Yes they can be ( http://www.murderousmaths.co.uk/games/primcal.htm http://www.murderousmaths.co.uk/games/primcal.htm, http://primes.utm.edu/notes/faq/six.html

Yes they can be ( http://www.murderousmaths.co.uk/games/primcal.htm http://www.murderousmaths.co.uk/games/primcal.htm, http://primes.utm.edu/notes/faq/six.html http://primes.utm.edu/notes/faq/six.html ) but the unfortunate thing is that while the ((p^2)-1 % 24 == 0) test is a necessary requirement for primality testing, it is not sufficient. Lots of non-primes like 25, 49, 155 etc. also have 0 remainders.