theorem Th15: :: ANALMETR:15

for V being RealLinearSpace
for u, u1, v, v1 being VECTOR of V holds
( [[u,v],[u1,v1]] in lambda (DirPar V) iff ex a, b being Real st
( a * (v - u) = b * (v1 - u1) & ( a <> 0 or b <> 0 ) ) )