src/HOL/Hahn_Banach/Hahn_Banach.thy
author wenzelm
Thu Mar 09 21:17:32 2017 +0100 (2017-03-09)
changeset 65166 f8aafbf2b02e
parent 63040 eb4ddd18d635
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tuned;
     1 (*  Title:      HOL/Hahn_Banach/Hahn_Banach.thy
     2     Author:     Gertrud Bauer, TU Munich
     3 *)
     4 
     5 section \<open>The Hahn-Banach Theorem\<close>
     6 
     7 theory Hahn_Banach
     8 imports Hahn_Banach_Lemmas
     9 begin
    10 
    11 text \<open>
    12   We present the proof of two different versions of the Hahn-Banach Theorem,
    13   closely following @{cite \<open>\S36\<close> "Heuser:1986"}.
    14 \<close>
    15 
    16 
    17 subsection \<open>The Hahn-Banach Theorem for vector spaces\<close>
    18 
    19 paragraph \<open>Hahn-Banach Theorem.\<close>
    20 text \<open>
    21   Let \<open>F\<close> be a subspace of a real vector space \<open>E\<close>, let \<open>p\<close> be a semi-norm on
    22   \<open>E\<close>, and \<open>f\<close> be a linear form defined on \<open>F\<close> such that \<open>f\<close> is bounded by
    23   \<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>
    24   on \<open>E\<close> such that \<open>h\<close> is norm-preserving, i.e. \<open>h\<close> is also bounded by \<open>p\<close>.
    25 \<close>
    26 
    27 paragraph \<open>Proof Sketch.\<close>
    28 text \<open>
    29   \<^enum> Define \<open>M\<close> as the set of norm-preserving extensions of \<open>f\<close> to subspaces of
    30   \<open>E\<close>. The linear forms in \<open>M\<close> are ordered by domain extension.
    31 
    32   \<^enum> We show that every non-empty chain in \<open>M\<close> has an upper bound in \<open>M\<close>.
    33 
    34   \<^enum> With Zorn's Lemma we conclude that there is a maximal function \<open>g\<close> in \<open>M\<close>.
    35 
    36   \<^enum> The domain \<open>H\<close> of \<open>g\<close> is the whole space \<open>E\<close>, as shown by classical
    37   contradiction:
    38 
    39     \<^item> Assuming \<open>g\<close> is not defined on whole \<open>E\<close>, it can still be extended in a
    40     norm-preserving way to a super-space \<open>H'\<close> of \<open>H\<close>.
    41 
    42     \<^item> Thus \<open>g\<close> can not be maximal. Contradiction!
    43 \<close>
    44 
    45 theorem Hahn_Banach:
    46   assumes E: "vectorspace E" and "subspace F E"
    47     and "seminorm E p" and "linearform F f"
    48   assumes fp: "\<forall>x \<in> F. f x \<le> p x"
    49   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)"
    50     \<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>
    51     \<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>
    52     \<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>
    53 proof -
    54   interpret vectorspace E by fact
    55   interpret subspace F E by fact
    56   interpret seminorm E p by fact
    57   interpret linearform F f by fact
    58   define M where "M = norm_pres_extensions E p F f"
    59   then have M: "M = \<dots>" by (simp only:)
    60   from E have F: "vectorspace F" ..
