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+++ b/src/HOL/Hahn_Banach/Hahn_Banach.thy Wed Jun 24 21:46:54 2009 +0200
@@ -0,0 +1,509 @@
+(* Title: HOL/Hahn_Banach/Hahn_Banach.thy
+ Author: Gertrud Bauer, TU Munich
+*)
+
+header {* The Hahn-Banach Theorem *}
+
+theory Hahn_Banach
+imports Hahn_Banach_Lemmas
+begin
+
+text {*
+ We present the proof of two different versions of the Hahn-Banach
+ Theorem, closely following \cite[\S36]{Heuser:1986}.
+*}
+
+subsection {* The Hahn-Banach Theorem for vector spaces *}
+
+text {*
+ \textbf{Hahn-Banach Theorem.} Let @{text F} be a subspace of a real
+ vector space @{text E}, let @{text p} be a semi-norm on @{text E},
+ and @{text f} be a linear form defined on @{text F} such that @{text
+ f} is bounded by @{text p}, i.e. @{text "\<forall>x \<in> F. f x \<le> p x"}. Then
+ @{text f} can be extended to a linear form @{text h} on @{text E}
+ such that @{text h} is norm-preserving, i.e. @{text h} is also
+ bounded by @{text p}.
+
+ \bigskip
+ \textbf{Proof Sketch.}
+ \begin{enumerate}
+
+ \item Define @{text M} as the set of norm-preserving extensions of
+ @{text f} to subspaces of @{text E}. The linear forms in @{text M}
+ are ordered by domain extension.
+
+ \item We show that every non-empty chain in @{text M} has an upper
+ bound in @{text M}.
+
+ \item With Zorn's Lemma we conclude that there is a maximal function
+ @{text g} in @{text M}.
+
+ \item The domain @{text H} of @{text g} is the whole space @{text
+ E}, as shown by classical contradiction:
+
+ \begin{itemize}
+
+ \item Assuming @{text g} is not defined on whole @{text E}, it can
+ still be extended in a norm-preserving way to a super-space @{text
+ H'} of @{text H}.
+
+ \item Thus @{text g} can not be maximal. Contradiction!
+
+ \end{itemize}
+ \end{enumerate}
+*}
+
+theorem Hahn_Banach:
+ assumes E: "vectorspace E" and "subspace F E"
+ and "seminorm E p" and "linearform F f"
+ assumes fp: "\<forall>x \<in> F. f x \<le> p x"
+ shows "\<exists>h. linearform E h \<and> (\<forall>x \<in> F. h x = f x) \<and> (\<forall>x \<in> E. h x \<le> p x)"
+ -- {* Let @{text E} be a vector space, @{text F} a subspace of @{text E}, @{text p} a seminorm on @{text E}, *}
+ -- {* and @{text f} a linear form on @{text F} such that @{text f} is bounded by @{text p}, *}
+ -- {* then @{text f} can be extended to a linear form @{text h} on @{text E} in a norm-preserving way. \skp *}
+proof -
+ interpret vectorspace E by fact
+ interpret subspace F E by fact
+ interpret seminorm E p by fact
+ interpret linearform F f by fact
+ def M \<equiv> "norm_pres_extensions E p F f"
+ then have M: "M = \<dots>" by (simp only:)
+ from E have F: "vectorspace F" ..
