theorem Th28: :: BKMODEL4:37

for A being non empty convex Subset of (TOP-REAL 3)
for n, u, v being Element of (TOP-REAL 3) st ( for w being Element of (TOP-REAL 3) st w in A holds
|(n,w)| <> 0 ) & u in A & v in A holds
0 < |(n,u)| * |(n,v)|