src/HOL/Hahn_Banach/Hahn_Banach_Ext_Lemmas.thy
author wenzelm
Mon Oct 19 17:45:36 2015 +0200 (2015-10-19)
changeset 61486 3590367b0ce9
parent 60458 0d10ae17e3ee
child 61539 a29295dac1ca
permissions -rw-r--r--
tuned document;
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(*  Title:      HOL/Hahn_Banach/Hahn_Banach_Ext_Lemmas.thy
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    Author:     Gertrud Bauer, TU Munich
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*)
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section \<open>Extending non-maximal functions\<close>
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theory Hahn_Banach_Ext_Lemmas
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imports Function_Norm
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begin
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text \<open>
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  In this section the following context is presumed.  Let @{text E} be
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  a real vector space with a seminorm @{text q} on @{text E}. @{text
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  F} is a subspace of @{text E} and @{text f} a linear function on
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  @{text F}. We consider a subspace @{text H} of @{text E} that is a
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  superspace of @{text F} and a linear form @{text h} on @{text
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  H}. @{text H} is a not equal to @{text E} and @{text "x\<^sub>0"} is
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  an element in @{text "E - H"}.  @{text H} is extended to the direct
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  sum @{text "H' = H + lin x\<^sub>0"}, so for any @{text "x \<in> H'"}
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  the decomposition of @{text "x = y + a \<cdot> x"} with @{text "y \<in> H"} is
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  unique. @{text h'} is defined on @{text H'} by @{text "h' x = h y +
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  a \<cdot> \<xi>"} for a certain @{text \<xi>}.
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  Subsequently we show some properties of this extension @{text h'} of
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  @{text h}.
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  \<^medskip>
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  This lemma will be used to show the existence of a linear
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  extension of @{text f} (see page \pageref{ex-xi-use}). It is a
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  consequence of the completeness of @{text \<real>}. To show
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  \begin{center}
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  \begin{tabular}{l}
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  @{text "\<exists>\<xi>. \<forall>y \<in> F. a y \<le> \<xi> \<and> \<xi> \<le> b y"}
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  \end{tabular}
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  \end{center}
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  \noindent it suffices to show that
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  \begin{center}
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  \begin{tabular}{l}
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  @{text "\<forall>u \<in> F. \<forall>v \<in> F. a u \<le> b v"}
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  \end{tabular}
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  \end{center}
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\<close>
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lemma ex_xi:
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  assumes "vectorspace F"
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  assumes r: "\<And>u v. u \<in> F \<Longrightarrow> v \<in> F \<Longrightarrow> a u \<le> b v"
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  shows "\<exists>xi::real. \<forall>y \<in> F. a y \<le> xi \<and> xi \<le> b y"
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proof -
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  interpret vectorspace F by fact
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  txt \<open>From the completeness of the reals follows:
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    The set @{text "S = {a u. u \<in> F}"} has a supremum, if it is
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    non-empty and has an upper bound.\<close>
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  let ?S = "{a u | u. u \<in> F}"
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  have "\<exists>xi. lub ?S xi"
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  proof (rule real_complete)
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    have "a 0 \<in> ?S" by blast
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    then show "\<exists>X. X \<in> ?S" ..
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    have "\<forall>y \<in> ?S. y \<le> b 0"
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    proof
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      fix y assume y: "y \<in> ?S"
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      then obtain u where u: "u \<in> F" and y: "y = a u" by blast
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      from u and zero have "a u \<le> b 0" by (rule r)
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      with y show "y \<le> b 0" by (simp only:)
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    qed
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    then show "\<exists>u. \<forall>y \<in> ?S. y \<le> u" ..
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  qed
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  then obtain xi where xi: "lub ?S xi" ..
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  {
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    fix y assume "y \<in> F"
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    then have "a y \<in> ?S" by blast
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    with xi have "a y \<le> xi" by (rule lub.upper)
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  }
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  moreover {
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    fix y assume y: "y \<in> F"
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    from xi have "xi \<le> b y"
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    proof (rule lub.least)
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      fix au assume "au \<in> ?S"
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      then obtain u where u: "u \<in> F" and au: "au = a u" by blast
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      from u y have "a u \<le> b y" by (rule r)
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      with au show "au \<le> b y" by (simp only:)
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    qed
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  }
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  ultimately show "\<exists>xi. \<forall>y \<in> F. a y \<le> xi \<and> xi \<le> b y" by blast
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qed
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text \<open>
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  \<^medskip>
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  The function @{text h'} is defined as a @{text "h' x = h y
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  + a \<cdot> \<xi>"} where @{text "x = y + a \<cdot> \<xi>"} is a linear extension of
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  @{text h} to @{text H'}.