+ note FE = `F \<unlhd> E`
+ {
+ fix c assume cM: "c \<in> chain M" and ex: "\<exists>x. x \<in> c"
+ have "\<Union>c \<in> M"
+ -- {* Show that every non-empty chain @{text c} of @{text M} has an upper bound in @{text M}: *}
+ -- {* @{text "\<Union>c"} is greater than any element of the chain @{text c}, so it suffices to show @{text "\<Union>c \<in> M"}. *}
+ unfolding M_def
+ proof (rule norm_pres_extensionI)
+ let ?H = "domain (\<Union>c)"
+ let ?h = "funct (\<Union>c)"
+
+ have a: "graph ?H ?h = \<Union>c"
+ proof (rule graph_domain_funct)
+ fix x y z assume "(x, y) \<in> \<Union>c" and "(x, z) \<in> \<Union>c"
+ with M_def cM show "z = y" by (rule sup_definite)
+ qed
+ moreover from M cM a have "linearform ?H ?h"
+ by (rule sup_lf)
+ moreover from a M cM ex FE E have "?H \<unlhd> E"
+ by (rule sup_subE)
+ moreover from a M cM ex FE have "F \<unlhd> ?H"
+ by (rule sup_supF)
+ moreover from a M cM ex have "graph F f \<subseteq> graph ?H ?h"
+ by (rule sup_ext)
+ moreover from a M cM have "\<forall>x \<in> ?H. ?h x \<le> p x"
+ by (rule sup_norm_pres)
+ ultimately show "\<exists>H h. \<Union>c = graph H h
+ \<and> linearform H h
+ \<and> H \<unlhd> E
+ \<and> F \<unlhd> H
+ \<and> graph F f \<subseteq> graph H h
+ \<and> (\<forall>x \<in> H. h x \<le> p x)" by blast
+ qed
+ }
+ then have "\<exists>g \<in> M. \<forall>x \<in> M. g \<subseteq> x \<longrightarrow> g = x"
+ -- {* With Zorn's Lemma we can conclude that there is a maximal element in @{text M}. \skp *}
+ proof (rule Zorn's_Lemma)
+ -- {* We show that @{text M} is non-empty: *}
+ show "graph F f \<in> M"
+ unfolding M_def
+ proof (rule norm_pres_extensionI2)
+ show "linearform F f" by fact
+ show "F \<unlhd> E" by fact
+ from F show "F \<unlhd> F" by (rule vectorspace.subspace_refl)
+ show "graph F f \<subseteq> graph F f" ..
+ show "\<forall>x\<in>F. f x \<le> p x" by fact
+ qed
+ qed
+ then obtain g where gM: "g \<in> M" and gx: "\<forall>x \<in> M. g \<subseteq> x \<longrightarrow> g = x"
+ by blast
+ from gM obtain H h where
+ g_rep: "g = graph H h"
+ and linearform: "linearform H h"
+ and HE: "H \<unlhd> E" and FH: "F \<unlhd> H"
+ and graphs: "graph F f \<subseteq> graph H h"
+ and hp: "\<forall>x \<in> H. h x \<le> p x" unfolding M_def ..
+ -- {* @{text g} is a norm-preserving extension of @{text f}, in other words: *}
+ -- {* @{text g} is the graph of some linear form @{text h} defined on a subspace @{text H} of @{text E}, *}
+ -- {* and @{text h} is an extension of @{text f} that is again bounded by @{text p}. \skp *}
+ from HE E have H: "vectorspace H"
+ by (rule subspace.vectorspace)
+
+ have HE_eq: "H = E"
+ -- {* We show that @{text h} is defined on whole @{text E} by classical contradiction. \skp *}
+ proof (rule classical)
+ assume neq: "H \<noteq> E"
+ -- {* Assume @{text h} is not defined on whole @{text E}. Then show that @{text h} can be extended *}
+ -- {* in a norm-preserving way to a function @{text h'} with the graph @{text g'}. \skp *}
+ have "\<exists>g' \<in> M. g \<subseteq> g' \<and> g \<noteq> g'"
+ proof -
+ from HE have "H \<subseteq> E" ..
+ with neq obtain x' where x'E: "x' \<in> E" and "x' \<notin> H" by blast
+ obtain x': "x' \<noteq> 0"
+ proof
+ show "x' \<noteq> 0"
+ proof
+ assume "x' = 0"
+ with H have "x' \<in> H" by (simp only: vectorspace.zero)
+ with `x' \<notin> H` show False by contradiction
+ qed
+ qed
+
+ def H' \<equiv> "H + lin x'"
+ -- {* Define @{text H'} as the direct sum of @{text H} and the linear closure of @{text x'}. \skp *}
+ have HH': "H \<unlhd> H'"
+ proof (unfold H'_def)
+ from x'E have "vectorspace (lin x')" ..
+ with H show "H \<unlhd> H + lin x'" ..
+ qed
+
+ obtain xi where
+ xi: "\<forall>y \<in> H. - p (y + x') - h y \<le> xi
+ \<and> xi \<le> p (y + x') - h y"
+ -- {* Pick a real number @{text \<xi>} that fulfills certain inequations; this will *}
+ -- {* be used to establish that @{text h'} is a norm-preserving extension of @{text h}.
