Given N, count the number of ordered triplets(a,b,c) whose product $abc \leq N$. I have found the series here . But I am not sure I understand Benoit Cloitre work which proposed an efficent way to compute. $O(N)$ computation is simply not good enough, I need something sublinear . Possibly between $O(N^{8/9})$ and $O(N^{2/3})$.
EDIT: As discussed with @henry here is my code from my broken interpretation of Andrew Lelechenko's formula