摘要： 求∑1<=i<=n∑1<=j<=ngcd(i,j) % P P = 10^9 + 7 2 <= n <= 10^10 这道题，明显就是杜教筛 推一下公式： 利用∑d|nphi(d) = n ans = ∑1<=i<=n∑1<=j<=n∑d|(i,j)phi(d) = ∑1<=d<=n∑1<=i<=n 阅读全文