:: Brouwer Fixed Point Theorem for Disks on the Plane
:: by Artur Korni{\l}owicz and Yasunari Shidama
::
:: Received February 22, 2005
:: Copyright (c) 2005 Association of Mizar Users

begin

set T2 = TOP-REAL 2;

definition

let S, T be non empty TopSpace;
func DiffElems S,T -> Subset of [:S,T:] equals :: BROUWER:def 1
{ [s,t] where s is Point of S, t is Point of T : s <> t } ;
coherence
{ [s,t] where s is Point of S, t is Point of T : s <> t } is Subset of [:S,T:]

proof end;

end;

:: deftheorem defines DiffElems BROUWER:def 1 :

for S, T being non empty TopSpace holds DiffElems S,T = { [s,t] where s is Point of S, t is Point of T : s <> t } ;

theorem :: BROUWER:1

for S, T being non empty TopSpace
for x being set holds
( x in DiffElems S,T iff ex s being Point of S ex t being Point of T st
( x = [s,t] & s <> t ) ) ;

registration

let S be non empty non trivial TopSpace;
let T be non empty TopSpace;
cluster DiffElems S,T -> non empty ;
coherence
not DiffElems S,T is empty

proof end;

end;

registration

let S be non empty TopSpace;
let T be non empty non trivial TopSpace;
cluster DiffElems S,T -> non empty ;
coherence
not DiffElems S,T is empty

proof end;

end;

theorem Th2: :: BROUWER:2

for n being Element of NAT
for x being Point of (TOP-REAL n) holds cl_Ball x,0 = {x}

proof end;

definition

let n be Element of NAT ;
let x be Point of (TOP-REAL n);
let r be real number ;
func Tdisk x,r -> SubSpace of TOP-REAL n equals :: BROUWER:def 2
(TOP-REAL n) | (cl_Ball x,r);
coherence
(TOP-REAL n) | (cl_Ball x,r) is SubSpace of TOP-REAL n ;

end;

:: deftheorem defines Tdisk BROUWER:def 2 :

for n being Element of NAT
for x being Point of (TOP-REAL n)
for r being real number holds Tdisk x,r = (TOP-REAL n) | (cl_Ball x,r);

registration

let n be Element of NAT ;
let x be Point of (TOP-REAL n);
let r be non negative real number ;
cluster Tdisk x,r -> non empty ;
coherence
not Tdisk x,r is empty ;

end;

theorem Th3: :: BROUWER:3

for n being Element of NAT
for r being real number
for x being Point of (TOP-REAL n) holds the carrier of (Tdisk x,r) = cl_Ball x,r

proof end;

registration

let n be Element of NAT ;
let x be Point of (TOP-REAL n);
let r be real number ;
cluster Tdisk x,r -> convex ;
coherence
Tdisk x,r is convex

proof end;

end;

theorem Th4: :: BROUWER:4

for n being Element of NAT
for r being non negative real number
for s, t, x being Point of (TOP-REAL n) st s <> t & s is Point of (Tdisk x,r) & s is not Point of (Tcircle x,r) holds
ex e being Point of (Tcircle x,r) st {e} = (halfline s,t) /\ (Sphere x,r)

proof end;

theorem Th5: :: BROUWER:5

for n being Element of NAT
for r being non negative real number
for s, t, x being Point of (TOP-REAL n) st s <> t & s in the carrier of (Tcircle x,r) & t is Point of (Tdisk x,r) holds
ex e being Point of (Tcircle x,r) st
( e <> s & {s,e} = (halfline s,t) /\ (Sphere x,r) )

proof end;

definition

let n be non empty Element of NAT ;
let o be Point of (TOP-REAL n);
let s, t be Point of (TOP-REAL n);
let r be non negative real number ;
assume that
A1: s is Point of (Tdisk o,r) and
A2: t is Point of (Tdisk o,r) and
A3: s <> t ;
func HC s,t,o,r -> Point of (TOP-REAL n) means :Def3: :: BROUWER:def 3
( it in (halfline s,t) /\ (Sphere o,r) & it <> s );
existence
ex b₁ being Point of (TOP-REAL n) st
( b₁ in (halfline s,t) /\ (Sphere o,r) & b₁ <> s )

proof end;

uniqueness
for b₁, b₂ being Point of (TOP-REAL n) st b₁ in (halfline s,t) /\ (Sphere o,r) & b₁ <> s & b₂ in (halfline s,t) /\ (Sphere o,r) & b₂ <> s holds
b₁ = b₂

proof end;

end;

