\(id_n = n\)
\(1_n = 1\)
\(\epsilon_n = [n == 1]\)
\(\sigma_n\)为\(n\)的约数和
\(d_n\)为\(n\)的约数个数
\((f \times g) (n) = \sum_{d | n} f(d) \times g(\frac{n}{d})\)
\((\mu \times I) = \epsilon\)
\(\phi \times I = id_n\)
\(id_n \times \mu = \phi_n\)
原文：https://www.cnblogs.com/Hock/p/13095147.html