hi All, what is the proper name of expressions r = (a/b +- c/d) where a,b,c,d are integers or square roots of integers? Their count a[n] of non-negative r given the a,b,c,d have values <= n is 2, 115, 1471, 7065, 30215, 66303 for n= 1...6 nothing on superseek. Anyone care to extend or to suggest pointers / links? W.