "eventually (\<lambda>x. dist (f x) a < r) net"
using tendstoD [OF f r1] by fast
hence "eventually (\<lambda>x. norm (inverse (f x)) \<le> inverse (norm a - r)) net"
proof (rule eventually_elim1)
fix x
assume "dist (f x) a < r"
hence 1: "norm (f x - a) < r"
by (simp add: dist_norm)
hence 2: "f x \<noteq> 0" using r2 by auto
hence "norm (inverse (f x)) = inverse (norm (f x))"
by (rule nonzero_norm_inverse)
also have "\<dots> \<le> inverse (norm a - r)"
proof (rule le_imp_inverse_le)
show "0 < norm a - r" using r2 by simp
next
have "norm a - norm (f x) \<le> norm (a - f x)"
by (rule norm_triangle_ineq2)
also have "\<dots> = norm (f x - a)"
by (rule norm_minus_commute)
also have "\<dots> < r" using 1 .
finally show "norm a - r \<le> norm (f x)" by simp
qed
finally show "norm (inverse (f x)) \<le> inverse (norm a - r)" .
qed
thus ?thesis by (rule BfunI)
qed
lemma tendsto_inverse_lemma:
fixes a :: "'a::real_normed_div_algebra"
shows "\<lbrakk>(f ---> a) net; a \<noteq> 0; eventually (\<lambda>x. f x \<noteq> 0) net\<rbrakk>
\<Longrightarrow> ((\<lambda>x. inverse (f x)) ---> inverse a) net"
apply (subst tendsto_Zfun_iff)
apply (rule Zfun_ssubst)
apply (erule eventually_elim1)
apply (erule (1) inverse_diff_inverse)
apply (rule Zfun_minus)
apply (rule Zfun_mult_left)
apply (rule mult.Bfun_prod_Zfun)
apply (erule (1) Bfun_inverse)
apply (simp add: tendsto_Zfun_iff)
done
lemma tendsto_inverse [tendsto_intros]:
fixes a :: "'a::real_normed_div_algebra"
assumes f: "(f ---> a) net"
assumes a: "a \<noteq> 0"
shows "((\<lambda>x. inverse (f x)) ---> inverse a) net"
proof -
from a have "0 < norm a" by simp
with f have "eventually (\<lambda>x. dist (f x) a < norm a) net"
by (rule tendstoD)
then have "eventually (\<lambda>x. f x \<noteq> 0) net"
unfolding dist_norm by (auto elim!: eventually_elim1)
with f a show ?thesis
by (rule tendsto_inverse_lemma)
qed
lemma tendsto_divide [tendsto_intros]:
fixes a b :: "'a::real_normed_field"
shows "\<lbrakk>(f ---> a) net; (g ---> b) net; b \<noteq> 0\<rbrakk>
\<Longrightarrow> ((\<lambda>x. f x / g x) ---> a / b) net"
by (simp add: mult.tendsto tendsto_inverse divide_inverse)
end