A prime number is a positive integer greater than 1 that can be divided without a remainder only by 1 and by itself. The list of prime numbers start with: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29... This list is infinite.

Essentially, a prime number calculator can determine if a number is prime or not. It may also be able to find the next prime number after and/or before a specific value. AddUp uses three prime number functions to implement these facilities:

- prime: returns 1 if its argument is prime, 0 otherwise.
- prNext: return the next prime number after its argument.
- prPrev: return the previous prime number after its argument.

Since determining with certainty if a number is prime can be an extremely long process, AddUp has a limit on values that can be examined with these functions (which is still reasonably high).