# Theory Hahn_Banach_Ext_Lemmas

theory Hahn_Banach_Ext_Lemmas
imports Function_Norm
(*  Title:      HOL/Hahn_Banach/Hahn_Banach_Ext_Lemmas.thy
Author:     Gertrud Bauer, TU Munich
*)

section ‹Extending non-maximal functions›

theory Hahn_Banach_Ext_Lemmas
imports Function_Norm
begin

text ‹
In this section the following context is presumed. Let ‹E› be a real vector
space with a seminorm ‹q› on ‹E›. ‹F› is a subspace of ‹E› and ‹f› a linear
function on ‹F›. We consider a subspace ‹H› of ‹E› that is a superspace of
‹F› and a linear form ‹h› on ‹H›. ‹H› is a not equal to ‹E› and ‹x⇩0› is an
element in ‹E - H›. ‹H› is extended to the direct sum ‹H' = H + lin x⇩0›, so
for any ‹x ∈ H'› the decomposition of ‹x = y + a ⋅ x› with ‹y ∈ H› is
unique. ‹h'› is defined on ‹H'› by ‹h' x = h y + a ⋅ ξ› for a certain ‹ξ›.

Subsequently we show some properties of this extension ‹h'› of ‹h›.

┉
This lemma will be used to show the existence of a linear extension of ‹f›
(see page \pageref{ex-xi-use}). It is a consequence of the completeness of
‹ℝ›. To show
\begin{center}
\begin{tabular}{l}
‹∃ξ. ∀y ∈ F. a y ≤ ξ ∧ ξ ≤ b y›
\end{tabular}
\end{center}
⇤ it suffices to show that
\begin{center}
\begin{tabular}{l}
‹∀u ∈ F. ∀v ∈ F. a u ≤ b v›
\end{tabular}
\end{center}
›

lemma ex_xi:
assumes "vectorspace F"
assumes r: "⋀u v. u ∈ F ⟹ v ∈ F ⟹ a u ≤ b v"
shows "∃xi::real. ∀y ∈ F. a y ≤ xi ∧ xi ≤ b y"
proof -
interpret vectorspace F by fact
txt ‹From the completeness of the reals follows:
The set ‹S = {a u. u ∈ F}› has a supremum, if it is
non-empty and has an upper bound.›

let ?S = "{a u | u. u ∈ F}"
have "∃xi. lub ?S xi"
proof (rule real_complete)
have "a 0 ∈ ?S" by blast
then show "∃X. X ∈ ?S" ..
have "∀y ∈ ?S. y ≤ b 0"
proof
fix y assume y: "y ∈ ?S"
then obtain u where u: "u ∈ F" and y: "y = a u" by blast
from u and zero have "a u ≤ b 0" by (rule r)
with y show "y ≤ b 0" by (simp only:)
qed
then show "∃u. ∀y ∈ ?S. y ≤ u" ..
qed
then obtain xi where xi: "lub ?S xi" ..
{
fix y assume "y ∈ F"
then have "a y ∈ ?S" by blast
with xi have "a y ≤ xi" by (rule lub.upper)
}
moreover {
fix y assume y: "y ∈ F"
from xi have "xi ≤ b y"
proof (rule lub.least)
fix au assume "au ∈ ?S"
then obtain u where u: "u ∈ F" and au: "au = a u" by blast
from u y have "a u ≤ b y" by (rule r)
with au show "au ≤ b y" by (simp only:)
qed
}
ultimately show "∃xi. ∀y ∈ F. a y ≤ xi ∧ xi ≤ b y" by blast
qed

text ‹
┉
The function ‹h'› is defined as a ‹h' x = h y + a ⋅ ξ› where ‹x = y + a ⋅ ξ›
is a linear extension of ‹h› to ‹H'›.
›

