摘要：题目 d(k)表示k的所有约数的和。d(6) = 1 + 2 + 3 + 6 = 12。 定义S(N) = ∑1 include include include include include include include const int maxlongint=2147483647; cons 阅读全文

摘要：题目 已知 $f(x)=\sum_{d|x}μ(d)∗d$ 现在请求出下面式子的值 $\sum_{i=1}^{n}\sum_{j=1}^{n}f(gcd(i,j))∗f(lcm(i,j))$ 由于值可能过大所以请对 10^9+7 取模 $n≤10^9$ 分析 $f$为积性函数， 因为$lcm(i,j 阅读全文