    61   note FE = \<open>F \<unlhd> E\<close>
    62   {
    63     fix c assume cM: "c \<in> chains M" and ex: "\<exists>x. x \<in> c"
    64     have "\<Union>c \<in> M"
    65       \<comment> \<open>Show that every non-empty chain \<open>c\<close> of \<open>M\<close> has an upper bound in \<open>M\<close>:\<close>
    66       \<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>
    67       unfolding M_def
    68     proof (rule norm_pres_extensionI)
    69       let ?H = "domain (\<Union>c)"
    70       let ?h = "funct (\<Union>c)"
    71 
    72       have a: "graph ?H ?h = \<Union>c"
    73       proof (rule graph_domain_funct)
    74         fix x y z assume "(x, y) \<in> \<Union>c" and "(x, z) \<in> \<Union>c"
    75         with M_def cM show "z = y" by (rule sup_definite)
    76       qed
    77       moreover from M cM a have "linearform ?H ?h"
    78         by (rule sup_lf)
    79       moreover from a M cM ex FE E have "?H \<unlhd> E"
    80         by (rule sup_subE)
    81       moreover from a M cM ex FE have "F \<unlhd> ?H"
    82         by (rule sup_supF)
    83       moreover from a M cM ex have "graph F f \<subseteq> graph ?H ?h"
    84         by (rule sup_ext)
    85       moreover from a M cM have "\<forall>x \<in> ?H. ?h x \<le> p x"
    86         by (rule sup_norm_pres)
    87       ultimately show "\<exists>H h. \<Union>c = graph H h
    88           \<and> linearform H h
    89           \<and> H \<unlhd> E
    90           \<and> F \<unlhd> H
    91           \<and> graph F f \<subseteq> graph H h
    92           \<and> (\<forall>x \<in> H. h x \<le> p x)" by blast
    93     qed
    94   }
    95   then have "\<exists>g \<in> M. \<forall>x \<in> M. g \<subseteq> x \<longrightarrow> x = g"
    96   \<comment> \<open>With Zorn's Lemma we can conclude that there is a maximal element in \<open>M\<close>. \<^smallskip>\<close>
    97   proof (rule Zorn's_Lemma)
    98       \<comment> \<open>We show that \<open>M\<close> is non-empty:\<close>
    99     show "graph F f \<in> M"
   100       unfolding M_def
   101     proof (rule norm_pres_extensionI2)
   102       show "linearform F f" by fact
   103       show "F \<unlhd> E" by fact
   104       from F show "F \<unlhd> F" by (rule vectorspace.subspace_refl)
   105       show "graph F f \<subseteq> graph F f" ..
   106       show "\<forall>x\<in>F. f x \<le> p x" by fact
   107     qed
   108   qed
   109   then obtain g where gM: "g \<in> M" and gx: "\<forall>x \<in> M. g \<subseteq> x \<longrightarrow> g = x"
   110     by blast
   111   from gM obtain H h where
   112       g_rep: "g = graph H h"
   113     and linearform: "linearform H h"
   114     and HE: "H \<unlhd> E" and FH: "F \<unlhd> H"
   115     and graphs: "graph F f \<subseteq> graph H h"
   116     and hp: "\<forall>x \<in> H. h x \<le> p x" unfolding M_def ..
   117       \<comment> \<open>\<open>g\<close> is a norm-preserving extension of \<open>f\<close>, in other words:\<close>
   118       \<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>
   119       \<comment> \<open>and \<open>h\<close> is an extension of \<open>f\<close> that is again bounded by \<open>p\<close>. \<^smallskip>\<close>
   120   from HE E have H: "vectorspace H"
   121     by (rule subspace.vectorspace)
   122 
   123   have HE_eq: "H = E"
   124     \<comment> \<open>We show that \<open>h\<close> is defined on whole \<open>E\<close> by classical contradiction. \<^smallskip>\<close>
   125   proof (rule classical)
   126     assume neq: "H \<noteq> E"
   127       \<comment> \<open>Assume \<open>h\<close> is not defined on whole \<open>E\<close>. Then show that \<open>h\<close> can be extended\<close>
   128       \<comment> \<open>in a norm-preserving way to a function \<open>h'\<close> with the graph \<open>g'\<close>. \<^smallskip>\<close>
   129     have "\<exists>g' \<in> M. g \<subseteq> g' \<and> g \<noteq> g'"
   130     proof -
   131       from HE have "H \<subseteq> E" ..