:: deftheorem Def3 defines HC BROUWER:def 3 :

for n being non empty Element of NAT
for o, s, t being Point of (TOP-REAL n)
for r being non negative real number st s is Point of (Tdisk o,r) & t is Point of (Tdisk o,r) & s <> t holds
for b₆ being Point of (TOP-REAL n) holds
( b₆ = HC s,t,o,r iff ( b₆ in (halfline s,t) /\ (Sphere o,r) & b₆ <> s ) );

theorem Th6: :: BROUWER:6

for r being non negative real number
for n being non empty Element of NAT
for s, o, t being Point of (TOP-REAL n) st s is Point of (Tdisk o,r) & t is Point of (Tdisk o,r) & s <> t holds
HC s,t,o,r is Point of (Tcircle o,r)

proof end;

theorem :: BROUWER:7

for a being real number
for r being non negative real number
for n being non empty Element of NAT
for s, t, o being Point of (TOP-REAL n)
for S, T, O being Element of REAL n st S = s & T = t & O = o & s is Point of (Tdisk o,r) & t is Point of (Tdisk o,r) & s <> t & a = ((- |((t - s),(s - o))|) + (sqrt ((|((t - s),(s - o))| ^2 ) - ((Sum (sqr (T - S))) * ((Sum (sqr (S - O))) - (r ^2 )))))) / (Sum (sqr (T - S))) holds
HC s,t,o,r = ((1 - a) * s) + (a * t)

proof end;

theorem Th8: :: BROUWER:8

for a being real number
for r being non negative real number
for r1, r2, s1, s2 being real number
for s, t, o being Point of (TOP-REAL 2) st s is Point of (Tdisk o,r) & t is Point of (Tdisk o,r) & s <> t & r1 = (t `1 ) - (s `1 ) & r2 = (t `2 ) - (s `2 ) & s1 = (s `1 ) - (o `1 ) & s2 = (s `2 ) - (o `2 ) & a = ((- ((s1 * r1) + (s2 * r2))) + (sqrt ((((s1 * r1) + (s2 * r2)) ^2 ) - (((r1 ^2 ) + (r2 ^2 )) * (((s1 ^2 ) + (s2 ^2 )) - (r ^2 )))))) / ((r1 ^2 ) + (r2 ^2 )) holds
HC s,t,o,r = |[((s `1 ) + (a * r1)),((s `2 ) + (a * r2))]|

proof end;

definition

let n be non empty Element of NAT ;
let o be Point of (TOP-REAL n);
let r be non negative real number ;
let x be Point of (Tdisk o,r);
let f be Function of (Tdisk o,r),(Tdisk o,r);
assume A1: not x is_a_fixpoint_of f ;
func HC x,f -> Point of (Tcircle o,r) means :Def4: :: BROUWER:def 4
ex y, z being Point of (TOP-REAL n) st
( y = x & z = f . x & it = HC z,y,o,r );
existence
ex b₁ being Point of (Tcircle o,r) ex y, z being Point of (TOP-REAL n) st
( y = x & z = f . x & b₁ = HC z,y,o,r )

proof end;

uniqueness
for b₁, b₂ being Point of (Tcircle o,r) st ex y, z being Point of (TOP-REAL n) st
( y = x & z = f . x & b₁ = HC z,y,o,r ) & ex y, z being Point of (TOP-REAL n) st
( y = x & z = f . x & b₂ = HC z,y,o,r ) holds
b₁ = b₂ ;

end;

:: deftheorem Def4 defines HC BROUWER:def 4 :

for n being non empty Element of NAT
for o being Point of (TOP-REAL n)
for r being non negative real number
for x being Point of (Tdisk o,r)
for f being Function of (Tdisk o,r),(Tdisk o,r) st not x is_a_fixpoint_of f holds
for b₆ being Point of (Tcircle o,r) holds
( b₆ = HC x,f iff ex y, z being Point of (TOP-REAL n) st
( y = x & z = f . x & b₆ = HC z,y,o,r ) );

theorem Th9: :: BROUWER:9

for r being non negative real number
for n being non empty Element of NAT
for o being Point of (TOP-REAL n)
for x being Point of (Tdisk o,r)
for f being Function of (Tdisk o,r),(Tdisk o,r) st not x is_a_fixpoint_of f & x is Point of (Tcircle o,r) holds
HC x,f = x

proof end;

theorem Th10: :: BROUWER:10

for r being positive real number
for o being Point of (TOP-REAL 2)
for Y being non empty SubSpace of Tdisk o,r st Y = Tcircle o,r holds
not Y is_a_retract_of Tdisk o,r

proof end;

