This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

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