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\<close>
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lemma h'_lf:
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  assumes h'_def: "h' \<equiv> \<lambda>x. let (y, a) =
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      SOME (y, a). x = y + a \<cdot> x0 \<and> y \<in> H in h y + a * xi"
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    and H'_def: "H' \<equiv> H + lin x0"
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    and HE: "H \<unlhd> E"
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  assumes "linearform H h"
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  assumes x0: "x0 \<notin> H"  "x0 \<in> E"  "x0 \<noteq> 0"
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  assumes E: "vectorspace E"
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  shows "linearform H' h'"
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proof -
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  interpret linearform H h by fact
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  interpret vectorspace E by fact
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  show ?thesis
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  proof
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    note E = \<open>vectorspace E\<close>
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    have H': "vectorspace H'"
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    proof (unfold H'_def)
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      from \<open>x0 \<in> E\<close>
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      have "lin x0 \<unlhd> E" ..
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      with HE show "vectorspace (H + lin x0)" using E ..
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    qed
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    {
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      fix x1 x2 assume x1: "x1 \<in> H'" and x2: "x2 \<in> H'"
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      show "h' (x1 + x2) = h' x1 + h' x2"
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      proof -
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        from H' x1 x2 have "x1 + x2 \<in> H'"
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          by (rule vectorspace.add_closed)
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        with x1 x2 obtain y y1 y2 a a1 a2 where
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          x1x2: "x1 + x2 = y + a \<cdot> x0" and y: "y \<in> H"
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          and x1_rep: "x1 = y1 + a1 \<cdot> x0" and y1: "y1 \<in> H"
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          and x2_rep: "x2 = y2 + a2 \<cdot> x0" and y2: "y2 \<in> H"
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          unfolding H'_def sum_def lin_def by blast
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        have ya: "y1 + y2 = y \<and> a1 + a2 = a" using E HE _ y x0
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        proof (rule decomp_H') text_raw \<open>\label{decomp-H-use}\<close>
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          from HE y1 y2 show "y1 + y2 \<in> H"
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            by (rule subspace.add_closed)
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          from x0 and HE y y1 y2
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          have "x0 \<in> E"  "y \<in> E"  "y1 \<in> E"  "y2 \<in> E" by auto
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          with x1_rep x2_rep have "(y1 + y2) + (a1 + a2) \<cdot> x0 = x1 + x2"
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            by (simp add: add_ac add_mult_distrib2)
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          also note x1x2
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          finally show "(y1 + y2) + (a1 + a2) \<cdot> x0 = y + a \<cdot> x0" .
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        qed
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        from h'_def x1x2 E HE y x0
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        have "h' (x1 + x2) = h y + a * xi"
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          by (rule h'_definite)
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        also have "\<dots> = h (y1 + y2) + (a1 + a2) * xi"
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          by (simp only: ya)
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        also from y1 y2 have "h (y1 + y2) = h y1 + h y2"
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          by simp
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        also have "\<dots> + (a1 + a2) * xi = (h y1 + a1 * xi) + (h y2 + a2 * xi)"
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          by (simp add: distrib_right)
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        also from h'_def x1_rep E HE y1 x0
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        have "h y1 + a1 * xi = h' x1"
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          by (rule h'_definite [symmetric])
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        also from h'_def x2_rep E HE y2 x0
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        have "h y2 + a2 * xi = h' x2"
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          by (rule h'_definite [symmetric])
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        finally show ?thesis .
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      qed
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    next
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      fix x1 c assume x1: "x1 \<in> H'"
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      show "h' (c \<cdot> x1) = c * (h' x1)"
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      proof -
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        from H' x1 have ax1: "c \<cdot> x1 \<in> H'"
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          by (rule vectorspace.mult_closed)
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        with x1 obtain y a y1 a1 where
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            cx1_rep: "c \<cdot> x1 = y + a \<cdot> x0" and y: "y \<in> H"
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          and x1_rep: "x1 = y1 + a1 \<cdot> x0" and y1: "y1 \<in> H"
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          unfolding H'_def sum_def lin_def by blast
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        have ya: "c \<cdot> y1 = y \<and> c * a1 = a" using E HE _ y x0
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        proof (rule decomp_H')
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          from HE y1 show "c \<cdot> y1 \<in> H"
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            by (rule subspace.mult_closed)
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          from x0 and HE y y1
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          have "x0 \<in> E"  "y \<in> E"  "y1 \<in> E" by auto
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          with x1_rep have "c \<cdot> y1 + (c * a1) \<cdot> x0 = c \<cdot> x1"
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            by (simp add: mult_assoc add_mult_distrib1)
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          also note cx1_rep
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          finally show "c \<cdot> y1 + (c * a1) \<cdot> x0 = y + a \<cdot> x0" .
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        qed
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        from h'_def cx1_rep E HE y x0 have "h' (c \<cdot> x1) = h y + a * xi"
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          by (rule h'_definite)
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        also have "\<dots> = h (c \<cdot> y1) + (c * a1) * xi"
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          by (simp only: ya)
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        also from y1 have "h (c \<cdot> y1) = c * h y1"
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          by simp
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        also have "\<dots> + (c * a1) * xi = c * (h y1 + a1 * xi)"
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          by (simp only: distrib_left)
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        also from h'_def x1_rep E HE y1 x0 have "h y1 + a1 * xi = h' x1"
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          by (rule h'_definite [symmetric])
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        finally show ?thesis .
