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A129727 Primes p for which the period length of 1/p is a semiprime. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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).

LINKS

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

EXAMPLE

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.

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 *)

CROSSREFS

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

Sequence in context: A048449 A147812 A043884 * A275897 A227786 A270792

Adjacent sequences:  A129724 A129725 A129726 * A129728 A129729 A129730

KEYWORD

base,easy,nonn

AUTHOR

Jonathan Vos Post, May 12 2007

STATUS

approved

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Last modified May 23 07:46 EDT 2019. Contains 323508 sequences. (Running on oeis4.)