:: Construction of a bilinear symmetric form in orthogonal vector space
:: by Eugeniusz Kusak, Wojciech Leo\'nczuk and Micha{\l} Muzalewski
::
:: Copyright (c) 1990-2019 Association of Mizar Users

reconsider X = as non empty set ;

reconsider o = 0 as Element of X by TARSKI:def 1;

deffunc H1( Element of X, Element of X) -> Element of X = o;

consider md being BinOp of X such that
Lm1: for x, y being Element of X holds md . (x,y) = H1(x,y) from
Lm2: now :: thesis: for F being Field ex mo being Relation of X st
for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) )
defpred S1[ object ] means ex a, b being Element of X st
( \$1 = [a,b] & b = o );
set CV = [:X,X:];
let F be Field; :: thesis: ex mo being Relation of X st
for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) )

consider mo being set such that
A1: for x being object holds
( x in mo iff ( x in [:X,X:] & S1[x] ) ) from mo c= [:X,X:] by A1;
then reconsider mo = mo as Relation of X ;
take mo = mo; :: thesis: for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) )

thus for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) by A1; :: thesis: verum
end;

Lm3: for F being Field
for mF being Function of [: the carrier of F,X:],X
for mo being Relation of X holds
( SymStr(# X,md,o,mF,mo #) is Abelian & SymStr(# X,md,o,mF,mo #) is add-associative & SymStr(# X,md,o,mF,mo #) is right_zeroed & SymStr(# X,md,o,mF,mo #) is right_complementable )

proof end;

Lm4: now :: thesis: for F being Field
for mF being Function of [: the carrier of F,X:],X st ( for a being Element of F
for x being Element of X holds mF . [a,x] = o ) holds
for mo being Relation of X
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital )
let F be Field; :: thesis: for mF being Function of [: the carrier of F,X:],X st ( for a being Element of F
for x being Element of X holds mF . [a,x] = o ) holds
for mo being Relation of X
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital )

let mF be Function of [: the carrier of F,X:],X; :: thesis: ( ( for a being Element of F
for x being Element of X holds mF . [a,x] = o ) implies for mo being Relation of X
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital ) )

assume A1: for a being Element of F
for x being Element of X holds mF . [a,x] = o ; :: thesis: for mo being Relation of X
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital )

let mo be Relation of X; :: thesis: for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital )

let MPS be non empty right_complementable Abelian add-associative right_zeroed SymStr over F; :: thesis: ( MPS = SymStr(# X,md,o,mF,mo #) implies ( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital ) )
assume A2: MPS = SymStr(# X,md,o,mF,mo #) ; :: thesis: ( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital )
for x, y being Element of F
for v, w being Element of MPS holds
( x * (v + w) = (x * v) + (x * w) & (x + y) * v = (x * v) + (y * v) & (x * y) * v = x * (y * v) & (1_ F) * v = v )
proof
let x, y be Element of F; :: thesis: for v, w being Element of MPS holds
( x * (v + w) = (x * v) + (x * w) & (x + y) * v = (x * v) + (y * v) & (x * y) * v = x * (y * v) & (1_ F) * v = v )

