A129727 Primes p for which the period length of 1/p is a semiprime.

7, 13, 23, 31, 43, 47, 59, 67, 71, 101, 103, 139, 167, 179, 191, 263, 277, 283, 293, 311, 383, 431, 439, 443, 503, 547, 557, 599, 607, 613, 653, 683, 787, 809, 827, 853, 859, 863, 887, 947, 983, 997, 1013, 1019, 1039, 1163, 1213, 1237, 1321, 1367, 1399, 1423

Offset: 1

Author

Jonathan Vos Post, May 12 2007

Keywords

• base
• easy
• nonn

Comments

The prime index of A122183. Semiprime analog of A072859 = primes p for which the period length of 1/p is prime. Based upon A002371 = period of decimal expansion of 1/(n-th prime).

Examples

			a(1) = 7 because A000040(4) Period of decimal expansion of 1/7 = 6 = 2*3, a semiprime.
a(2) = 13 because A000040(6) = 6 = 2*3.
a(3) = 23 because A000040(9) = 22 = 2*11.
a(4) = 31 because A000040(11) = 15 = 3*5.
a(5) = 43 because A000040(14) = 21 = 3*7.
a(6) = 47 because A000040(15) = 46 = 2*23.
a(7) = 59 because A000040(17) = 58 = 2*29.
		

Links

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

Crossrefs

Cf. A000040, A001358, A002371, A072859, A128948.

Programs

Mathematica

fQ[p_] := Plus @@ Last /@ FactorInteger@Length@RealDigits[1/p][[1, 1]] == 2;; lst = {}; Do[ p = Prime@n; If[ fQ@p, AppendTo[lst, p]], {n, 230}] (* Robert G. Wilson v *)