| Conjecturedno. of terms | Infinite |
|---|---|
| First terms | 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, 97, 113, 131, 199 |
| Largest known term | 108177207 − 1/9 |
| OEIS index |
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Apermutable prime, also known asanagrammatic prime, is aprime number which, in a givenbase, can have its digits' positions switched through anypermutation and still be a prime number.H. E. Richert, who is supposedly the first to study these primes, called them permutable primes,[1] but later they were also calledabsolute primes.[2]
In base 2, onlyrepunits can be permutable primes, because any 0 permuted to the ones place results in an even number. Therefore, the base 2 permutable primes are theMersenne primes. The generalization can safely be made that for anypositional number system, permutable primes with more than one digit can only have digits that arecoprime with theradix of the number system. One-digit primes, meaning any prime below the radix, are always trivially permutable.
Inbase 10, all the permutable primes with fewer than 49,081 digits are known
Where Rn := is a repunit, a number consisting only ofn ones (inbase 10). Any repunit prime is a permutable prime with the above definition, but some definitions require at least two distinct digits.[3]
Of the above, there are 16 unique permutation sets, with smallest elements
All permutable primes of two or more digits are composed from the digits 1, 3, 7, 9, because no prime number except 2 is even, and no prime number besides 5 is divisible by 5. It is proven[4] that no permutable prime exists which contains three different of the four digits 1, 3, 7, 9, as well as that there exists no permutable prime composed of two or more of each of two digits selected from 1, 3, 7, 9.
There is non-digit permutable prime for 3 <n < 6·10175 which is not a repunit.[1] It isconjectured that there are no non-repunit permutable primes other than the eighteen listed above. They can be split into seven permutation sets:
Inbase 12, the smallest elements of the unique permutation sets of the permutable primes with fewer than 9,739 digits are known (using inverted two and three for ten and eleven, respectively)
There is non-digit permutable prime in base 12 for 4 <n < 12144 which is not a repunit. It is conjectured that there are no non-repunit permutable primes in base 12 other than those listed above.
In base 10 and base 12, every permutable prime is a repunit or a near-repdigit, that is, it is a permutation of the integerP(b,n,x,y) =xxxx...xxxyb (n digits, in baseb)wherex andy are digits which is coprime tob. Besides,x andy must be also coprime (since if there is a primep divides bothx andy, thenp also divides the number), so ifx =y, thenx =y = 1. (This is not true in all bases, but exceptions are rare and could be finite in any given base; the only exceptions below 109 in bases up to 20 are: 13911, 36A11, 24713, 78A13, 29E19 (M. Fiorentini, 2015).)