(* Title: HOL/Hahn_Banach/Hahn_Banach.thy
Author: Gertrud Bauer, TU Munich
*)
section \<open>The Hahn-Banach Theorem\<close>
theory Hahn_Banach
imports Hahn_Banach_Lemmas
begin
text \<open>
We present the proof of two different versions of the Hahn-Banach Theorem,
closely following \<^cite>\<open>\<open>\S36\<close> in "Heuser:1986"\<close>.
\<close>
subsection \<open>The Hahn-Banach Theorem for vector spaces\<close>
paragraph \<open>Hahn-Banach Theorem.\<close>
text \<open>
Let \<open>F\<close> be a subspace of a real vector space \<open>E\<close>, let \<open>p\<close> be a semi-norm on
\<open>E\<close>, and \<open>f\<close> be a linear form defined on \<open>F\<close> such that \<open>f\<close> is bounded by
\<open>p\<close>, i.e. \<open>\<forall>x \<in> F. f x \<le> p x\<close>. Then \<open>f\<close> can be extended to a linear form \<open>h\<close>
on \<open>E\<close> such that \<open>h\<close> is norm-preserving, i.e. \<open>h\<close> is also bounded by \<open>p\<close>.
\<close>
paragraph \<open>Proof Sketch.\<close>
text \<open>
\<^enum> Define \<open>M\<close> as the set of norm-preserving extensions of \<open>f\<close> to subspaces of
\<open>E\<close>. The linear forms in \<open>M\<close> are ordered by domain extension.
\<^enum> We show that every non-empty chain in \<open>M\<close> has an upper bound in \<open>M\<close>.
\<^enum> With Zorn's Lemma we conclude that there is a maximal function \<open>g\<close> in \<open>M\<close>.
\<^enum> The domain \<open>H\<close> of \<open>g\<close> is the whole space \<open>E\<close>, as shown by classical
contradiction:
\<^item> Assuming \<open>g\<close> is not defined on whole \<open>E\<close>, it can still be extended in a
norm-preserving way to a super-space \<open>H'\<close> of \<open>H\<close>.
\<^item> Thus \<open>g\<close> can not be maximal. Contradiction!
\<close>
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)"
\<comment> \<open>Let \<open>E\<close> be a vector space, \<open>F\<close> a subspace of \<open>E\<close>, \<open>p\<close> a seminorm on \<open>E\<close>,\<close>
\<comment> \<open>and \<open>f\<close> a linear form on \<open>F\<close> such that \<open>f\<close> is bounded by \<open>p\<close>,\<close>
\<comment> \<open>then \<open>f\<close> can be extended to a linear form \<open>h\<close> on \<open>E\<close> in a norm-preserving way. \<^smallskip>\<close>
proof -
interpret vectorspace E by fact
interpret subspace F E by fact
interpret seminorm E p by fact
interpret linearform F f by fact
define M where "M = norm_pres_extensions E p F f"
then have M: "M = \<dots>" by (simp only:)
from E have F: "vectorspace F" ..
note FE = \<open>F \<unlhd> E\<close>
{
fix c assume cM: "c \<in> chains M" and ex: "\<exists>x. x \<in> c"
have "\<Union>c \<in> M"
\<comment> \<open>Show that every non-empty chain \<open>c\<close> of \<open>M\<close> has an upper bound in \<open>M\<close>:\<close>
\<comment> \<open>\<open>\<Union>c\<close> is greater than any element of the chain \<open>c\<close>, so it suffices to show \<open>\<Union>c \<in> M\<close>.\<close>
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> x = g"
\<comment> \<open>With Zorn's Lemma we can conclude that there is a maximal element in \<open>M\<close>. \<^smallskip>\<close>
proof (rule Zorn's_Lemma)
\<comment> \<open>We show that \<open>M\<close> is non-empty:\<close>
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 ..
\<comment> \<open>\<open>g\<close> is a norm-preserving extension of \<open>f\<close>, in other words:\<close>
\<comment> \<open>\<open>g\<close> is the graph of some linear form \<open>h\<close> defined on a subspace \<open>H\<close> of \<open>E\<close>,\<close>
\<comment> \<open>and \<open>h\<close> is an extension of \<open>f\<close> that is again bounded by \<open>p\<close>. \<^smallskip>\<close>
from HE E have H: "vectorspace H"
by (rule subspace.vectorspace)
have HE_eq: "H = E"
\<comment> \<open>We show that \<open>h\<close> is defined on whole \<open>E\<close> by classical contradiction. \<^smallskip>\<close>
proof (rule classical)
assume neq: "H \<noteq> E"
\<comment> \<open>Assume \<open>h\<close> is not defined on whole \<open>E\<close>. Then show that \<open>h\<close> can be extended\<close>
\<comment> \<open>in a norm-preserving way to a function \<open>h'\<close> with the graph \<open>g'\<close>. \<^smallskip>\<close>
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 \<open>x' \<notin> H\<close> show False by contradiction
qed
qed
define H' where "H' = H + lin x'"
\<comment> \<open>Define \<open>H'\<close> as the direct sum of \<open>H\<close> and the linear closure of \<open>x'\<close>. \<^smallskip>\<close>
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"
\<comment> \<open>Pick a real number \<open>\<xi>\<close> that fulfills certain inequality; this will\<close>
\<comment> \<open>be used to establish that \<open>h'\<close> is a norm-preserving extension of \<open>h\<close>.
