let K be Field; :: thesis: for V being VectSp of K
for W1, W2 being Subspace of V
for I1 being Basis of W1
for I2 being Basis of W2
for I being Subset of V st V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 holds
I is linearly-independent

let V be VectSp of K; :: thesis: for W1, W2 being Subspace of V
for I1 being Basis of W1
for I2 being Basis of W2
for I being Subset of V st V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 holds
I is linearly-independent

let W1, W2 be Subspace of V; :: thesis: for I1 being Basis of W1
for I2 being Basis of W2
for I being Subset of V st V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 holds
I is linearly-independent

let I1 be Basis of W1; :: thesis: for I2 being Basis of W2
for I being Subset of V st V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 holds
I is linearly-independent

let I2 be Basis of W2; :: thesis: for I being Subset of V st V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 holds
I is linearly-independent

let I be Subset of V; :: thesis: ( V is_the_direct_sum_of W1,W2 & I = I1 \/ I2 implies I is linearly-independent )
assume that
A1: V is_the_direct_sum_of W1,W2 and
A2: I = I1 \/ I2 ; :: thesis:
assume I is linearly-dependent ; :: thesis: contradiction
then consider l being Linear_Combination of I such that
A3: Sum l = 0. V and
A4: Carrier l <> {} ;
A5A: I1 /\ I2 = {} by ;
A5B: I1 misses I2 by ;
( the carrier of W1 c= the carrier of V & the carrier of W2 c= the carrier of V ) by VECTSP_4:def 2;
then reconsider II1 = I1, II2 = I2 as Subset of V by XBOOLE_1:1;
consider l1 being Linear_Combination of II1, l2 being Linear_Combination of II2 such that
A6: l = l1 + l2 by ;
reconsider ll1 = l1 as Linear_Combination of I by ;
reconsider ll2 = l2 as Linear_Combination of I by ;
A7: Sum l = (Sum ll1) + (Sum ll2) by ;
set v1 = Sum ll1;
set v2 = Sum ll2;
( Carrier ll1 c= I1 & Carrier ll2 c= I2 ) by VECTSP_6:def 4;
then A8: (Carrier ll1) /\ (Carrier ll2) = {} by ;
A10: Sum ll1 <> 0. V
proof end;
A13: Sum ll1 = - (Sum ll2) by ;
Sum ll1 in Lin II1 by VECTSP_7:7;
then Sum ll1 in Lin I1 by VECTSP_9:17;
then Sum ll1 in ModuleStr(# the carrier of W1, the addF of W1, the ZeroF of W1, the lmult of W1 #) by VECTSP_7:def 3;
then A14: Sum ll1 in W1 ;
Sum ll2 in Lin II2 by VECTSP_7:7;
then Sum ll2 in Lin I2 by VECTSP_9:17;
then Sum ll2 in ModuleStr(# the carrier of W2, the addF of W2, the ZeroF of W2, the lmult of W2 #) by VECTSP_7:def 3;
then Sum ll2 in W2 ;
then A15: Sum ll1 in W2 by ;
W1 /\ W2 = (0). V by ;
hence contradiction by A10, A14, A15, VECTSP_5:3, VECTSP_4:35; :: thesis: verum