This app lets you
• check numbers to see if they are prime numbers,
• find next prime,
• find primes within an interval (range), and
• determine prime factors.
The input numbers need be between 0 and 9,223,372,036,854,775,807, which is 2^63-1, or approximately 9.223·10^18. Just keep numbers less than 19 digits; a validation mechanism prevents illegal input.
This is my first iOS app, so it is quite simple. I developed it in a few days in January 2014, and it has been updated a few times since its version 1.0. It is very stable, and the few people who use it, have rated it 5.0.