+ \label{ex-xi-use}\skp *}
+ proof -
+ from H have "\<exists>xi. \<forall>y \<in> H. - p (y + x') - h y \<le> xi
+ \<and> xi \<le> p (y + x') - h y"
+ proof (rule ex_xi)
+ fix u v assume u: "u \<in> H" and v: "v \<in> H"
+ with HE have uE: "u \<in> E" and vE: "v \<in> E" by auto
+ from H u v linearform have "h v - h u = h (v - u)"
+ by (simp add: linearform.diff)
+ also from hp and H u v have "\<dots> \<le> p (v - u)"
+ by (simp only: vectorspace.diff_closed)
+ also from x'E uE vE have "v - u = x' + - x' + v + - u"
+ by (simp add: diff_eq1)
+ also from x'E uE vE have "\<dots> = v + x' + - (u + x')"
+ by (simp add: add_ac)
+ also from x'E uE vE have "\<dots> = (v + x') - (u + x')"
+ by (simp add: diff_eq1)
+ also from x'E uE vE E have "p \<dots> \<le> p (v + x') + p (u + x')"
+ by (simp add: diff_subadditive)
+ finally have "h v - h u \<le> p (v + x') + p (u + x')" .
+ then show "- p (u + x') - h u \<le> p (v + x') - h v" by simp
+ qed
+ then show thesis by (blast intro: that)
+ qed
+
+ def h' \<equiv> "\<lambda>x. let (y, a) =
+ SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H in h y + a * xi"
+ -- {* Define the extension @{text h'} of @{text h} to @{text H'} using @{text \<xi>}. \skp *}
+
+ have "g \<subseteq> graph H' h' \<and> g \<noteq> graph H' h'"
+ -- {* @{text h'} is an extension of @{text h} \dots \skp *}
+ proof
+ show "g \<subseteq> graph H' h'"
+ proof -
+ have "graph H h \<subseteq> graph H' h'"
+ proof (rule graph_extI)
+ fix t assume t: "t \<in> H"
+ from E HE t have "(SOME (y, a). t = y + a \<cdot> x' \<and> y \<in> H) = (t, 0)"
+ using `x' \<notin> H` `x' \<in> E` `x' \<noteq> 0` by (rule decomp_H'_H)
+ with h'_def show "h t = h' t" by (simp add: Let_def)
+ next
+ from HH' show "H \<subseteq> H'" ..
+ qed
+ with g_rep show ?thesis by (simp only:)
+ qed
+
+ show "g \<noteq> graph H' h'"
+ proof -
+ have "graph H h \<noteq> graph H' h'"
+ proof
+ assume eq: "graph H h = graph H' h'"
+ have "x' \<in> H'"
+ unfolding H'_def
+ proof
+ from H show "0 \<in> H" by (rule vectorspace.zero)
+ from x'E show "x' \<in> lin x'" by (rule x_lin_x)
+ from x'E show "x' = 0 + x'" by simp
+ qed
+ then have "(x', h' x') \<in> graph H' h'" ..
+ with eq have "(x', h' x') \<in> graph H h" by (simp only:)
+ then have "x' \<in> H" ..
+ with `x' \<notin> H` show False by contradiction
+ qed
+ with g_rep show ?thesis by simp
+ qed
+ qed
+ moreover have "graph H' h' \<in> M"
+ -- {* and @{text h'} is norm-preserving. \skp *}
+ proof (unfold M_def)
+ show "graph H' h' \<in> norm_pres_extensions E p F f"
+ proof (rule norm_pres_extensionI2)
+ show "linearform H' h'"
+ using h'_def H'_def HE linearform `x' \<notin> H` `x' \<in> E` `x' \<noteq> 0` E
+ by (rule h'_lf)
+ show "H' \<unlhd> E"
+ unfolding H'_def
+ proof
+ show "H \<unlhd> E" by fact
+ show "vectorspace E" by fact
+ from x'E show "lin x' \<unlhd> E" ..
+ qed
+ from H `F \<unlhd> H` HH' show FH': "F \<unlhd> H'"
+ by (rule vectorspace.subspace_trans)
+ show "graph F f \<subseteq> graph H' h'"
+ proof (rule graph_extI)
+ fix x assume x: "x \<in> F"
+ with graphs have "f x = h x" ..
