On the Discriminant x'Ax.y'Ay-(x'Ay)2
Hans Schwerdtfeger · Canadian Mathematical Bulletin · 1958
Let x, y be column matrices of n real homogeneous coordinates xj, yj (j = 1,..., n) representing points in (n - 1) - dimensional real projective space Pn - 1. Let A be an n × n real symmetric matrix. The equation x' Ax = 0 represents a quadric in Pn - 1 and the equation