Anyone care to elaborate on the downvote? Given that the parent seemed surprised that it was possible to factorise a &#x27;big&#x27; number quickly on a modern PC, I thought I&#x27;d elaborate.<p>Brute force search is O(N), the sieve of Eratosthenes is O(N log(log(N))). The Quadratic sieve is good up to 100 digits and runs in O(exp(sqrt(log n log log n))). Some utilities you can use for this are YAFU, MSIEVE or GGNFS.