摘要：
$$
F(n) = \sum_{d | n}f(d) \Leftrightarrow f(n) = \sum_{d | n}\mu(d)F(\dfrac nd) \\
F(n) = \sum_{n | d}f(d) \Leftrightarrow f(n) = \sum_{n | d}\mu(d)F(\dfrac nd)
$$ 阅读全文
摘要：
$$
\varphi(n) = \sum_{i = 1}^n[\gcd(i, n) = 1]
$$ 阅读全文