It really bothered me. So, I googled. All links give the same solution: one, two, three.

Find the largest prime factor of a composite number.

The problem of integer factorization is one of the most important in the number theory. Last time I implemented a classical algorithm with complexity. The author of the Project Euler website suggests a small improvement --- iterating over only odd numbers, but I consider this as too tiny thing to do.

As regards the problem of factorization, I'd rather look in special literature, what are the known approaches (and I'll do that later, let's switch to the next one).