(* Title: HOL/Real/HahnBanach/HahnBanachExtLemmas.thy
ID: $Id$
Author: Gertrud Bauer, TU Munich
*)
header {* Extending non-maximal functions *}
theory HahnBanachExtLemmas = FunctionNorm:
text {*
In this section the following context is presumed. Let @{text E} be
a real vector space with a seminorm @{text q} on @{text E}. @{text
F} is a subspace of @{text E} and @{text f} a linear function on
@{text F}. We consider a subspace @{text H} of @{text E} that is a
superspace of @{text F} and a linear form @{text h} on @{text
H}. @{text H} is a not equal to @{text E} and @{text "x\<^sub>0"} is
an element in @{text "E - H"}. @{text H} is extended to the direct
sum @{text "H' = H + lin x\<^sub>0"}, so for any @{text "x \<in> H'"}
the decomposition of @{text "x = y + a \<cdot> x"} with @{text "y \<in> H"} is
unique. @{text h'} is defined on @{text H'} by @{text "h' x = h y +
a \<cdot> \<xi>"} for a certain @{text \<xi>}.
Subsequently we show some properties of this extension @{text h'} of
@{text h}.
\medskip This lemma will be used to show the existence of a linear
extension of @{text f} (see page \pageref{ex-xi-use}). It is a
consequence of the completeness of @{text \<real>}. To show
\begin{center}
\begin{tabular}{l}
@{text "\<exists>\<xi>. \<forall>y \<in> F. a y \<le> \<xi> \<and> \<xi> \<le> b y"}
\end{tabular}
\end{center}
\noindent it suffices to show that
\begin{center}
\begin{tabular}{l}
@{text "\<forall>u \<in> F. \<forall>v \<in> F. a u \<le> b v"}
\end{tabular}
\end{center}
*}
lemma ex_xi:
includes vectorspace F
assumes r: "\<And>u v. u \<in> F \<Longrightarrow> v \<in> F \<Longrightarrow> a u \<le> b v"
shows "\<exists>xi::real. \<forall>y \<in> F. a y \<le> xi \<and> xi \<le> b y"
proof -
txt {* From the completeness of the reals follows:
The set @{text "S = {a u. u \<in> F}"} has a supremum, if it is
non-empty and has an upper bound. *}
let ?S = "{a u | u. u \<in> F}"
have "\<exists>xi. lub ?S xi"
proof (rule real_complete)
have "a 0 \<in> ?S" by blast
then show "\<exists>X. X \<in> ?S" ..
have "\<forall>y \<in> ?S. y \<le> b 0"
proof
fix y assume y: "y \<in> ?S"
then obtain u where u: "u \<in> F" and y: "y = a u" by blast
from u and zero have "a u \<le> b 0" by (rule r)
with y show "y \<le> b 0" by (simp only:)
qed
then show "\<exists>u. \<forall>y \<in> ?S. y \<le> u" ..
qed
then obtain xi where xi: "lub ?S xi" ..
{
fix y assume "y \<in> F"
then have "a y \<in> ?S" by blast
with xi have "a y \<le> xi" by (rule lub.upper)
} moreover {
fix y assume y: "y \<in> F"
from xi have "xi \<le> b y"
proof (rule lub.least)
fix au assume "au \<in> ?S"
then obtain u where u: "u \<in> F" and au: "au = a u" by blast
from u y have "a u \<le> b y" by (rule r)
with au show "au \<le> b y" by (simp only:)
qed
} ultimately show "\<exists>xi. \<forall>y \<in> F. a y \<le> xi \<and> xi \<le> b y" by blast
qed
text {*
\medskip The function @{text h'} is defined as a @{text "h' x = h y
+ a \<cdot> \<xi>"} where @{text "x = y + a \<cdot> \<xi>"} is a linear extension of
@{text h} to @{text H'}.