   132       with neq obtain x' where x'E: "x' \<in> E" and "x' \<notin> H" by blast
   133       obtain x': "x' \<noteq> 0"
   134       proof
   135         show "x' \<noteq> 0"
   136         proof
   137           assume "x' = 0"
   138           with H have "x' \<in> H" by (simp only: vectorspace.zero)
   139           with \<open>x' \<notin> H\<close> show False by contradiction
   140         qed
   141       qed
   142 
   143       define H' where "H' = H + lin x'"
   144         \<comment> \<open>Define \<open>H'\<close> as the direct sum of \<open>H\<close> and the linear closure of \<open>x'\<close>. \<^smallskip>\<close>
   145       have HH': "H \<unlhd> H'"
   146       proof (unfold H'_def)
   147         from x'E have "vectorspace (lin x')" ..
   148         with H show "H \<unlhd> H + lin x'" ..
   149       qed
   150 
   151       obtain xi where
   152         xi: "\<forall>y \<in> H. - p (y + x') - h y \<le> xi
   153           \<and> xi \<le> p (y + x') - h y"
   154         \<comment> \<open>Pick a real number \<open>\<xi>\<close> that fulfills certain inequality; this will\<close>
   155         \<comment> \<open>be used to establish that \<open>h'\<close> is a norm-preserving extension of \<open>h\<close>.
   156            \label{ex-xi-use}\<^smallskip>\<close>
   157       proof -
   158         from H have "\<exists>xi. \<forall>y \<in> H. - p (y + x') - h y \<le> xi
   159             \<and> xi \<le> p (y + x') - h y"
   160         proof (rule ex_xi)
   161           fix u v assume u: "u \<in> H" and v: "v \<in> H"
   162           with HE have uE: "u \<in> E" and vE: "v \<in> E" by auto
   163           from H u v linearform have "h v - h u = h (v - u)"
   164             by (simp add: linearform.diff)
   165           also from hp and H u v have "\<dots> \<le> p (v - u)"
   166             by (simp only: vectorspace.diff_closed)
   167           also from x'E uE vE have "v - u = x' + - x' + v + - u"
   168             by (simp add: diff_eq1)
   169           also from x'E uE vE have "\<dots> = v + x' + - (u + x')"
   170             by (simp add: add_ac)
   171           also from x'E uE vE have "\<dots> = (v + x') - (u + x')"
   172             by (simp add: diff_eq1)
   173           also from x'E uE vE E have "p \<dots> \<le> p (v + x') + p (u + x')"
   174             by (simp add: diff_subadditive)
   175           finally have "h v - h u \<le> p (v + x') + p (u + x')" .
   176           then show "- p (u + x') - h u \<le> p (v + x') - h v" by simp
   177         qed
   178         then show thesis by (blast intro: that)
   179       qed
   180 
   181       define h' where "h' x = (let (y, a) =
   182           SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H in h y + a * xi)" for x
   183         \<comment> \<open>Define the extension \<open>h'\<close> of \<open>h\<close> to \<open>H'\<close> using \<open>\<xi>\<close>. \<^smallskip>\<close>
   184 
   185       have "g \<subseteq> graph H' h' \<and> g \<noteq> graph H' h'"
   186         \<comment> \<open>\<open>h'\<close> is an extension of \<open>h\<close> \dots \<^smallskip>\<close>
   187       proof
   188         show "g \<subseteq> graph H' h'"
   189         proof -
   190           have "graph H h \<subseteq> graph H' h'"
   191           proof (rule graph_extI)
   192             fix t assume t: "t \<in> H"
   193             from E HE t have "(SOME (y, a). t = y + a \<cdot> x' \<and> y \<in> H) = (t, 0)"
   194               using \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> by (rule decomp_H'_H)
   195             with h'_def show "h t = h' t" by (simp add: Let_def)
   196           next
   197             from HH' show "H \<subseteq> H'" ..
