This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Numbers in the form of 2 n − 1 are only prime if n is prime. Since 91 can be factorised into 1 3 × 7 , the number is not prime. However numbers in this form that are prime are called Mersenne Primes .