Emirp

An emirp is a prime number that results in a different prime when its decimal digits are reversed. This definition excludes the related palindromic primes. The term reversible prime is used to mean the same as emirp, but may also, ambiguously, include the palindromic primes.
The sequence of emirps begins 13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, 157, 167, 179, 199, 311, 337, 347, 359, 389, 701, 709, 733, 739, 743, 751, 761, 769, 907, 937, 941, 953, 967, 971, 983, 991,....
The difference in all pairs of emirps is always a multiple of 18. This follows from all primes bigger than 2 being odd and from differences between pairs of natural numbers with reversed digits being multiples of 9.
All non-palindromic permutable primes are emirps.
It is not known whether there are infinitely many emirps.