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A122183 Primes p_i by index i for which the period length of 1/p_i is a semiprime. 1
4, 6, 9, 11, 14, 15, 17, 19, 20, 26, 27, 34, 39, 41, 43, 56, 59, 61, 62, 64, 76, 83, 85, 86, 96, 101, 102, 109, 111, 112, 119, 124, 138, 140, 144, 147, 149, 150, 154, 161, 166, 168, 170, 171, 175, 192, 198, 203, 216, 219, 222, 224, 225, 235, 236, 239, 240, 246, 251 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Semiprime analog of A072859 based on A002371.

Numbers n such that A002371(n) is an element of A001358.

LINKS

Table of n, a(n) for n=1..59.

EXAMPLE

a(1) = 4 because A002371(4) Period of decimal expansion of 1/(4th prime) = 6 = 2*3, a semiprime.

a(2) = 6 because A002371(6) = 6 = 2*3.

a(3) = 9 because A002371(9) = 22 = 2*11.

a(4) = 11 because A002371(11) = 15 = 3*5.

a(5) = 14 because A002371(14) = 21 = 3*7.

a(6) = 15 because A002371(15) = 46 = 2*23.

a(7) = 17 because A002371(17) = 58 = 2*29.

MATHEMATICA

semiprimeQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; PrimePi /@ Select[Prime@ Range@ 254, semiprimeQ@ MultiplicativeOrder[10, # ] &] (* Robert G. Wilson v *)

CROSSREFS

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

Sequence in context: A190001 A189532 A094550 * A234373 A189756 A133578

Adjacent sequences:  A122180 A122181 A122182 * A122184 A122185 A122186

KEYWORD

base,easy,nonn

AUTHOR

Jonathan Vos Post, May 10 2007

EXTENSIONS

Edited, corrected and extended by Robert G. Wilson v, May 22 2007

STATUS

approved

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Last modified April 22 06:14 EDT 2019. