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://roosephu.github.io/2013/06/16/PE-432/
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
