source 1986 Chudnovsky--Chudnovsky "Sequences of numbers generated by addition in formal groups and new primality and factorization tests", page 415, formula (4.4i) compute U1 = X1 Z2 compute U2 = X2 Z1 compute S1 = Y1 Z2 compute S2 = Y2 Z1 compute W = Z1 Z2 compute P = U2-U1 compute R = S2-S1 compute X3 = P(-(U1+U2)P^2+W R^2) compute Y3 = (R(-2 W R^2+3(U1+U2)P^2)-P^3(S1+S2))/2 compute Z3 = W P^3