definition

let n be non empty Element of NAT ;
let r be non negative real number ;
let o be Point of (TOP-REAL n);
let f be Function of (Tdisk o,r),(Tdisk o,r);
func BR-map f -> Function of (Tdisk o,r),(Tcircle o,r) means :Def5: :: BROUWER:def 5
for x being Point of (Tdisk o,r) holds it . x = HC x,f;
existence
ex b₁ being Function of (Tdisk o,r),(Tcircle o,r) st
for x being Point of (Tdisk o,r) holds b₁ . x = HC x,f

proof end;

uniqueness
for b₁, b₂ being Function of (Tdisk o,r),(Tcircle o,r) st ( for x being Point of (Tdisk o,r) holds b₁ . x = HC x,f ) & ( for x being Point of (Tdisk o,r) holds b₂ . x = HC x,f ) holds
b₁ = b₂

proof end;

end;

:: deftheorem Def5 defines BR-map BROUWER:def 5 :

for n being non empty Element of NAT
for r being non negative real number
for o being Point of (TOP-REAL n)
for f being Function of (Tdisk o,r),(Tdisk o,r)
for b₅ being Function of (Tdisk o,r),(Tcircle o,r) holds
( b₅ = BR-map f iff for x being Point of (Tdisk o,r) holds b₅ . x = HC x,f );

theorem Th11: :: BROUWER:11

for r being non negative real number
for n being non empty Element of NAT
for o being Point of (TOP-REAL n)
for x being Point of (Tdisk o,r)
for f being Function of (Tdisk o,r),(Tdisk o,r) st not x is_a_fixpoint_of f & x is Point of (Tcircle o,r) holds
(BR-map f) . x = x

proof end;

theorem :: BROUWER:12

for r being non negative real number
for n being non empty Element of NAT
for o being Point of (TOP-REAL n)
for f being continuous Function of (Tdisk o,r),(Tdisk o,r) st f has_no_fixpoint holds
(BR-map f) | (Sphere o,r) = id (Tcircle o,r)

proof end;

Lm1: now

let T be non empty TopSpace; :: thesis: for a being Real holds the carrier of T --> a is continuous
let a be Real; :: thesis: the carrier of T --> a is continuous
set c = the carrier of T;
set f = the carrier of T --> a;
thus the carrier of T --> a is continuous :: thesis: verum

proof

let Y be Subset of REAL ; :: according to PSCOMP_1:def 25 :: thesis: ( not Y is closed or (the carrier of T --> a) " Y is closed )
A1: dom (the carrier of T --> a) = the carrier of T by FUNCT_2:def 1;
assume Y is closed ; :: thesis: (the carrier of T --> a) " Y is closed
A2: rng (the carrier of T --> a) = {a} by FUNCOP_1:14;

per cases ( a in Y or not a in Y ) ;

suppose a in Y ; :: thesis: (the carrier of T --> a) " Y is closed

then A3: rng (the carrier of T --> a) c= Y by A2, ZFMISC_1:37;
(the carrier of T --> a) " Y = (the carrier of T --> a) " ((rng (the carrier of T --> a)) /\ Y) by RELAT_1:168
.= (the carrier of T --> a) " (rng (the carrier of T --> a)) by A3, XBOOLE_1:28
.= [#] T by A1, RELAT_1:169 ;
hence (the carrier of T --> a) " Y is closed ; :: thesis: verum

end;

suppose not a in Y ; :: thesis: (the carrier of T --> a) " Y is closed

then A4: rng (the carrier of T --> a) misses Y by A2, ZFMISC_1:56;
(the carrier of T --> a) " Y = (the carrier of T --> a) " ((rng (the carrier of T --> a)) /\ Y) by RELAT_1:168
.= (the carrier of T --> a) " {} by A4, XBOOLE_0:def 7
.= {} T by RELAT_1:171 ;
hence (the carrier of T --> a) " Y is closed ; :: thesis: verum

end;

end;

end;

end;

theorem Th13: :: BROUWER:13

for r being positive real number
for o being Point of (TOP-REAL 2)
for f being continuous Function of (Tdisk o,r),(Tdisk o,r) st f has_no_fixpoint holds
BR-map f is continuous

proof end;

theorem Th14: :: BROUWER:14

for r being non negative real number
for o being Point of (TOP-REAL 2)
for f being continuous Function of (Tdisk o,r),(Tdisk o,r) holds f has_a_fixpoint

proof end;

theorem :: BROUWER:15

for r being non negative real number
for o being Point of (TOP-REAL 2)
for f being continuous Function of (Tdisk o,r),(Tdisk o,r) ex x being Point of (Tdisk o,r) st f . x = x

proof end;