source 2007 Bernstein--Lange assume Z1=1 compute XX = X1^2 compute YY = Y1^2 compute YYYY = YY^2 compute S = 2((X1+YY)^2-XX-YYYY) compute M = 3 XX+a compute T = M^2-2 S compute X3 = T compute Y3 = M(S-T)-8 YYYY compute Z3 = 2 Y1