Prime numbers

Hi!I got a question.How can i tell the engine that if a / b = a rational number,something happens?Or more directly,how can i make the engine check if a number is prime?

engine cannot decide is the number prime or not. You can start here http://en.wikipedia.org/wiki/Primality_test

Ok,thanks,but can u answer to my first question?"How can i tell the engine that if a / b = a rational number,something happens?"

for instance let's say that a = 5 and b = 2

if(a/b == integer(a/b))
{
do somethign
}

or:
if(!fraction(a/b))
{
do something
}

Thank you!This is exacly what i was looking for!