theorem Th21: :: ANPROJ_9:25

for N being invertible Matrix of 3,F_Real
for a, b, c being non zero Element of F_Real st N = <*<*a,0,0*>,<*0,b,0*>,<*0,0,c*>*> holds
( (homography N) . Dir100 = Dir100 & (homography N) . Dir010 = Dir010 & (homography N) . Dir001 = Dir001 )