+ also have "\<dots> = h x + 0 * xi" by simp
+ also have "\<dots> = (let (y, a) = (x, 0) in h y + a * xi)"
+ by (simp add: Let_def)
+ also have "(x, 0) =
+ (SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H)"
+ using E HE
+ proof (rule decomp_H'_H [symmetric])
+ from FH x show "x \<in> H" ..
+ from x' show "x' \<noteq> 0" .
+ show "x' \<notin> H" by fact
+ show "x' \<in> E" by fact
+ qed
+ also have
+ "(let (y, a) = (SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H)
+ in h y + a * xi) = h' x" by (simp only: h'_def)
+ finally show "f x = h' x" .
+ next
+ from FH' show "F \<subseteq> H'" ..
+ qed
+ show "\<forall>x \<in> H'. h' x \<le> p x"
+ using h'_def H'_def `x' \<notin> H` `x' \<in> E` `x' \<noteq> 0` E HE
+ `seminorm E p` linearform and hp xi
+ by (rule h'_norm_pres)
+ qed
+ qed
+ ultimately show ?thesis ..
+ qed
+ then have "\<not> (\<forall>x \<in> M. g \<subseteq> x \<longrightarrow> g = x)" by simp
+ -- {* So the graph @{text g} of @{text h} cannot be maximal. Contradiction! \skp *}
+ with gx show "H = E" by contradiction
+ qed
+
+ from HE_eq and linearform have "linearform E h"
+ by (simp only:)
+ moreover have "\<forall>x \<in> F. h x = f x"
+ proof
+ fix x assume "x \<in> F"
+ with graphs have "f x = h x" ..
+ then show "h x = f x" ..
+ qed
+ moreover from HE_eq and hp have "\<forall>x \<in> E. h x \<le> p x"
+ by (simp only:)
+ ultimately show ?thesis by blast
+qed
+
+
+subsection {* Alternative formulation *}
+
+text {*
+ The following alternative formulation of the Hahn-Banach
+ Theorem\label{abs-Hahn-Banach} uses the fact that for a real linear
+ form @{text f} and a seminorm @{text p} the following inequations
+ are equivalent:\footnote{This was shown in lemma @{thm [source]
+ abs_ineq_iff} (see page \pageref{abs-ineq-iff}).}
+ \begin{center}
+ \begin{tabular}{lll}
+ @{text "\<forall>x \<in> H. \<bar>h x\<bar> \<le> p x"} & and &
+ @{text "\<forall>x \<in> H. h x \<le> p x"} \\
+ \end{tabular}
+ \end{center}
+*}
+
+theorem abs_Hahn_Banach:
+ assumes E: "vectorspace E" and FE: "subspace F E"
+ and lf: "linearform F f" and sn: "seminorm E p"
+ assumes fp: "\<forall>x \<in> F. \<bar>f x\<bar> \<le> p x"
+ shows "\<exists>g. linearform E g
+ \<and> (\<forall>x \<in> F. g x = f x)
+ \<and> (\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x)"
+proof -
+ interpret vectorspace E by fact
+ interpret subspace F E by fact
+ interpret linearform F f by fact
+ interpret seminorm E p by fact
+ have "\<exists>g. linearform E g \<and> (\<forall>x \<in> F. g x = f x) \<and> (\<forall>x \<in> E. g x \<le> p x)"
+ using E FE sn lf
+ proof (rule Hahn_Banach)
+ show "\<forall>x \<in> F. f x \<le> p x"
+ using FE E sn lf and fp by (rule abs_ineq_iff [THEN iffD1])
+ qed
+ then obtain g where lg: "linearform E g" and *: "\<forall>x \<in> F. g x = f x"
+ and **: "\<forall>x \<in> E. g x \<le> p x" by blast
+ have "\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x"
+ using _ E sn lg **
+ proof (rule abs_ineq_iff [THEN iffD2])
+ show "E \<unlhd> E" ..
+ qed
+ with lg * show ?thesis by blast
+qed
+
+
+subsection {* The Hahn-Banach Theorem for normed spaces *}
+
+text {*
+ Every continuous linear form @{text f} on a subspace @{text F} of a
+ norm space @{text E}, can be extended to a continuous linear form
+ @{text g} on @{text E} such that @{text "\<parallel>f\<parallel> = \<parallel>g\<parallel>"}.
