Here's a way to find small factors. I think that finding large factors (up to the square root of $n$) is as hard as factoring in general.

Here's a way to find small factors. I think that finding large factors (up to the square root of $n$) is as hard as factoring in general.

Edit: Maybe the method in the paper I linked is not so great because it looks for small factors in a set of number. But it may cite more relevant papers.