I am trying to solve problem 401 of Project Euler, without giving much away, I have broken down the problem into several summations and I am trying to calculate one part, which is:
$$ \sum_{i=1}^{N/2} i (N \mod i)$$
The N is really large $10^{15}$ and it is not computationally feasible to loop so many times. I want to avoid multi-threading because that feels like cheating, so I am looking for a way to reduce this problem into something easier.