lemma h'_lf:
assumes h'_def: "⋀x. h' x = (let (y, a) =
SOME (y, a). x = y + a ⋅ x0 ∧ y ∈ H in h y + a * xi)"
and H'_def: "H' = H + lin x0"
and HE: "H ⊴ E"
assumes "linearform H h"
assumes x0: "x0 ∉ H"  "x0 ∈ E"  "x0 ≠ 0"
assumes E: "vectorspace E"
shows "linearform H' h'"
proof -
interpret linearform H h by fact
interpret vectorspace E by fact
show ?thesis
proof
note E = ‹vectorspace E›
have H': "vectorspace H'"
proof (unfold H'_def)
from ‹x0 ∈ E›
have "lin x0 ⊴ E" ..
with HE show "vectorspace (H + lin x0)" using E ..
qed
{
fix x1 x2 assume x1: "x1 ∈ H'" and x2: "x2 ∈ H'"
show "h' (x1 + x2) = h' x1 + h' x2"
proof -
from H' x1 x2 have "x1 + x2 ∈ H'"
with x1 x2 obtain y y1 y2 a a1 a2 where
x1x2: "x1 + x2 = y + a ⋅ x0" and y: "y ∈ H"
and x1_rep: "x1 = y1 + a1 ⋅ x0" and y1: "y1 ∈ H"
and x2_rep: "x2 = y2 + a2 ⋅ x0" and y2: "y2 ∈ H"
unfolding H'_def sum_def lin_def by blast

have ya: "y1 + y2 = y ∧ a1 + a2 = a" using E HE _ y x0
proof (rule decomp_H') text_raw ‹\label{decomp-H-use}›
from HE y1 y2 show "y1 + y2 ∈ H"
from x0 and HE y y1 y2
have "x0 ∈ E"  "y ∈ E"  "y1 ∈ E"  "y2 ∈ E" by auto
with x1_rep x2_rep have "(y1 + y2) + (a1 + a2) ⋅ x0 = x1 + x2"
also note x1x2
finally show "(y1 + y2) + (a1 + a2) ⋅ x0 = y + a ⋅ x0" .
qed

from h'_def x1x2 E HE y x0
have "h' (x1 + x2) = h y + a * xi"
by (rule h'_definite)
also have "… = 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 "… + (a1 + a2) * xi = (h y1 + a1 * xi) + (h y2 + a2 * xi)"
also from h'_def x1_rep E HE y1 x0
have "h y1 + a1 * xi = h' x1"
by (rule h'_definite [symmetric])
also from h'_def x2_rep E 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 ∈ H'"
show "h' (c ⋅ x1) = c * (h' x1)"
proof -
from H' x1 have ax1: "c ⋅ x1 ∈ H'"
by (rule vectorspace.mult_closed)
with x1 obtain y a y1 a1 where
cx1_rep: "c ⋅ x1 = y + a ⋅ x0" and y: "y ∈ H"
and x1_rep: "x1 = y1 + a1 ⋅ x0" and y1: "y1 ∈ H"
unfolding H'_def sum_def lin_def by blast

have ya: "c ⋅ y1 = y ∧ c * a1 = a" using E HE _ y x0
proof (rule decomp_H')
from HE y1 show "c ⋅ y1 ∈ H"
by (rule subspace.mult_closed)
from x0 and HE y y1
have "x0 ∈ E"  "y ∈ E"  "y1 ∈ E" by auto
with x1_rep have "c ⋅ y1 + (c * a1) ⋅ x0 = c ⋅ x1"
also note cx1_rep
finally show "c ⋅ y1 + (c * a1) ⋅ x0 = y + a ⋅ x0" .
qed

from h'_def cx1_rep E HE y x0 have "h' (c ⋅ x1) = h y + a * xi"
by (rule h'_definite)
also have "… = h (c ⋅ y1) + (c * a1) * xi"
by (simp only: ya)
also from y1 have "h (c ⋅ y1) = c * h y1"
by simp
also have "… + (c * a1) * xi = c * (h y1 + a1 * xi)"
by (simp only: distrib_left)
also from h'_def x1_rep E HE y1 x0 have "h y1 + a1 * xi = h' x1"
by (rule h'_definite [symmetric])
finally show ?thesis .
qed
}
qed
qed

