怎么在 O(nlog2n)O(n\log^2n)O(nlog2n) 或 O(nlogn)O(n\log n)O(nlogn) 时复之内求出:
∑i=1n∑j=i+1ngcd(ai,aj)\sum_{i=1}^{n}\sum_{j=i+1}^n \gcd(a_i,a_j)∑i=1n∑j=i+1ngcd(ai,aj)