   198           qed
   199           with g_rep show ?thesis by (simp only:)
   200         qed
   201 
   202         show "g \<noteq> graph H' h'"
   203         proof -
   204           have "graph H h \<noteq> graph H' h'"
   205           proof
   206             assume eq: "graph H h = graph H' h'"
   207             have "x' \<in> H'"
   208               unfolding H'_def
   209             proof
   210               from H show "0 \<in> H" by (rule vectorspace.zero)
   211               from x'E show "x' \<in> lin x'" by (rule x_lin_x)
   212               from x'E show "x' = 0 + x'" by simp
   213             qed
   214             then have "(x', h' x') \<in> graph H' h'" ..
   215             with eq have "(x', h' x') \<in> graph H h" by (simp only:)
   216             then have "x' \<in> H" ..
   217             with \<open>x' \<notin> H\<close> show False by contradiction
   218           qed
   219           with g_rep show ?thesis by simp
   220         qed
   221       qed
   222       moreover have "graph H' h' \<in> M"
   223         \<comment> \<open>and \<open>h'\<close> is norm-preserving. \<^smallskip>\<close>
   224       proof (unfold M_def)
   225         show "graph H' h' \<in> norm_pres_extensions E p F f"
   226         proof (rule norm_pres_extensionI2)
   227           show "linearform H' h'"
   228             using h'_def H'_def HE linearform \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> E
   229             by (rule h'_lf)
   230           show "H' \<unlhd> E"
   231           unfolding H'_def
   232           proof
   233             show "H \<unlhd> E" by fact
   234             show "vectorspace E" by fact
   235             from x'E show "lin x' \<unlhd> E" ..
   236           qed
   237           from H \<open>F \<unlhd> H\<close> HH' show FH': "F \<unlhd> H'"
   238             by (rule vectorspace.subspace_trans)
   239           show "graph F f \<subseteq> graph H' h'"
   240           proof (rule graph_extI)
   241             fix x assume x: "x \<in> F"
   242             with graphs have "f x = h x" ..
   243             also have "\<dots> = h x + 0 * xi" by simp
   244             also have "\<dots> = (let (y, a) = (x, 0) in h y + a * xi)"
   245               by (simp add: Let_def)
   246             also have "(x, 0) =
   247                 (SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H)"
   248               using E HE
   249             proof (rule decomp_H'_H [symmetric])
   250               from FH x show "x \<in> H" ..
   251               from x' show "x' \<noteq> 0" .
   252               show "x' \<notin> H" by fact
   253               show "x' \<in> E" by fact
   254             qed
   255             also have
   256               "(let (y, a) = (SOME (y, a). x = y + a \<cdot> x' \<and> y \<in> H)
   257               in h y + a * xi) = h' x" by (simp only: h'_def)
   258             finally show "f x = h' x" .
   259           next
   260             from FH' show "F \<subseteq> H'" ..
   261           qed
   262           show "\<forall>x \<in> H'. h' x \<le> p x"
   263             using h'_def H'_def \<open>x' \<notin> H\<close> \<open>x' \<in> E\<close> \<open>x' \<noteq> 0\<close> E HE
   264               \<open>seminorm E p\<close> linearform and hp xi
   265             by (rule h'_norm_pres)
   266         qed
   267       qed
   268       ultimately show ?thesis ..
   269     qed
   270     then have "\<not> (\<forall>x \<in> M. g \<subseteq> x \<longrightarrow> g = x)" by simp
   271       \<comment> \<open>So the graph \<open>g\<close> of \<open>h\<close> cannot be maximal. Contradiction! \<^smallskip>\<close>
   272     with gx show "H = E" by contradiction
   273   qed
   274 
   275   from HE_eq and linearform have "linearform E h"
   276     by (simp only:)
   277   moreover have "\<forall>x \<in> F. h x = f x"
   278   proof
   279     fix x assume "x \<in> F"
   280     with graphs have "f x = h x" ..
   281     then show "h x = f x" ..
