source 1987 Montgomery "Speeding the Pollard and elliptic curve methods of factorization", page 261, sixth display parameter a24 assume 4 a24 = a+2 compute X3 = (X1+Z1)^2 (X1-Z1)^2 compute Z3 = (4 X1 Z1)((X1-Z1)^2 + a24(4 X1 Z1))