text ‹
┉
The linear extension ‹h'› of ‹h› is bounded by the seminorm ‹p›.
›

lemma h'_norm_pres:
assumes h'_def: "⋀x. h' x = (let (y, a) =
SOME (y, a). x = y + a ⋅ x0 ∧ y ∈ H in h y + a * xi)"
and H'_def: "H' = H + lin x0"
and x0: "x0 ∉ H"  "x0 ∈ E"  "x0 ≠ 0"
assumes E: "vectorspace E" and HE: "subspace H E"
and "seminorm E p" and "linearform H h"
assumes a: "∀y ∈ H. h y ≤ p y"
and a': "∀y ∈ H. - p (y + x0) - h y ≤ xi ∧ xi ≤ p (y + x0) - h y"
shows "∀x ∈ H'. h' x ≤ p x"
proof -
interpret vectorspace E by fact
interpret subspace H E by fact
interpret seminorm E p by fact
interpret linearform H h by fact
show ?thesis
proof
fix x assume x': "x ∈ H'"
show "h' x ≤ p x"
proof -
from a' have a1: "∀ya ∈ H. - p (ya + x0) - h ya ≤ xi"
and a2: "∀ya ∈ H. xi ≤ p (ya + x0) - h ya" by auto
from x' obtain y a where
x_rep: "x = y + a ⋅ x0" and y: "y ∈ H"
unfolding H'_def sum_def lin_def by blast
from y have y': "y ∈ E" ..
from y have ay: "inverse a ⋅ y ∈ H" by simp

from h'_def x_rep E HE y x0 have "h' x = h y + a * xi"
by (rule h'_definite)
also have "… ≤ p (y + a ⋅ x0)"
proof (rule linorder_cases)
assume z: "a = 0"
then have "h y + a * xi = h y" by simp
also from a y have "… ≤ p y" ..
also from x0 y' z have "p y = p (y + a ⋅ x0)" by simp
finally show ?thesis .
next
txt ‹In the case ‹a < 0›, we use ‹a⇩1›
with ‹ya› taken as ‹y / a›:›
assume lz: "a < 0" then have nz: "a ≠ 0" by simp
from a1 ay
have "- p (inverse a ⋅ y + x0) - h (inverse a ⋅ y) ≤ xi" ..
with lz have "a * xi ≤
a * (- p (inverse a ⋅ y + x0) - h (inverse a ⋅ y))"

also have "… =
- a * (p (inverse a ⋅ y + x0)) - a * (h (inverse a ⋅ y))"
also from lz x0 y' have "- a * (p (inverse a ⋅ y + x0)) =
p (a ⋅ (inverse a ⋅ y + x0))"
also from nz x0 y' have "… = p (y + a ⋅ x0)"
also from nz y have "a * (h (inverse a ⋅ y)) =  h y"
by simp
finally have "a * xi ≤ p (y + a ⋅ x0) - h y" .
then show ?thesis by simp
next
txt ‹In the case ‹a > 0›, we use ‹a⇩2›
with ‹ya› taken as ‹y / a›:›
assume gz: "0 < a" then have nz: "a ≠ 0" by simp
from a2 ay
have "xi ≤ p (inverse a ⋅ y + x0) - h (inverse a ⋅ y)" ..
with gz have "a * xi ≤
a * (p (inverse a ⋅ y + x0) - h (inverse a ⋅ y))"
by simp
also have "… = a * p (inverse a ⋅ y + x0) - a * h (inverse a ⋅ y)"
also from gz x0 y'
have "a * p (inverse a ⋅ y + x0) = p (a ⋅ (inverse a ⋅ y + x0))"
also from nz x0 y' have "… = p (y + a ⋅ x0)"
also from nz y have "a * h (inverse a ⋅ y) = h y"
by simp
finally have "a * xi ≤ p (y + a ⋅ x0) - h y" .
then show ?thesis by simp
qed
also from x_rep have "… = p x" by (simp only:)
finally show ?thesis .
qed
qed
qed

end