   282   qed
   283   moreover from HE_eq and hp have "\<forall>x \<in> E. h x \<le> p x"
   284     by (simp only:)
   285   ultimately show ?thesis by blast
   286 qed
   287 
   288 
   289 subsection \<open>Alternative formulation\<close>
   290 
   291 text \<open>
   292   The following alternative formulation of the Hahn-Banach
   293   Theorem\label{abs-Hahn-Banach} uses the fact that for a real linear form \<open>f\<close>
   294   and a seminorm \<open>p\<close> the following inequality are equivalent:\footnote{This
   295   was shown in lemma @{thm [source] abs_ineq_iff} (see page
   296   \pageref{abs-ineq-iff}).}
   297   \begin{center}
   298   \begin{tabular}{lll}
   299   \<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> \\
   300   \end{tabular}
   301   \end{center}
   302 \<close>
   303 
   304 theorem abs_Hahn_Banach:
   305   assumes E: "vectorspace E" and FE: "subspace F E"
   306     and lf: "linearform F f" and sn: "seminorm E p"
   307   assumes fp: "\<forall>x \<in> F. \<bar>f x\<bar> \<le> p x"
   308   shows "\<exists>g. linearform E g
   309     \<and> (\<forall>x \<in> F. g x = f x)
   310     \<and> (\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x)"
   311 proof -
   312   interpret vectorspace E by fact
   313   interpret subspace F E by fact
   314   interpret linearform F f by fact
   315   interpret seminorm E p by fact
   316   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)"
   317     using E FE sn lf
   318   proof (rule Hahn_Banach)
   319     show "\<forall>x \<in> F. f x \<le> p x"
   320       using FE E sn lf and fp by (rule abs_ineq_iff [THEN iffD1])
   321   qed
   322   then obtain g where lg: "linearform E g" and *: "\<forall>x \<in> F. g x = f x"
   323       and **: "\<forall>x \<in> E. g x \<le> p x" by blast
   324   have "\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x"
   325     using _ E sn lg **
   326   proof (rule abs_ineq_iff [THEN iffD2])
   327     show "E \<unlhd> E" ..
   328   qed
   329   with lg * show ?thesis by blast
   330 qed
   331 
   332 
   333 subsection \<open>The Hahn-Banach Theorem for normed spaces\<close>
   334 
   335 text \<open>
   336   Every continuous linear form \<open>f\<close> on a subspace \<open>F\<close> of a norm space \<open>E\<close>, can
   337   be extended to a continuous linear form \<open>g\<close> on \<open>E\<close> such that \<open>\<parallel>f\<parallel> = \<parallel>g\<parallel>\<close>.
   338 \<close>
   339 
   340 theorem norm_Hahn_Banach:
   341   fixes V and norm ("\<parallel>_\<parallel>")
   342   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}"
   343   fixes fn_norm ("\<parallel>_\<parallel>\<hyphen>_" [0, 1000] 999)
   344   defines "\<And>V f. \<parallel>f\<parallel>\<hyphen>V \<equiv> \<Squnion>(B V f)"
   345   assumes E_norm: "normed_vectorspace E norm" and FE: "subspace F E"
   346     and linearform: "linearform F f" and "continuous F f norm"
   347   shows "\<exists>g. linearform E g
   348      \<and> continuous E g norm
   349      \<and> (\<forall>x \<in> F. g x = f x)
   350      \<and> \<parallel>g\<parallel>\<hyphen>E = \<parallel>f\<parallel>\<hyphen>F"
   351 proof -
   352   interpret normed_vectorspace E norm by fact
   353   interpret normed_vectorspace_with_fn_norm E norm B fn_norm
   354     by (auto simp: B_def fn_norm_def) intro_locales
   355   interpret subspace F E by fact
   356   interpret linearform F f by fact
   357   interpret continuous F f norm by fact
   358   have E: "vectorspace E" by intro_locales
   359   have F: "vectorspace F" by rule intro_locales
   360   have F_norm: "normed_vectorspace F norm"
   361     using FE E_norm by (rule subspace_normed_vs)
   362   have ge_zero: "0 \<le> \<parallel>f\<parallel>\<hyphen>F"
   363     by (rule normed_vectorspace_with_fn_norm.fn_norm_ge_zero
   364       [OF normed_vectorspace_with_fn_norm.intro,
   365        OF F_norm \<open>continuous F f norm\<close> , folded B_def fn_norm_def])
   366   txt \<open>We define a function \<open>p\<close> on \<open>E\<close> as follows:
   367     \<open>p x = \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>\<close>\<close>
   368   define p where "p x = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>" for x
   369 
   370   txt \<open>\<open>p\<close> is a seminorm on \<open>E\<close>:\<close>
   371   have q: "seminorm E p"
   372   proof
   373     fix x y a assume x: "x \<in> E" and y: "y \<in> E"
   374     
   375     txt \<open>\<open>p\<close> is positive definite:\<close>
   376     have "0 \<le> \<parallel>f\<parallel>\<hyphen>F" by (rule ge_zero)
   377     moreover from x have "0 \<le> \<parallel>x\<parallel>" ..