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      qed
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    }
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  qed
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qed
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text \<open>
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  \<^medskip>
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  The linear extension @{text h'} of @{text h}
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  is bounded by the seminorm @{text p}.\<close>
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lemma h'_norm_pres:
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  assumes h'_def: "h' \<equiv> \<lambda>x. let (y, a) =
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      SOME (y, a). x = y + a \<cdot> x0 \<and> y \<in> H in h y + a * xi"
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    and H'_def: "H' \<equiv> H + lin x0"
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    and x0: "x0 \<notin> H"  "x0 \<in> E"  "x0 \<noteq> 0"
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  assumes E: "vectorspace E" and HE: "subspace H E"
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    and "seminorm E p" and "linearform H h"
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  assumes a: "\<forall>y \<in> H. h y \<le> p y"
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    and a': "\<forall>y \<in> H. - p (y + x0) - h y \<le> xi \<and> xi \<le> p (y + x0) - h y"
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  shows "\<forall>x \<in> H'. h' x \<le> p x"
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proof -
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  interpret vectorspace E by fact
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  interpret subspace H E by fact
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  interpret seminorm E p by fact
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  interpret linearform H h by fact
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  show ?thesis
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  proof
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    fix x assume x': "x \<in> H'"
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    show "h' x \<le> p x"
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    proof -
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      from a' have a1: "\<forall>ya \<in> H. - p (ya + x0) - h ya \<le> xi"
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        and a2: "\<forall>ya \<in> H. xi \<le> p (ya + x0) - h ya" by auto
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      from x' obtain y a where
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          x_rep: "x = y + a \<cdot> x0" and y: "y \<in> H"
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        unfolding H'_def sum_def lin_def by blast
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      from y have y': "y \<in> E" ..
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      from y have ay: "inverse a \<cdot> y \<in> H" by simp
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      from h'_def x_rep E HE y x0 have "h' x = h y + a * xi"
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        by (rule h'_definite)
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      also have "\<dots> \<le> p (y + a \<cdot> x0)"
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      proof (rule linorder_cases)
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        assume z: "a = 0"
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        then have "h y + a * xi = h y" by simp
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        also from a y have "\<dots> \<le> p y" ..
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        also from x0 y' z have "p y = p (y + a \<cdot> x0)" by simp
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        finally show ?thesis .
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      next
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        txt \<open>In the case @{text "a < 0"}, we use @{text "a\<^sub>1"}
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          with @{text ya} taken as @{text "y / a"}:\<close>
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        assume lz: "a < 0" then have nz: "a \<noteq> 0" by simp
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        from a1 ay
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        have "- p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y) \<le> xi" ..
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        with lz have "a * xi \<le>
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          a * (- p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y))"
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          by (simp add: mult_left_mono_neg order_less_imp_le)
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        also have "\<dots> =
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          - a * (p (inverse a \<cdot> y + x0)) - a * (h (inverse a \<cdot> y))"
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          by (simp add: right_diff_distrib)
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        also from lz x0 y' have "- a * (p (inverse a \<cdot> y + x0)) =
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          p (a \<cdot> (inverse a \<cdot> y + x0))"
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          by (simp add: abs_homogenous)
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        also from nz x0 y' have "\<dots> = p (y + a \<cdot> x0)"
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          by (simp add: add_mult_distrib1 mult_assoc [symmetric])
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        also from nz y have "a * (h (inverse a \<cdot> y)) =  h y"
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          by simp
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        finally have "a * xi \<le> p (y + a \<cdot> x0) - h y" .
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        then show ?thesis by simp
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      next
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        txt \<open>In the case @{text "a > 0"}, we use @{text "a\<^sub>2"}
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          with @{text ya} taken as @{text "y / a"}:\<close>
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        assume gz: "0 < a" then have nz: "a \<noteq> 0" by simp
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        from a2 ay
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        have "xi \<le> p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y)" ..
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        with gz have "a * xi \<le>
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          a * (p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y))"
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          by simp
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        also have "\<dots> = a * p (inverse a \<cdot> y + x0) - a * h (inverse a \<cdot> y)"
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          by (simp add: right_diff_distrib)
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        also from gz x0 y'
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        have "a * p (inverse a \<cdot> y + x0) = p (a \<cdot> (inverse a \<cdot> y + x0))"
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          by (simp add: abs_homogenous)
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        also from nz x0 y' have "\<dots> = p (y + a \<cdot> x0)"
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          by (simp add: add_mult_distrib1 mult_assoc [symmetric])
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        also from nz y have "a * h (inverse a \<cdot> y) = h y"
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          by simp
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        finally have "a * xi \<le> p (y + a \<cdot> x0) - h y" .
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        then show ?thesis by simp
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      qed
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      also from x_rep have "\<dots> = p x" by (simp only:)
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   281
      finally show ?thesis .
wenzelm@10007
   282
    qed
wenzelm@10007
   283
  qed
wenzelm@13515
   284
qed
wenzelm@7917
   285
wenzelm@10007
   286
end