let F be Field; :: thesis: 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))

let S be OrtSp of F; :: thesis: 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))

let a, b, c be Element of S; :: thesis: ( not b _|_ & not b _|_ implies ProJ (c,b,a) = ((ProJ (b,a,c)) ") * (ProJ (a,b,c)) )
set 1F = 1_ F;
assume that
A1: not b _|_ and
A2: not b _|_ ; :: thesis: ProJ (c,b,a) = ((ProJ (b,a,c)) ") * (ProJ (a,b,c))
ProJ (b,a,c) <> 0. F by A1, A2, Th20;
then A3: - ((ProJ (b,a,c)) ") <> 0. F by VECTSP_1:25;
ProJ (b,a,c) <> 0. F by A1, A2, Th20;
then (- (1_ F)) * (((ProJ (b,a,c)) ") * (ProJ (b,a,c))) = (- (1_ F)) * (1_ F) by VECTSP_1:def 10;
then ((- (1_ F)) * ((ProJ (b,a,c)) ")) * (ProJ (b,a,c)) = (- (1_ F)) * (1_ F) by GROUP_1:def 3;
then ((- (1_ F)) * ((ProJ (b,a,c)) ")) * (ProJ (b,a,c)) = - (1_ F) ;
then (- (((ProJ (b,a,c)) ") * (1_ F))) * (ProJ (b,a,c)) = - (1_ F) by VECTSP_1:9;
then (- ((ProJ (b,a,c)) ")) * (ProJ (b,a,c)) = - (1_ F) ;
then ProJ (b,a,((- ((ProJ (b,a,c)) ")) * c)) = - (1_ F) by ;
then b _|_ by ;
then b _|_ by VECTSP_1:14;
then b _|_ by RLVECT_1:17;
then A4: ((- ((ProJ (b,a,c)) ")) * c) + a _|_ by Th2;
not a _|_ by ;
then ProJ (a,b,((- ((ProJ (b,a,c)) ")) * c)) = - (ProJ (((- ((ProJ (b,a,c)) ")) * c),b,a)) by ;
then ProJ (a,b,((- ((ProJ (b,a,c)) ")) * c)) = - (ProJ (c,b,a)) by A2, A3, Th2, Th15;
then (- ((ProJ (b,a,c)) ")) * (ProJ (a,b,c)) = - (ProJ (c,b,a)) by ;
then (- (((ProJ (b,a,c)) ") * (ProJ (a,b,c)))) * (- (1_ F)) = (- (ProJ (c,b,a))) * (- (1_ F)) by VECTSP_1:9;
then (((ProJ (b,a,c)) ") * (ProJ (a,b,c))) * (1_ F) = (- (ProJ (c,b,a))) * (- (1_ F)) by VECTSP_1:10;
then (((ProJ (b,a,c)) ") * (ProJ (a,b,c))) * (1_ F) = (ProJ (c,b,a)) * (1_ F) by VECTSP_1:10;
then ((ProJ (b,a,c)) ") * (ProJ (a,b,c)) = (ProJ (c,b,a)) * (1_ F) ;
hence ProJ (c,b,a) = ((ProJ (b,a,c)) ") * (ProJ (a,b,c)) ; :: thesis: verum