|Number of known terms||20|
|Conjectured number of terms||Infinite|
|First terms||2, 3, 5, 7, 11, 13, 17, 37, 79, 113, 199|
|Largest known term||(10270343-1)/9|
A permutable prime, also known as anagrammatic prime, is a prime number which, in a given base, can have its digits' positions switched through any permutation and still be a prime number. H. E. Richert, who is supposed to be the first to study these primes, called them permutable primes, but later they were also called absolute primes.
In base 10, all the permutable primes with fewer than 49,081 digits are known
- 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, 97, 113, 131, 199, 311, 337, 373, 733, 919, 991, R19 (1111111111111111111), R23, R317, R1031, ... (sequence A003459 in OEIS)
Of the above, there are 16 unique permutation sets, with smallest elements
- 2, 3, 5, 7, R2, 13, 17, 37, 79, 113, 199, 337, R19, R23, R317, R1031, ... (sequence A258706 in OEIS)
Note Rn = is a repunit, a number consisting only of n ones (in base 10). Any repunit prime is a permutable prime with the above definition, but some definitions require at least two distinct digits.
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 proved 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.
In base 2, only repunits can be permutable primes, because any 0 permuted to the ones place results in an even number. Therefore the base 2 permutable primes are the Mersenne primes. The generalization can safely be made that for any positional number system, permutable primes with more than one digit can only have digits that are coprime with the radix of the number system. One-digit primes, meaning any prime below the radix, are always trivially permutable.
In duodecimal, the permutable primes are (unique permutation sets, with smallest elements)
- 2, 3, 5, 7, Ɛ, R2, 15, 57, 5Ɛ, R3, 117, 11Ɛ, 555Ɛ, R5, R17, R81, R91, R225, R255, R4ᘔ5, ...
In base 10, every permutable prime is a repunit or a near-repdigit, that is, it is a permutation of the integer P(b, n, x, y) = xxxx...xxxyb (n digits, in base b) where x and y are digits which is coprime to b, if x = y, then x = 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).
Let P(b, n, x, y) be a permutable prime in base b and let p be a prime such that n > p. If b is a primitive root of p, and p does not divide x, then n is a multiple of p - 1.
- Richert, Hans-Egon (1951). "On permutable primtall". Norsk Matematiske Tiddskrift 33: 50–54. Zbl 0054.02305.
- Bhargava, T.N.; Doyle, P.H. (1974). "On the existence of absolute primes". Math. Mag. 47: 233. Zbl 0293.10006.
- Chris Caldwell, The Prime Glossary: permutable prime at The Prime Pages.
- A.W. Johnson, "Absolute primes," Mathematics Magazine 50 (1977), 100–103.