+*}
+
+theorem norm_Hahn_Banach:
+ fixes V and norm ("\<parallel>_\<parallel>")
+ fixes B defines "\<And>V f. B V f \<equiv> {0} \<union> {\<bar>f x\<bar> / \<parallel>x\<parallel> | x. x \<noteq> 0 \<and> x \<in> V}"
+ fixes fn_norm ("\<parallel>_\<parallel>\<hyphen>_" [0, 1000] 999)
+ defines "\<And>V f. \<parallel>f\<parallel>\<hyphen>V \<equiv> \<Squnion>(B V f)"
+ assumes E_norm: "normed_vectorspace E norm" and FE: "subspace F E"
+ and linearform: "linearform F f" and "continuous F norm f"
+ shows "\<exists>g. linearform E g
+ \<and> continuous E norm g
+ \<and> (\<forall>x \<in> F. g x = f x)
+ \<and> \<parallel>g\<parallel>\<hyphen>E = \<parallel>f\<parallel>\<hyphen>F"
+proof -
+ interpret normed_vectorspace E norm by fact
+ interpret normed_vectorspace_with_fn_norm E norm B fn_norm
+ by (auto simp: B_def fn_norm_def) intro_locales
+ interpret subspace F E by fact
+ interpret linearform F f by fact
+ interpret continuous F norm f by fact
+ have E: "vectorspace E" by intro_locales
+ have F: "vectorspace F" by rule intro_locales
+ have F_norm: "normed_vectorspace F norm"
+ using FE E_norm by (rule subspace_normed_vs)
+ have ge_zero: "0 \<le> \<parallel>f\<parallel>\<hyphen>F"
+ by (rule normed_vectorspace_with_fn_norm.fn_norm_ge_zero
+ [OF normed_vectorspace_with_fn_norm.intro,
+ OF F_norm `continuous F norm f` , folded B_def fn_norm_def])
+ txt {* We define a function @{text p} on @{text E} as follows:
+ @{text "p x = \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>"} *}
+ def p \<equiv> "\<lambda>x. \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
+
+ txt {* @{text p} is a seminorm on @{text E}: *}
+ have q: "seminorm E p"
+ proof
+ fix x y a assume x: "x \<in> E" and y: "y \<in> E"
+
+ txt {* @{text p} is positive definite: *}
+ have "0 \<le> \<parallel>f\<parallel>\<hyphen>F" by (rule ge_zero)
+ moreover from x have "0 \<le> \<parallel>x\<parallel>" ..
+ ultimately show "0 \<le> p x"
+ by (simp add: p_def zero_le_mult_iff)
+
+ txt {* @{text p} is absolutely homogenous: *}
+
+ show "p (a \<cdot> x) = \<bar>a\<bar> * p x"
+ proof -
+ have "p (a \<cdot> x) = \<parallel>f\<parallel>\<hyphen>F * \<parallel>a \<cdot> x\<parallel>" by (simp only: p_def)
+ also from x have "\<parallel>a \<cdot> x\<parallel> = \<bar>a\<bar> * \<parallel>x\<parallel>" by (rule abs_homogenous)
+ also have "\<parallel>f\<parallel>\<hyphen>F * (\<bar>a\<bar> * \<parallel>x\<parallel>) = \<bar>a\<bar> * (\<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>)" by simp
+ also have "\<dots> = \<bar>a\<bar> * p x" by (simp only: p_def)
+ finally show ?thesis .
+ qed
+
+ txt {* Furthermore, @{text p} is subadditive: *}
+
+ show "p (x + y) \<le> p x + p y"
+ proof -
+ have "p (x + y) = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x + y\<parallel>" by (simp only: p_def)
+ also have a: "0 \<le> \<parallel>f\<parallel>\<hyphen>F" by (rule ge_zero)
+ from x y have "\<parallel>x + y\<parallel> \<le> \<parallel>x\<parallel> + \<parallel>y\<parallel>" ..
+ with a have " \<parallel>f\<parallel>\<hyphen>F * \<parallel>x + y\<parallel> \<le> \<parallel>f\<parallel>\<hyphen>F * (\<parallel>x\<parallel> + \<parallel>y\<parallel>)"
+ by (simp add: mult_left_mono)
+ also have "\<dots> = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel> + \<parallel>f\<parallel>\<hyphen>F * \<parallel>y\<parallel>" by (simp only: right_distrib)
+ also have "\<dots> = p x + p y" by (simp only: p_def)
+ finally show ?thesis .