\label{ex-xi-use}\<^smallskip>\<close>
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
define h' where "h' x = (let (y, a) =
SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H in h y + a * xi)" for x
\<comment> \<open>Define the extension \<open>h'\<close> of \<open>h\<close> to \<open>H'\<close> using \<open>\<xi>\<close>. \<^smallskip>\<close>
have "g \<subseteq> graph H' h' \<and> g \<noteq> graph H' h'"
\<comment> \<open>\<open>h'\<close> is an extension of \<open>h\<close> \dots \<^smallskip>\<close>
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 \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> 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 \<open>x' \<notin> H\<close> show False by contradiction
qed
with g_rep show ?thesis by simp
qed
qed
moreover have "graph H' h' \<in> M"
\<comment> \<open>and \<open>h'\<close> is norm-preserving. \<^smallskip>\<close>
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 \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> 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 \<open>F \<unlhd> H\<close> 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 \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> E HE
\<open>seminorm E p\<close> 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
\<comment> \<open>So the graph \<open>g\<close> of \<open>h\<close> cannot be maximal. Contradiction! \<^smallskip>\<close>
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 \<open>Alternative formulation\<close>
text \<open>
The following alternative formulation of the Hahn-Banach
Theorem\label{abs-Hahn-Banach} uses the fact that for a real linear form \<open>f\<close>
and a seminorm \<open>p\<close> the following inequality are equivalent:\footnote{This
was shown in lemma @{thm [source] abs_ineq_iff} (see page
\pageref{abs-ineq-iff}).}
\begin{center}
\begin{tabular}{lll}
\<open>\<forall>x \<in> H. \<bar>h x\<bar> \<le> p x\<close> & and & \<open>\<forall>x \<in> H. h x \<le> p x\<close> \\
\end{tabular}
\end{center}
\<close>
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 \<open>The Hahn-Banach Theorem for normed spaces\<close>
text \<open>
Every continuous linear form \<open>f\<close> on a subspace \<open>F\<close> of a norm space \<open>E\<close>, can
be extended to a continuous linear form \<open>g\<close> on \<open>E\<close> such that \<open>\<parallel>f\<parallel> = \<parallel>g\<parallel>\<close>.
\<close>
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 f norm"
shows "\<exists>g. linearform E g
\<and> continuous E g norm
\<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 f norm 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 \<open>continuous F f norm\<close> , folded B_def fn_norm_def])
txt \<open>We define a function \<open>p\<close> on \<open>E\<close> as follows:
\<open>p x = \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>\<close>\<close>
define p where "p x = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>" for x
txt \<open>\<open>p\<close> is a seminorm on \<open>E\<close>:\<close>
have q: "seminorm E p"
proof
fix x y a assume x: "x \<in> E" and y: "y \<in> E"
txt \<open>\<open>p\<close> is positive definite:\<close>
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 \<open>\<open>p\<close> is absolutely homogeneous:\<close>
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 \<open>Furthermore, \<open>p\<close> is subadditive:\<close>
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: distrib_left)
also have "\<dots> = p x + p y" by (simp only: p_def)
finally show ?thesis .
qed
qed
txt \<open>\<open>f\<close> is bounded by \<open>p\<close>.\<close>
have "\<forall>x \<in> F. \<bar>f x\<bar> \<le> p x"
proof
fix x assume "x \<in> F"
with \<open>continuous F f norm\<close> 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 \<open>Using the fact that \<open>p\<close> is a seminorm and \<open>f\<close> is bounded by \<open>p\<close> we can
apply the Hahn-Banach Theorem for real vector spaces. So \<open>f\<close> can be
extended in a norm-preserving way to some function \<open>g\<close> on the whole vector
space \<open>E\<close>.\<close>
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 \<open>We furthermore have to show that \<open>g\<close> is also continuous:\<close>
have g_cont: "continuous E g norm" 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 \<open>To complete the proof, we show that \<open>\<parallel>g\<parallel> = \<parallel>f\<parallel>\<close>.\<close>
have "\<parallel>g\<parallel>\<hyphen>E = \<parallel>f\<parallel>\<hyphen>F"
proof (rule order_antisym)
txt \<open>
First we show \<open>\<parallel>g\<parallel> \<le> \<parallel>f\<parallel>\<close>. The function norm \<open>\<parallel>g\<parallel>\<close> is defined as the
smallest \<open>c \<in> \<real>\<close> such that
\begin{center}
\begin{tabular}{l}
\<open>\<forall>x \<in> E. \<bar>g x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>\<close>
\end{tabular}
\end{center}
\<^noindent> Furthermore holds
\begin{center}
\begin{tabular}{l}
\<open>\<forall>x \<in> E. \<bar>g x\<bar> \<le> \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>\<close>
\end{tabular}
\end{center}
\<close>
from g_cont _ ge_zero
show "\<parallel>g\<parallel>\<hyphen>E \<le> \<parallel>f\<parallel>\<hyphen>F"
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 \<open>The other direction is achieved by a similar argument.\<close>
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])
fix x assume x: "x \<in> F"
show "\<bar>f x\<bar> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>"
proof -
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 ?thesis .
qed
next
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 f norm" by fact
qed
qed
with linearformE a g_cont show ?thesis by blast
qed
end