- https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
- https://projecteuler.net/problem=10
-
https://www.shuizilong.com/house/archives/project-euler-521-smallest-prime-factor/
- http://yhylord.logdown.com/posts/2014/04/06/linear-sieve-and-multiplicative-function
- http://quartergeek.com/eight-gcd-problems/
- http://acm.hust.edu.cn/vjudge/contest/view.action?cid=80079#overview