   378     ultimately show "0 \<le> p x"  
   379       by (simp add: p_def zero_le_mult_iff)
   380 
   381     txt \<open>\<open>p\<close> is absolutely homogeneous:\<close>
   382 
   383     show "p (a \<cdot> x) = \<bar>a\<bar> * p x"
   384     proof -
   385       have "p (a \<cdot> x) = \<parallel>f\<parallel>\<hyphen>F * \<parallel>a \<cdot> x\<parallel>" by (simp only: p_def)
   386       also from x have "\<parallel>a \<cdot> x\<parallel> = \<bar>a\<bar> * \<parallel>x\<parallel>" by (rule abs_homogenous)
   387       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
   388       also have "\<dots> = \<bar>a\<bar> * p x" by (simp only: p_def)
   389       finally show ?thesis .
   390     qed
   391 
   392     txt \<open>Furthermore, \<open>p\<close> is subadditive:\<close>
   393 
   394     show "p (x + y) \<le> p x + p y"
   395     proof -
   396       have "p (x + y) = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x + y\<parallel>" by (simp only: p_def)
   397       also have a: "0 \<le> \<parallel>f\<parallel>\<hyphen>F" by (rule ge_zero)
   398       from x y have "\<parallel>x + y\<parallel> \<le> \<parallel>x\<parallel> + \<parallel>y\<parallel>" ..
   399       with a have " \<parallel>f\<parallel>\<hyphen>F * \<parallel>x + y\<parallel> \<le> \<parallel>f\<parallel>\<hyphen>F * (\<parallel>x\<parallel> + \<parallel>y\<parallel>)"
   400         by (simp add: mult_left_mono)
   401       also have "\<dots> = \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel> + \<parallel>f\<parallel>\<hyphen>F * \<parallel>y\<parallel>" by (simp only: distrib_left)
   402       also have "\<dots> = p x + p y" by (simp only: p_def)
   403       finally show ?thesis .
