http://en.wikipedia.org/wiki/Gothic_Chess
http://mathworld.wolfram.com/PowerTower.html
http://math.eretrandre.org/tetrationforum/index.php
http://math.eretrandre.org/tetrationforum/member.php?action=profile&uid=47
http://en.wikipedia.org/wiki/Lucky_number
http://en.wikipedia.org/wiki/Prime_twins
http://arxiv.org/
http://mathoverflow.net/