> {5, 13, 21, 29, 53, ... I'm confused. For m = 21, x^2 - 21*y^2 == 4 has minimum solution (5,1) but x^2 - 21*y^2 == -4 has no solutions. For m = 45, x^2 - 45*y^2 == 4 has minimum solution (7,1) but x^2 - 45*y^2 == -4 has no solutions. So why is 21 included but 45 is not?