let V be torsion-free Z_Module; :: thesis: for W1, W2, W3 being free finite-rank Subspace of V
for a being Element of INT.Ring st a <> 0. INT.Ring & W3 = a (*) W1 holds
rank (W3 + W2) = rank (W1 + W2)
let W1, W2, W3 be free finite-rank Subspace of V; :: thesis: for a being Element of INT.Ring st a <> 0. INT.Ring & W3 = a (*) W1 holds
rank (W3 + W2) = rank (W1 + W2)
let a be Element of INT.Ring; :: thesis: ( a <> 0. INT.Ring & W3 = a (*) W1 implies rank (W3 + W2) = rank (W1 + W2) )
assume A1: ( a <> 0. INT.Ring & W3 = a (*) W1 ) ; :: thesis: rank (W3 + W2) = rank (W1 + W2)
for v being Vector of V st v in W3 + W2 holds
v in W1 + W2

proof

let v be Vector of V; :: thesis: ( v in W3 + W2 implies v in W1 + W2 )
assume B1: v in W3 + W2 ; :: thesis: v in W1 + W2
consider v1, v2 being Vector of V such that
B2: ( v1 in W3 & v2 in W2 & v = v1 + v2 ) by B1, ZMODUL01:92;
v1 in W1 by B2, A1;
hence v in W1 + W2 by B2, ZMODUL01:92; :: thesis: verum

end;

then W3 + W2 is Subspace of W1 + W2 by ZMODUL01:44;
then A2: rank (W3 + W2) <= rank (W1 + W2) by ZMODUL05:2;
reconsider aW = a (*) (W1 + W2) as free finite-rank Subspace of V by ZMODUL01:42;
for v being Vector of V st v in a (*) (W1 + W2) holds
v in W3 + W2

proof

let v be Vector of V; :: thesis: ( v in a (*) (W1 + W2) implies v in W3 + W2 )
assume B1: v in a (*) (W1 + W2) ; :: thesis: v in W3 + W2
consider vx being Vector of (W1 + W2) such that
B2: v = a * vx by B1;
reconsider vvx = vx as Vector of V by ZMODUL01:25;
vvx in W1 + W2 ;
then consider v1, v2 being Vector of V such that
B3: ( v1 in W1 & v2 in W2 & vvx = v1 + v2 ) by ZMODUL01:92;
B4: v = a * vvx by B2, ZMODUL01:29
.= (a * v1) + (a * v2) by VECTSP_1:def 14, B3 ;
reconsider vv1 = v1 as Vector of W1 by B3;
a * vv1 in a * W1 ;
then B5: a * v1 in W3 by A1, ZMODUL01:29;
a * v2 in W2 by B3, ZMODUL01:37;
hence v in W3 + W2 by B4, B5, ZMODUL01:92; :: thesis: verum

end;

then aW is Subspace of W3 + W2 by ZMODUL01:44;
then rank (a (*) (W1 + W2)) <= rank (W3 + W2) by ZMODUL05:2;
then rank (W1 + W2) <= rank (W3 + W2) by A1, ThRankS1;
hence rank (W3 + W2) = rank (W1 + W2) by A2, XXREAL_0:1; :: thesis: verum