Factorials and Prime Numbers

[manochsa]

Is there a way to determine whether n! +/- 1 will be a prime number?

For example if n = 4:
4! + 1 = 25 --> not a prime number
4! -1 = 23 --> a prime number

Thanks,

Sanjay

[Ben Ku]

To determine whether a number is prime is hard enough; determining whether one more than a factorial is prime is very challenging. I'm sure you can find some proof for this somewhere, but it is not needed for the GMAT.

If you need more clarification, please give an example of a problem where this idea is needed might be needed so we can provide a helpful response.