如果 N = p1^c1 * p2^c2 * ... *pk^ck约数个数: (c1 + 1) * (c2 + 1) * ... * (ck + 1)约数之和: (p1^0 + p1^1 + ... + p1^c1) * ... * (pk^0 + pk^1 + ... + pk^ck)