An emirp (prime spelled backwards) 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 may be used to mean the same as emirp, but may also, ambiguously, include the palindromic primes.
Contents
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, ... (sequence A006567 in the OEIS).
All non-palindromic permutable primes are emirps.
As of November 2009, the largest known emirp is 1010006+941992101×104999+1, found by Jens Kruse Andersen in October 2007.
The term 'emirpimes' (singular) is used also in places to treat semiprimes in a similar way. That is, an emirpimes is a semiprime that is also a (distinct) semiprime upon reversing its digits.
Other bases
The emirps in base 12 are (using A and B for ten and eleven, respectively)
15, 51, 57, 5B, 75, B5, 107, 117, 11B, 12B, 13B, 145, 157, 16B, 17B, 195, 19B, 1A7, 1B5, 507, 51B, 541, 577, 587, 591, 59B, 5B1, 5BB, 701, 705, 711, 751, 76B, 775, 785, 7A1, 7BB, B11, B15, B21, B31, B61, B67, B71, B91, B95, BB5, BB7, ...Emirps with added mirror properties
There is a subset of emirps x, with mirror xm, such that x is the yth prime, and xm is the ymth prime. (E.g. 73 is the 21st prime number; its mirror, 37, is the 12th prime number; 12 is the mirror of 21.)