let v, w be Element of MPS; :: thesis: ( x * (v + w) = (x * v) + (x * w) & (x + y) * v = (x * v) + (y * v) & (x * y) * v = x * (y * v) & (1_ F) * v = v )
A3: (x * y) * v = x * (y * v)
proof
set z = x * y;
A4: (x * y) * v = mF . ((x * y),v) by ;
reconsider v = v as Element of MPS ;
reconsider v = v as Element of MPS ;
A5: (x * y) * v = o by A1, A2, A4;
reconsider v = v as Element of MPS ;
A6: mF . (y,v) = o by A1, A2;
reconsider v = v as Element of MPS ;
y * v = o by ;
then x * (y * v) = mF . (x,o) by ;
hence (x * y) * v = x * (y * v) by A1, A5; :: thesis: verum
end;
A7: x * (v + w) = (x * v) + (x * w)
proof
reconsider v = v, w = w as Element of MPS ;
A8: o = 0. MPS by ;
reconsider v = v, w = w as Element of X by A2;
A9: md . (v,w) = o by Lm1;
reconsider v = v, w = w as Element of MPS ;
x * (v + w) = mF . (x,o) by ;
then A10: x * (v + w) = o by A1;
reconsider w = w as Element of MPS ;
reconsider v = v as Element of MPS ;
A11: mF . (x,v) = o by A1;
reconsider v = v as Element of MPS ;
A12: mF . (x,w) = o by A1;
reconsider w = w as Element of MPS ;
A13: x * w = o by ;
x * v = o by ;
hence x * (v + w) = (x * v) + (x * w) by ; :: thesis: verum
end;
A14: (x + y) * v = (x * v) + (y * v)
proof
set z = x + y;
A15: (x + y) * v = mF . ((x + y),v) by ;
reconsider v = v as Element of MPS ;
reconsider v = v as Element of MPS ;
A16: (x + y) * v = o by A1, A2, A15;
reconsider v = v as Element of MPS ;
A17: mF . (x,v) = o by A1, A2;
reconsider v = v as Element of MPS ;
A18: x * v = o by ;
reconsider v = v as Element of MPS ;
A19: mF . (y,v) = o by A1, A2;
A20: o = 0. MPS by ;
reconsider v = v as Element of MPS ;
y * v = o by ;
hence (x + y) * v = (x * v) + (y * v) by ; :: thesis: verum
end;
(1_ F) * v = v
proof
set one1 = 1_ F;
A21: (1_ F) * v = mF . ((1_ F),v) by ;
reconsider v = v as Element of MPS ;
mF . ((1_ F),v) = o by A1, A2;
hence (1_ F) * v = v by ; :: thesis: verum
end;
hence ( x * (v + w) = (x * v) + (x * w) & (x + y) * v = (x * v) + (y * v) & (x * y) * v = x * (y * v) & (1_ F) * v = v ) by A7, A14, A3; :: thesis: verum
end;
hence ( MPS is vector-distributive & MPS is scalar-distributive & MPS is scalar-associative & MPS is scalar-unital ) by ; :: thesis: verum
end;

Lm5: now :: thesis: for F being Field
for mF being Function of [: the carrier of F,X:],X
for mo being Relation of X st ( for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) ) holds
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )
set CV = [:X,X:];
let F be Field; :: thesis: for mF being Function of [: the carrier of F,X:],X
for mo being Relation of X st ( for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) ) holds
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )

let mF be Function of [: the carrier of F,X:],X; :: thesis: for mo being Relation of X st ( for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) ) holds
for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )

let mo be Relation of X; :: thesis: ( ( for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) ) implies for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) ) )

assume A1: for x being set holds
( x in mo iff ( x in [:X,X:] & ex a, b being Element of X st
( x = [a,b] & b = o ) ) ) ; :: thesis: for MPS being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st MPS = SymStr(# X,md,o,mF,mo #) holds
( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )

let MPS be non empty right_complementable Abelian add-associative right_zeroed SymStr over F; :: thesis: ( MPS = SymStr(# X,md,o,mF,mo #) implies ( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) ) )

assume A2: MPS = SymStr(# X,md,o,mF,mo #) ; :: thesis: ( ( for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )

0. MPS = o by ;
hence for a, b, c, d being Element of MPS st a <> 0. MPS & b <> 0. MPS & c <> 0. MPS & d <> 0. MPS holds
ex p being Element of MPS st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) by ; :: thesis: ( ( for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ ) & ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )

A3: for a, b being Element of MPS holds
( b _|_ iff ( [a,b] in [:X,X:] & ex c, d being Element of X st
( [a,b] = [c,d] & d = o ) ) )
proof
let a, b be Element of MPS; :: thesis: ( b _|_ iff ( [a,b] in [:X,X:] & ex c, d being Element of X st
( [a,b] = [c,d] & d = o ) ) )

( b _|_ iff [a,b] in mo ) by ;
hence ( b _|_ iff ( [a,b] in [:X,X:] & ex c, d being Element of X st
( [a,b] = [c,d] & d = o ) ) ) by A1; :: thesis: verum
end;
A4: for a, b being Element of MPS holds
( b _|_ iff b = o )
proof
let a, b be Element of MPS; :: thesis: ( b _|_ iff b = o )
A5: ( b = o implies b _|_ )
proof
consider c, d being Element of MPS such that
A6: ( c = a & d = b ) ;
assume A7: b = o ; :: thesis: b _|_
[a,b] = [c,d] by A6;
hence b _|_ by A2, A3, A7; :: thesis: verum
end;
( b _|_ implies b = o )
proof
assume b _|_ ; :: thesis: b = o
then ex c, d being Element of X st
( [a,b] = [c,d] & d = o ) by A3;
hence b = o by XTUPLE_0:1; :: thesis: verum
end;
hence ( b _|_ iff b = o ) by A5; :: thesis: verum
end;
thus for a, b being Element of MPS
for l being Element of F st b _|_ holds
b _|_ :: thesis: ( ( for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ ) & ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )
proof
let a, b be Element of MPS; :: thesis: for l being Element of F st b _|_ holds
b _|_

let l be Element of F; :: thesis: ( b _|_ implies b _|_ )
assume b _|_ ; :: thesis: b _|_
then b = o by A4;
hence b _|_ by A4; :: thesis: verum
end;
thus for a, b, c being Element of MPS st a _|_ & a _|_ holds
a _|_ :: thesis: ( ( for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ ) & ( for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ ) )
proof
let a, b, c be Element of MPS; :: thesis: ( a _|_ & a _|_ implies a _|_ )
assume that
A8: a _|_ and
a _|_ ; :: thesis: a _|_
a = o by A4, A8;
hence a _|_ by A4; :: thesis: verum
end;
thus for a, b, x being Element of MPS st not a _|_ holds
ex k being Element of F st a _|_ :: thesis: for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_
proof
let a, b, x be Element of MPS; :: thesis: ( not a _|_ implies ex k being Element of F st a _|_ )
assume A9: not a _|_ ; :: thesis: ex k being Element of F st a _|_
assume for k being Element of F holds not a _|_ ; :: thesis: contradiction
a <> o by A4, A9;
hence contradiction by ; :: thesis: verum
end;
thus for a, b, c being Element of MPS st b - c _|_ & c - a _|_ holds
a - b _|_ :: thesis: verum
proof
let a, b, c be Element of MPS; :: thesis: ( b - c _|_ & c - a _|_ implies a - b _|_ )
assume that
b - c _|_ and
c - a _|_ ; :: thesis: a - b _|_
assume not a - b _|_ ; :: thesis: contradiction
then a - b <> o by A4;
hence contradiction by ; :: thesis: verum
end;
end;

:: 2. ORTHOGONAL VECTOR SPACE
definition
let F be Field;
let IT be non empty right_complementable Abelian add-associative right_zeroed SymStr over F;
attr IT is OrtSp-like means :Def1: :: ORTSP_1:def 1
for a, b, c, d, x being Element of IT
for l being Element of F holds
( ( a <> 0. IT & b <> 0. IT & c <> 0. IT & d <> 0. IT implies ex p being Element of IT st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( b _|_ implies b _|_ ) & ( a _|_ & a _|_ implies a _|_ ) & ( not a _|_ implies ex k being Element of F st a _|_ ) & ( b - c _|_ & c - a _|_ implies a - b _|_ ) );
end;

:: deftheorem Def1 defines OrtSp-like ORTSP_1:def 1 :
for F being Field
for IT being non empty right_complementable Abelian add-associative right_zeroed SymStr over F holds
( IT is OrtSp-like iff for a, b, c, d, x being Element of IT
for l being Element of F holds
( ( a <> 0. IT & b <> 0. IT & c <> 0. IT & d <> 0. IT implies ex p being Element of IT st
( not a _|_ & not b _|_ & not c _|_ & not d _|_ ) ) & ( b _|_ implies b _|_ ) & ( a _|_ & a _|_ implies a _|_ ) & ( not a _|_ implies ex k being Element of F st a _|_ ) & ( b - c _|_ & c - a _|_ implies a - b _|_ ) ) );

registration
let F be Field;
existence
ex b1 being non empty right_complementable Abelian add-associative right_zeroed SymStr over F st
( b1 is OrtSp-like & b1 is vector-distributive & b1 is scalar-distributive & b1 is scalar-associative & b1 is scalar-unital & b1 is strict )
proof end;
end;

definition end;

theorem Th1: :: ORTSP_1:1
for F being Field
for S being OrtSp of F
for a being Element of S holds a _|_
proof end;

theorem Th2: :: ORTSP_1:2
for F being Field
for S being OrtSp of F
for a, b being Element of S st b _|_ holds
a _|_
proof end;

theorem Th3: :: ORTSP_1:3
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not b _|_ & b _|_ holds
not b _|_
proof end;

theorem Th4: :: ORTSP_1:4
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not a _|_ & a _|_ holds
not a _|_
proof end;

theorem Th5: :: ORTSP_1:5
for F being Field
for S being OrtSp of F
for a, b being Element of S
for l being Element of F st not a _|_ & not l = 0. F holds
( not a _|_ & not l * a _|_ )
proof end;

theorem Th6: :: ORTSP_1:6
for F being Field
for S being OrtSp of F
for a, b being Element of S st b _|_ holds
b _|_
proof end;

theorem Th7: :: ORTSP_1:7
for F being Field
for S being OrtSp of F
for a, b, c, d being Element of S st d _|_ & d _|_ holds
d _|_
proof end;

theorem Th8: :: ORTSP_1:8
for F being Field
for S being OrtSp of F
for a, b, x being Element of S
for k, l being Element of F st not a _|_ & a _|_ & a _|_ holds
k = l
proof end;

theorem Th9: :: ORTSP_1:9
for F being Field
for S being OrtSp of F
for a, b being Element of S st a _|_ & b _|_ holds
a - b _|_
proof end;

theorem :: ORTSP_1:10
for F being Field
for S being OrtSp of F st (1_ F) + (1_ F) <> 0. F & ex a being Element of S st a <> 0. S holds
ex b being Element of S st not b _|_
proof end;

