In this case, the digits of 1999 add up to 28, which happens to be a perfect number. The answer is 1999. Pomerance calls any positive integer, n , that obeys the relationship n 11 -7 n a convenience store number. Indeed, number theory offers a host of problems that are remarkably easy to state but fiendishly difficult to solve. These so-called twins are scattered throughout the list of known prime numbers.
Statistical evidence suggests there are infinitely many twin primes.
No one, however, has yet proved that there is an infinite number of twin primes. Another series of problems involves iterated gaps between primes. Start with a list of prime numbers and calculate the difference between adjacent primes. Then calculate the differences between the differences, taking any negative values as positive. Continue the process.