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 A072859 Primes p for which the period length of 1/p is prime. 8
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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 LINKS Ray Chandler, Table of n, a(n) for n = 1..10000 EXAMPLE 1/37== 0.027027...with period length = 3, hence 37 is in the sequence. MATHEMATICA Select[Prime[Range[500]], PrimeQ[MultiplicativeOrder[10, #]]&] (* Ray Chandler, Oct 31 2011 *) PROG (PARI) a(n)=if(n<4, n==2, znorder(Mod(10, prime(n)))) ? for(n=1, 100, if(isprime(a(n))==1, print1(prime(n), ", "))) CROSSREFS Cf. A002371, A249330. Sequence in context: A195201 A233403 A054293 * A125744 A116057 A099227 Adjacent sequences:  A072856 A072857 A072858 * A072860 A072861 A072862 KEYWORD base,easy,nonn AUTHOR Benoit Cloitre, Jul 26 2002 STATUS approved

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Last modified June 19 11:41 EDT 2019. Contains 324219 sequences. (Running on oeis4.)