:: 5. ORTHOGONAL PROJECTION
definition
let F be Field;
let S be OrtSp of F;
let a, b, x be Element of S;
assume A1: not a _|_ ;
func ProJ (a,b,x) -> Element of F means :Def2: :: ORTSP_1:def 2
for l being Element of F st a _|_ holds
it = l;
existence
ex b1 being Element of F st
for l being Element of F st a _|_ holds
b1 = l
proof end;
uniqueness
for b1, b2 being Element of F st ( for l being Element of F st a _|_ holds
b1 = l ) & ( for l being Element of F st a _|_ holds
b2 = l ) holds
b1 = b2
proof end;
end;

:: deftheorem Def2 defines ProJ ORTSP_1:def 2 :
for F being Field
for S being OrtSp of F
for a, b, x being Element of S st not a _|_ holds
for b6 being Element of F holds
( b6 = ProJ (a,b,x) iff for l being Element of F st a _|_ holds
b6 = l );

theorem Th11: :: ORTSP_1:11
for F being Field
for S being OrtSp of F
for a, b, x being Element of S st not a _|_ holds
a _|_
proof end;

theorem Th12: :: ORTSP_1:12
for F being Field
for S being OrtSp of F
for a, b, x being Element of S
for l being Element of F st not a _|_ holds
ProJ (a,b,(l * x)) = l * (ProJ (a,b,x))
proof end;

theorem Th13: :: ORTSP_1:13
for F being Field
for S being OrtSp of F
for a, b, x, y being Element of S st not a _|_ holds
ProJ (a,b,(x + y)) = (ProJ (a,b,x)) + (ProJ (a,b,y))
proof end;

theorem :: ORTSP_1:14
for F being Field
for S being OrtSp of F
for a, b, x being Element of S
for l being Element of F st not a _|_ & l <> 0. F holds
ProJ (a,(l * b),x) = (l ") * (ProJ (a,b,x))
proof end;

theorem Th15: :: ORTSP_1:15
for F being Field
for S being OrtSp of F
for a, b, x being Element of S
for l being Element of F st not a _|_ & l <> 0. F holds
ProJ ((l * a),b,x) = ProJ (a,b,x)
proof end;

theorem :: ORTSP_1:16
for F being Field
for S being OrtSp of F
for a, b, c, p being Element of S st not a _|_ & a _|_ holds
( ProJ (a,(b + p),c) = ProJ (a,b,c) & ProJ (a,b,(c + p)) = ProJ (a,b,c) )
proof end;

theorem :: ORTSP_1:17
for F being Field
for S being OrtSp of F
for a, b, c, p being Element of S st not a _|_ & b _|_ & c _|_ holds
ProJ ((a + p),b,c) = ProJ (a,b,c)
proof end;

theorem Th18: :: ORTSP_1:18
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not a _|_ & a _|_ holds
ProJ (a,b,c) = 1_ F
proof end;

theorem Th19: :: ORTSP_1:19
for F being Field
for S being OrtSp of F
for a, b being Element of S st not a _|_ holds
ProJ (a,b,b) = 1_ F
proof end;

theorem Th20: :: ORTSP_1:20
for F being Field
for S being OrtSp of F
for a, b, x being Element of S st not a _|_ holds
( a _|_ iff ProJ (a,b,x) = 0. F )
proof end;

theorem Th21: :: ORTSP_1:21
for F being Field
for S being OrtSp of F
for a, b, p, q being Element of S st not a _|_ & not a _|_ holds
(ProJ (a,b,p)) * ((ProJ (a,b,q)) ") = ProJ (a,q,p)
proof end;

theorem Th22: :: ORTSP_1:22
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not a _|_ & not a _|_ holds
ProJ (a,b,c) = (ProJ (a,c,b)) "
proof end;

theorem Th23: :: ORTSP_1:23
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not a _|_ & c + a _|_ holds
ProJ (a,b,c) = - (ProJ (c,b,a))
proof end;

