为什么有
∏p∈P,p∣m!p−1p=∏p∈P,p≤mp−1p\prod\limits_{p \in\mathbb{P},p|m!}^{} \dfrac{p-1}{p} =\prod\limits_{p \in\mathbb{P},p \le m}^{} \dfrac{p-1}{p} p∈P,p∣m!∏pp−1=p∈P,p≤m∏pp−1
