let V be RealLinearSpace; :: thesis: for W1, W2 being Subspace of V st V is_the_direct_sum_of W1,W2 holds
V is_the_direct_sum_of W2,W1
let W1, W2 be Subspace of V; :: thesis: ( V is_the_direct_sum_of W1,W2 implies V is_the_direct_sum_of W2,W1 )
assume V is_the_direct_sum_of W1,W2 ; :: thesis: V is_the_direct_sum_of W2,W1
then ( RLSStruct(# the carrier of V,the U2 of V,the addF of V,the Mult of V #) = W1 + W2 & W1 /\ W2 = (0). V ) by Def4;
then ( RLSStruct(# the carrier of V,the U2 of V,the addF of V,the Mult of V #) = W2 + W1 & W2 /\ W1 = (0). V ) by Lm1, Th18;
hence V is_the_direct_sum_of W2,W1 by Def4; :: thesis: verum