   404     qed
   405   qed
   406 
   407   txt \<open>\<open>f\<close> is bounded by \<open>p\<close>.\<close>
   408 
   409   have "\<forall>x \<in> F. \<bar>f x\<bar> \<le> p x"
   410   proof
   411     fix x assume "x \<in> F"
   412     with \<open>continuous F f norm\<close> and linearform
   413     show "\<bar>f x\<bar> \<le> p x"
   414       unfolding p_def by (rule normed_vectorspace_with_fn_norm.fn_norm_le_cong
   415         [OF normed_vectorspace_with_fn_norm.intro,
   416          OF F_norm, folded B_def fn_norm_def])
   417   qed
   418 
   419   txt \<open>Using the fact that \<open>p\<close> is a seminorm and \<open>f\<close> is bounded by \<open>p\<close> we can
   420     apply the Hahn-Banach Theorem for real vector spaces. So \<open>f\<close> can be
   421     extended in a norm-preserving way to some function \<open>g\<close> on the whole vector
   422     space \<open>E\<close>.\<close>
   423 
   424   with E FE linearform q obtain g where
   425       linearformE: "linearform E g"
   426     and a: "\<forall>x \<in> F. g x = f x"
   427     and b: "\<forall>x \<in> E. \<bar>g x\<bar> \<le> p x"
   428     by (rule abs_Hahn_Banach [elim_format]) iprover
   429 
   430   txt \<open>We furthermore have to show that \<open>g\<close> is also continuous:\<close>
   431 
   432   have g_cont: "continuous E g norm" using linearformE
   433   proof
   434     fix x assume "x \<in> E"
   435     with b show "\<bar>g x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
   436       by (simp only: p_def)
   437   qed
   438 
   439   txt \<open>To complete the proof, we show that \<open>\<parallel>g\<parallel> = \<parallel>f\<parallel>\<close>.\<close>
   440 
   441   have "\<parallel>g\<parallel>\<hyphen>E = \<parallel>f\<parallel>\<hyphen>F"
   442   proof (rule order_antisym)
   443     txt \<open>
   444       First we show \<open>\<parallel>g\<parallel> \<le> \<parallel>f\<parallel>\<close>. The function norm \<open>\<parallel>g\<parallel>\<close> is defined as the
   445       smallest \<open>c \<in> \<real>\<close> such that
   446       \begin{center}
   447       \begin{tabular}{l}
   448       \<open>\<forall>x \<in> E. \<bar>g x\<bar> \<le> c \<cdot> \<parallel>x\<parallel>\<close>
   449       \end{tabular}
   450       \end{center}
   451       \<^noindent> Furthermore holds
   452       \begin{center}
   453       \begin{tabular}{l}
   454       \<open>\<forall>x \<in> E. \<bar>g x\<bar> \<le> \<parallel>f\<parallel> \<cdot> \<parallel>x\<parallel>\<close>
   455       \end{tabular}
   456       \end{center}
   457     \<close>
   458 
   459     from g_cont _ ge_zero
   460     show "\<parallel>g\<parallel>\<hyphen>E \<le> \<parallel>f\<parallel>\<hyphen>F"
   461     proof
   462       fix x assume "x \<in> E"
   463       with b show "\<bar>g x\<bar> \<le> \<parallel>f\<parallel>\<hyphen>F * \<parallel>x\<parallel>"
   464         by (simp only: p_def)
   465     qed
   466 
   467     txt \<open>The other direction is achieved by a similar argument.\<close>
   468 
   469     show "\<parallel>f\<parallel>\<hyphen>F \<le> \<parallel>g\<parallel>\<hyphen>E"
   470     proof (rule normed_vectorspace_with_fn_norm.fn_norm_least
   471         [OF normed_vectorspace_with_fn_norm.intro,
   472          OF F_norm, folded B_def fn_norm_def])
   473       fix x assume x: "x \<in> F"
   474       show "\<bar>f x\<bar> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>"
   475       proof -
   476         from a x have "g x = f x" ..
   477         then have "\<bar>f x\<bar> = \<bar>g x\<bar>" by (simp only:)
   478         also from g_cont have "\<dots> \<le> \<parallel>g\<parallel>\<hyphen>E * \<parallel>x\<parallel>"
   479         proof (rule fn_norm_le_cong [OF _ linearformE, folded B_def fn_norm_def])
   480           from FE x show "x \<in> E" ..
   481         qed
   482         finally show ?thesis .
   483       qed
   484     next
   485       show "0 \<le> \<parallel>g\<parallel>\<hyphen>E"
   486         using g_cont by (rule fn_norm_ge_zero [of g, folded B_def fn_norm_def])
   487       show "continuous F f norm" by fact
   488     qed
   489   qed
   490   with linearformE a g_cont show ?thesis by blast
   491 qed
   492 
   493 end