*}
lemma h'_lf:
includes var H + var h + var E
assumes h'_def: "h' \<equiv> \<lambda>x. let (y, a) =
SOME (y, a). x = y + a \<cdot> x0 \<and> y \<in> H in h y + a * xi"
and H'_def: "H' \<equiv> H + lin x0"
and HE: "H \<unlhd> E"
includes linearform H h
assumes x0: "x0 \<notin> H" "x0 \<in> E" "x0 \<noteq> 0"
includes vectorspace E
shows "linearform H' h'"
proof
have H': "vectorspace H'"
proof (unfold H'_def)
have "x0 \<in> E" .
then have "lin x0 \<unlhd> E" ..
with HE show "vectorspace (H + lin x0)" ..
qed
{
fix x1 x2 assume x1: "x1 \<in> H'" and x2: "x2 \<in> H'"
show "h' (x1 + x2) = h' x1 + h' x2"
proof -
from H' x1 x2 have "x1 + x2 \<in> H'"
by (rule vectorspace.add_closed)
with x1 x2 obtain y y1 y2 a a1 a2 where
x1x2: "x1 + x2 = y + a \<cdot> x0" and y: "y \<in> H"
and x1_rep: "x1 = y1 + a1 \<cdot> x0" and y1: "y1 \<in> H"
and x2_rep: "x2 = y2 + a2 \<cdot> x0" and y2: "y2 \<in> H"
by (unfold H'_def sum_def lin_def) blast
have ya: "y1 + y2 = y \<and> a1 + a2 = a" using _ HE _ y x0
proof (rule decomp_H') txt_raw {* \label{decomp-H-use} *}
from HE y1 y2 show "y1 + y2 \<in> H"
by (rule subspace.add_closed)
from x0 and HE y y1 y2
have "x0 \<in> E" "y \<in> E" "y1 \<in> E" "y2 \<in> E" by auto
with x1_rep x2_rep have "(y1 + y2) + (a1 + a2) \<cdot> x0 = x1 + x2"
by (simp add: add_ac add_mult_distrib2)
also note x1x2
finally show "(y1 + y2) + (a1 + a2) \<cdot> x0 = y + a \<cdot> x0" .
qed
from h'_def x1x2 _ HE y x0
have "h' (x1 + x2) = h y + a * xi"
by (rule h'_definite)
also have "\<dots> = h (y1 + y2) + (a1 + a2) * xi"
by (simp only: ya)
also from y1 y2 have "h (y1 + y2) = h y1 + h y2"
by simp
also have "\<dots> + (a1 + a2) * xi = (h y1 + a1 * xi) + (h y2 + a2 * xi)"
by (simp add: real_add_mult_distrib)
also from h'_def x1_rep _ HE y1 x0
have "h y1 + a1 * xi = h' x1"
by (rule h'_definite [symmetric])
also from h'_def x2_rep _ HE y2 x0
have "h y2 + a2 * xi = h' x2"
by (rule h'_definite [symmetric])
finally show ?thesis .
qed
next
fix x1 c assume x1: "x1 \<in> H'"
show "h' (c \<cdot> x1) = c * (h' x1)"
proof -
from H' x1 have ax1: "c \<cdot> x1 \<in> H'"
by (rule vectorspace.mult_closed)
with x1 obtain y a y1 a1 where
cx1_rep: "c \<cdot> x1 = y + a \<cdot> x0" and y: "y \<in> H"
and x1_rep: "x1 = y1 + a1 \<cdot> x0" and y1: "y1 \<in> H"
by (unfold H'_def sum_def lin_def) blast
have ya: "c \<cdot> y1 = y \<and> c * a1 = a" using _ HE _ y x0
proof (rule decomp_H')
from HE y1 show "c \<cdot> y1 \<in> H"
by (rule subspace.mult_closed)
from x0 and HE y y1
have "x0 \<in> E" "y \<in> E" "y1 \<in> E" by auto
with x1_rep have "c \<cdot> y1 + (c * a1) \<cdot> x0 = c \<cdot> x1"
by (simp add: mult_assoc add_mult_distrib1)
also note cx1_rep
finally show "c \<cdot> y1 + (c * a1) \<cdot> x0 = y + a \<cdot> x0" .
qed
from h'_def cx1_rep _ HE y x0 have "h' (c \<cdot> x1) = h y + a * xi"
by (rule h'_definite)
also have "\<dots> = h (c \<cdot> y1) + (c * a1) * xi"
by (simp only: ya)
also from y1 have "h (c \<cdot> y1) = c * h y1"
by simp
also have "\<dots> + (c * a1) * xi = c * (h y1 + a1 * xi)"
by (simp only: real_add_mult_distrib2)
also from h'_def x1_rep _ HE y1 x0 have "h y1 + a1 * xi = h' x1"
by (rule h'_definite [symmetric])
finally show ?thesis .
