For divisibility rules--http://en.wikipedia.org/wiki/Divisibility_rule
for downloading free science books-- www.iitdreams.co.in/
determining whether an input number is prime or not --http://en.wikipedia.org/wiki/Primality_test
http://en.wikipedia.org/wiki/Prime_number
mental calculation help:--http://en.wikibooks.org/wiki/Mental_Math
Vedic mathematics--http://en.wikipedia.org/wiki/Bharati_Krishna_Tirtha's_Vedic_mathematics
http://www.vedicmathsindia.org/
http://www.vedicmaths.org/
some other :-
http://betterexplained.com/
http://www.math.com/
