$$ (x + y) ^ p \equiv x ^ p + y ^ p (modp) $$
$$ (x + 0)(x + 1) \cdots (x + (p - 1)) \equiv x ^ p - x(mod p)$$
当x
变成x + 1
时,左边不变,右边
$$ (x + 1) ^ p - (x + 1) \equiv \sum_{k = 0} ^ p C ^ k _ p x ^ k - (x + 1) \ \equiv x ^ p + 1 - (x + 1) \ \equiv x ^ p - x (modp) $$