let V be RealUnitarySpace; for v being VECTOR of V
for W1, W2 being Subspace of V st V is_the_direct_sum_of W1,W2 holds
(v |-- (W1,W2)) `1 = (v |-- (W2,W1)) `2
let v be VECTOR of V; for W1, W2 being Subspace of V st V is_the_direct_sum_of W1,W2 holds
(v |-- (W1,W2)) `1 = (v |-- (W2,W1)) `2
let W1, W2 be Subspace of V; ( V is_the_direct_sum_of W1,W2 implies (v |-- (W1,W2)) `1 = (v |-- (W2,W1)) `2 )
assume A1:
V is_the_direct_sum_of W1,W2
; (v |-- (W1,W2)) `1 = (v |-- (W2,W1)) `2
then A2:
(v |-- (W1,W2)) `2 in W2
by Def6;
A3:
V is_the_direct_sum_of W2,W1
by A1, Lm15;
then A4:
( v = ((v |-- (W2,W1)) `2) + ((v |-- (W2,W1)) `1) & (v |-- (W2,W1)) `1 in W2 )
by Def6;
A5:
(v |-- (W2,W1)) `2 in W1
by A3, Def6;
( v = ((v |-- (W1,W2)) `1) + ((v |-- (W1,W2)) `2) & (v |-- (W1,W2)) `1 in W1 )
by A1, Def6;
hence
(v |-- (W1,W2)) `1 = (v |-- (W2,W1)) `2
by A1, A2, A4, A5, Th45; verum