let V be RealLinearSpace; :: thesis: for v1, v2 being VECTOR of V holds
( ( v1 <> v2 & {v1,v2} is linearly-independent ) iff for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 ) )

let v1, v2 be VECTOR of V; :: thesis: ( ( v1 <> v2 & {v1,v2} is linearly-independent ) iff for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 ) )

thus ( v1 <> v2 & {v1,v2} is linearly-independent implies for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 ) ) :: thesis: ( ( for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 ) ) implies ( v1 <> v2 & {v1,v2} is linearly-independent ) )
proof
assume A1: ( v1 <> v2 & {v1,v2} is linearly-independent ) ; :: thesis: for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 )

let a, b be Real; :: thesis: ( (a * v1) + (b * v2) = 0. V implies ( a = 0 & b = 0 ) )
assume that
A2: (a * v1) + (b * v2) = 0. V and
A3: ( a <> 0 or b <> 0 ) ; :: thesis: contradiction
per cases ( a <> 0 or b <> 0 ) by A3;
suppose A4: a <> 0 ; :: thesis: contradiction
0. V = (a ") * ((a * v1) + (b * v2)) by A2
.= ((a ") * (a * v1)) + ((a ") * (b * v2)) by RLVECT_1:def 5
.= (((a ") * a) * v1) + ((a ") * (b * v2)) by RLVECT_1:def 7
.= (((a ") * a) * v1) + (((a ") * b) * v2) by RLVECT_1:def 7
.= (1 * v1) + (((a ") * b) * v2) by
.= v1 + (((a ") * b) * v2) by RLVECT_1:def 8 ;
then v1 = - (((a ") * b) * v2) by RLVECT_1:6
.= (- 1) * (((a ") * b) * v2) by RLVECT_1:16
.= ((- 1) * ((a ") * b)) * v2 by RLVECT_1:def 7 ;
hence contradiction by A1, Th12; :: thesis: verum
end;
suppose A5: b <> 0 ; :: thesis: contradiction
0. V = (b ") * ((a * v1) + (b * v2)) by A2
.= ((b ") * (a * v1)) + ((b ") * (b * v2)) by RLVECT_1:def 5
.= (((b ") * a) * v1) + ((b ") * (b * v2)) by RLVECT_1:def 7
.= (((b ") * a) * v1) + (((b ") * b) * v2) by RLVECT_1:def 7
.= (((b ") * a) * v1) + (1 * v2) by
.= (((b ") * a) * v1) + v2 by RLVECT_1:def 8 ;
then v2 = - (((b ") * a) * v1) by RLVECT_1:def 10
.= (- 1) * (((b ") * a) * v1) by RLVECT_1:16
.= ((- 1) * ((b ") * a)) * v1 by RLVECT_1:def 7 ;
hence contradiction by A1, Th12; :: thesis: verum
end;
end;
end;
hence contradiction ; :: thesis: verum
end;
assume A6: for a, b being Real st (a * v1) + (b * v2) = 0. V holds
( a = 0 & b = 0 ) ; :: thesis: ( v1 <> v2 & {v1,v2} is linearly-independent )
A7: now :: thesis: for a being Real holds not v1 = a * v2
let a be Real; :: thesis: not v1 = a * v2
assume v1 = a * v2 ; :: thesis: contradiction
then v1 = (0. V) + (a * v2) ;
then 0. V = v1 - (a * v2) by RLSUB_2:61
.= v1 + (- (a * v2)) by RLVECT_1:def 11
.= v1 + (a * (- v2)) by RLVECT_1:25
.= v1 + ((- a) * v2) by RLVECT_1:24
.= (1 * v1) + ((- a) * v2) by RLVECT_1:def 8 ;
hence contradiction by A6; :: thesis: verum
end;
now :: thesis: not v2 = 0. V
assume A8: v2 = 0. V ; :: thesis: contradiction
0. V = (0. V) + (0. V)
.= (0 * v1) + (0. V) by RLVECT_1:10
.= (0 * v1) + (1 * v2) by A8 ;
hence contradiction by A6; :: thesis: verum
end;
hence ( v1 <> v2 & {v1,v2} is linearly-independent ) by ; :: thesis: verum