theorem Th24: :: ORTSP_1:24
for F being Field
for S being OrtSp of F
for a, b, c being Element of S st not b _|_ & not b _|_ holds
ProJ (c,b,a) = ((ProJ (b,a,c)) ") * (ProJ (a,b,c))
proof end;

theorem Th25: :: ORTSP_1:25
for F being Field
for S being OrtSp of F
for a, p, q, x being Element of S st not a _|_ & not x _|_ & not a _|_ & not x _|_ holds
(ProJ (a,q,p)) * (ProJ (p,a,x)) = (ProJ (q,a,x)) * (ProJ (x,q,p))
proof end;

theorem Th26: :: ORTSP_1:26
for F being Field
for S being OrtSp of F
for a, b, p, q, x, y being Element of S st not a _|_ & not x _|_ & not a _|_ & not x _|_ & not a _|_ holds
((ProJ (a,b,p)) * (ProJ (p,a,x))) * (ProJ (x,p,y)) = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y))
proof end;

theorem Th27: :: ORTSP_1:27
for F being Field
for S being OrtSp of F
for a, p, x, y being Element of S st not p _|_ & not p _|_ & not p _|_ holds
(ProJ (p,a,x)) * (ProJ (x,p,y)) = (ProJ (p,a,y)) * (ProJ (y,p,x))
proof end;

:: 6. BILINEAR SYMMETRIC FORM
definition
let F be Field;
let S be OrtSp of F;
let x, y, a, b be Element of S;
assume A1: not a _|_ ;
func PProJ (a,b,x,y) -> Element of F means :Def3: :: ORTSP_1:def 3
for q being Element of S st not a _|_ & not x _|_ holds
it = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y)) if ex p being Element of S st
( not a _|_ & not x _|_ )
otherwise it = 0. F;
existence
( ( ex p being Element of S st
( not a _|_ & not x _|_ ) implies ex b1 being Element of F st
for q being Element of S st not a _|_ & not x _|_ holds
b1 = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y)) ) & ( ( for p being Element of S holds
( a _|_ or x _|_ ) ) implies ex b1 being Element of F st b1 = 0. F ) )
proof end;
uniqueness
for b1, b2 being Element of F holds
( ( ex p being Element of S st
( not a _|_ & not x _|_ ) & ( for q being Element of S st not a _|_ & not x _|_ holds
b1 = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y)) ) & ( for q being Element of S st not a _|_ & not x _|_ holds
b2 = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y)) ) implies b1 = b2 ) & ( ( for p being Element of S holds
( a _|_ or x _|_ ) ) & b1 = 0. F & b2 = 0. F implies b1 = b2 ) )
proof end;
consistency
for b1 being Element of F holds verum
;
end;

:: deftheorem Def3 defines PProJ ORTSP_1:def 3 :
for F being Field
for S being OrtSp of F
for x, y, a, b being Element of S st not a _|_ holds
for b7 being Element of F holds
( ( ex p being Element of S st
( not a _|_ & not x _|_ ) implies ( b7 = PProJ (a,b,x,y) iff for q being Element of S st not a _|_ & not x _|_ holds
b7 = ((ProJ (a,b,q)) * (ProJ (q,a,x))) * (ProJ (x,q,y)) ) ) & ( ( for p being Element of S holds
( a _|_ or x _|_ ) ) implies ( b7 = PProJ (a,b,x,y) iff b7 = 0. F ) ) );

theorem Th28: :: ORTSP_1:28
for F being Field
for S being OrtSp of F
for a, b, x, y being Element of S st not a _|_ & x = 0. S holds
PProJ (a,b,x,y) = 0. F
proof end;

theorem Th29: :: ORTSP_1:29
for F being Field
for S being OrtSp of F
for a, b, x, y being Element of S st not a _|_ holds
( PProJ (a,b,x,y) = 0. F iff x _|_ )
proof end;

theorem :: ORTSP_1:30
for F being Field
for S being OrtSp of F
for a, b, x, y being Element of S st not a _|_ holds
PProJ (a,b,x,y) = PProJ (a,b,y,x)
proof end;

theorem :: ORTSP_1:31
for F being Field
for S being OrtSp of F
for a, b, x, y being Element of S
for l being Element of F st not a _|_ holds
PProJ (a,b,x,(l * y)) = l * (PProJ (a,b,x,y))
proof end;

theorem :: ORTSP_1:32
for F being Field
for S being OrtSp of F
for a, b, x, y, z being Element of S st not a _|_ holds
PProJ (a,b,x,(y + z)) = (PProJ (a,b,x,y)) + (PProJ (a,b,x,z))
proof end;