An emirp (prime spelled backwards) is a prime number that results in a different prime when its 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.
The sequence of emirps begins 13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, 157... (sequence A006567 in OEIS).
All non-palindromic permutable primes are emirps.
As of November 2009, the largest known emirp is 10+941992101×10+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.
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