The cross-ratio (z₁, z₂; z₃, z₄) of four distinct points on a projective line P¹ is the fundamental projective invariant: it is preserved by every Möbius transformation z ↦ (az+b)/(cz+d) with ad − bc ≠ 0. Takes 6 values under permutations…
The cross-ratio (z₁, z₂; z₃, z₄) of four distinct points on a projective line P¹ is the fundamental projective invariant: it is preserved by every Möbius transformation z ↦ (az+b)/(cz+d) with ad − bc ≠ 0. Takes 6 values under permutations…