let V be RealLinearSpace; :: thesis: for W1, W2 being Subspace of V
for v being VECTOR of V st V is_the_direct_sum_of W1,W2 holds
(v |-- W1,W2) `2 = (v |-- W2,W1) `1
let W1, W2 be Subspace of V; :: thesis: for v being VECTOR of V st V is_the_direct_sum_of W1,W2 holds
(v |-- W1,W2) `2 = (v |-- W2,W1) `1
let v be VECTOR of V; :: thesis: ( V is_the_direct_sum_of W1,W2 implies (v |-- W1,W2) `2 = (v |-- W2,W1) `1 )
assume A1: V is_the_direct_sum_of W1,W2 ; :: thesis: (v |-- W1,W2) `2 = (v |-- W2,W1) `1
then A2: (v |-- W1,W2) `2 in W2 by Def6;
A3: V is_the_direct_sum_of W2,W1 by A1, Lm16;
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) `2 = (v |-- W2,W1) `1 by A1, A2, A4, A5, Th53; :: thesis: verum