qed
}
qed
text {* \medskip The linear extension @{text h'} of @{text h}
is bounded by the seminorm @{text p}. *}
lemma h'_norm_pres:
includes var H + var h + var E
assumes h'_def: "h' \<equiv> \<lambda>x. let (y, a) =
SOME (y, a). x = y + a \<cdot> x0 \<and> y \<in> H in h y + a * xi"
and H'_def: "H' \<equiv> H + lin x0"
and x0: "x0 \<notin> H" "x0 \<in> E" "x0 \<noteq> 0"
includes vectorspace E + subvectorspace H E +
seminorm E p + linearform H h
assumes a: "\<forall>y \<in> H. h y \<le> p y"
and a': "\<forall>y \<in> H. - p (y + x0) - h y \<le> xi \<and> xi \<le> p (y + x0) - h y"
shows "\<forall>x \<in> H'. h' x \<le> p x"
proof
fix x assume x': "x \<in> H'"
show "h' x \<le> p x"
proof -
from a' have a1: "\<forall>ya \<in> H. - p (ya + x0) - h ya \<le> xi"
and a2: "\<forall>ya \<in> H. xi \<le> p (ya + x0) - h ya" by auto
from x' obtain y a where
x_rep: "x = y + a \<cdot> x0" and y: "y \<in> H"
by (unfold H'_def sum_def lin_def) blast
from y have y': "y \<in> E" ..
from y have ay: "inverse a \<cdot> y \<in> H" by simp
from h'_def x_rep _ _ y x0 have "h' x = h y + a * xi"
by (rule h'_definite)
also have "\<dots> \<le> p (y + a \<cdot> x0)"
proof (rule linorder_cases)
assume z: "a = 0"
then have "h y + a * xi = h y" by simp
also from a y have "\<dots> \<le> p y" ..
also from x0 y' z have "p y = p (y + a \<cdot> x0)" by simp
finally show ?thesis .
next
txt {* In the case @{text "a < 0"}, we use @{text "a\<^sub>1"}
with @{text ya} taken as @{text "y / a"}: *}
assume lz: "a < 0" hence nz: "a \<noteq> 0" by simp
from a1 ay
have "- p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y) \<le> xi" ..
with lz have "a * xi \<le>
a * (- p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y))"
by (rule real_mult_less_le_anti)
also have "\<dots> =
- a * (p (inverse a \<cdot> y + x0)) - a * (h (inverse a \<cdot> y))"
by (rule real_mult_diff_distrib)
also from lz x0 y' have "- a * (p (inverse a \<cdot> y + x0)) =
p (a \<cdot> (inverse a \<cdot> y + x0))"
by (simp add: abs_homogenous abs_minus_eqI2)
also from nz x0 y' have "\<dots> = p (y + a \<cdot> x0)"
by (simp add: add_mult_distrib1 mult_assoc [symmetric])
also from nz y have "a * (h (inverse a \<cdot> y)) = h y"
by simp
finally have "a * xi \<le> p (y + a \<cdot> x0) - h y" .
then show ?thesis by simp
next
txt {* In the case @{text "a > 0"}, we use @{text "a\<^sub>2"}
with @{text ya} taken as @{text "y / a"}: *}
assume gz: "0 < a" hence nz: "a \<noteq> 0" by simp
from a2 ay
have "xi \<le> p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y)" ..
with gz have "a * xi \<le>
a * (p (inverse a \<cdot> y + x0) - h (inverse a \<cdot> y))"
by (rule real_mult_less_le_mono)
also have "... = a * p (inverse a \<cdot> y + x0) - a * h (inverse a \<cdot> y)"
by (rule real_mult_diff_distrib2)
also from gz x0 y'
have "a * p (inverse a \<cdot> y + x0) = p (a \<cdot> (inverse a \<cdot> y + x0))"
by (simp add: abs_homogenous abs_eqI2)
also from nz x0 y' have "\<dots> = p (y + a \<cdot> x0)"
by (simp add: add_mult_distrib1 mult_assoc [symmetric])
also from nz y have "a * h (inverse a \<cdot> y) = h y"
by simp
finally have "a * xi \<le> p (y + a \<cdot> x0) - h y" .
then show ?thesis by simp
qed
also from x_rep have "\<dots> = p x" by (simp only:)
finally show ?thesis .
qed
qed
end