+ qed
+ qed
+
+ txt {* @{text f} is bounded by @{text p}. *}
+
+ have "\<forall>x \<in> F. \<bar>f x\<bar> \<le> p x"
+ proof
+ fix x assume "x \<in> F"
+ with `continuous F norm f` and linearform
+ show "\<bar>f x\<bar> \<le> p x"
+ unfolding p_def by (rule normed_vectorspace_with_fn_norm.fn_norm_le_cong
+ [OF normed_vectorspace_with_fn_norm.intro,
+ OF F_norm, folded B_def fn_norm_def])
+ qed
+
+ txt {* Using the fact that @{text p} is a seminorm and @{text f} is bounded
+ by @{text p} we can apply the Hahn-Banach Theorem for real vector
+ spaces. So @{text f} can be extended in a norm-preserving way to
+ some function @{text g} on the whole vector space @{text E}. *}
+
+ with E FE linearform q obtain g where
+ linearformE: "linearform E g"
+ and a: "\<forall>x \<in> F. g x = f x"
+ and b: "\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x"
+ by (rule abs_Hahn_Banach [elim_format]) iprover
+
+ txt {* We furthermore have to show that @{text g} is also continuous: *}
+
+ have g_cont: "continuous E norm g" using linearformE
+ proof
+ fix x assume "x \<in> E"
+ with b show "\<bar>g x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
+ by (simp only: p_def)
+ qed
+
+ txt {* To complete the proof, we show that @{text "\<parallel>g\<parallel> = \<parallel>f\<parallel>"}. *}
+
+ have "\<parallel>g\<parallel>\<hyphen>E = \<parallel>f\<parallel>\<hyphen>F"
+ proof (rule order_antisym)
+ txt {*
+ First we show @{text "\<parallel>g\<parallel> \<le> \<parallel>f\<parallel>"}. The function norm @{text
+ "\<parallel>g\<parallel>"} is defined as the smallest @{text "c \<in> \<real>"} such that
+ \begin{center}
+ \begin{tabular}{l}
+ @{text "\<forall>x \<in> E. \<bar>g x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>"}
+ \end{tabular}
+ \end{center}
+ \noindent Furthermore holds
+ \begin{center}
+ \begin{tabular}{l}
+ @{text "\<forall>x \<in> E. \<bar>g x\<bar> \<le> \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>"}
+ \end{tabular}
+ \end{center}
+ *}
+
+ have "\<forall>x \<in> E. \<bar>g x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
+ proof
+ fix x assume "x \<in> E"
+ with b show "\<bar>g x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
+ by (simp only: p_def)
+ qed
+ from g_cont this ge_zero
+ show "\<parallel>g\<parallel>\<hyphen>E \<le> \<parallel>f\<parallel>\<hyphen>F"
+ by (rule fn_norm_least [of g, folded B_def fn_norm_def])
+
+ txt {* The other direction is achieved by a similar argument. *}
+
+ show "\<parallel>f\<parallel>\<hyphen>F \<le> \<parallel>g\<parallel>\<hyphen>E"
+ proof (rule normed_vectorspace_with_fn_norm.fn_norm_least
+ [OF normed_vectorspace_with_fn_norm.intro,
+ OF F_norm, folded B_def fn_norm_def])
+ show "\<forall>x \<in> F. \<bar>f x\<bar> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>"
+ proof
+ fix x assume x: "x \<in> F"
+ from a x have "g x = f x" ..
+ then have "\<bar>f x\<bar> = \<bar>g x\<bar>" by (simp only:)
+ also from g_cont
+ have "\<dots> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>"
+ proof (rule fn_norm_le_cong [OF _ linearformE, folded B_def fn_norm_def])
+ from FE x show "x \<in> E" ..
+ qed
+ finally show "\<bar>f x\<bar> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>" .
+ qed
+ show "0 \<le> \<parallel>g\<parallel>\<hyphen>E"
+ using g_cont
+ by (rule fn_norm_ge_zero [of g, folded B_def fn_norm_def])
+ show "continuous F norm f" by fact
+ qed
+ qed
+ with linearformE a g_cont show ?thesis by blast
+qed
+
+end