an±1 fora=2,3,5,6,7,10,11,12 and large exponents n [http://homes.cerias.purdue.edu/~ssw/cun/index.html]an±1 fora≤13 anda not a perfect number [http://wwwmaths.anu.edu.au/~brent/factors.html]2n±1 forn≤100 [http://www.euronet.nl/users/bota/medium-p.htm]10n±1 forn≤100 [http://www.swox.com/gmp/repunit.html]pp±1 wherep is a prime number andp<180 . [http://homes.cerias.purdue.edu/~ssw/bell/r1]22n+1 (Fermat numbers) [http://www.prothsearch.net/fermat.html]