A072859 Primes p for which the period of 1/p is prime.

11, 37, 41, 53, 79, 83, 107, 173, 227, 239, 271, 317, 347, 359, 467, 479, 563, 587, 643, 719, 733, 773, 797, 839, 907, 1031, 1187, 1231, 1283, 1307, 1319, 1439, 1493, 1523, 1627, 1637, 1879, 1907, 1987, 2027, 2039, 2467, 2477, 2677, 2791, 2837, 2879, 2963

Offset: 1

Author

Benoit Cloitre, Jul 26 2002

Keywords

• base
• easy
• nonn

Comments

Primes p such that the multiplicative order of 10 (mod p) is prime. - Joerg Arndt, Oct 26 2014

Together with 3, complement in primes of A249330. - Arkadiusz Wesolowski, Oct 25 2014

Examples

			1/37 = 0. 027 027 ... with period = 3, hence 37 is in the sequence.
		

Links

• Ray Chandler, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002371, A249330.

Programs

Mathematica

Select[Prime[Range[500]],PrimeQ[MultiplicativeOrder[10,#]]&] (* Ray Chandler, Oct 31 2011 *)

PARI

f(n)=if(n<4,n==2,znorder(Mod(10, prime(n))));
for(n=1,100,if(isprime(f(n))==1,print1(prime(n),", ")))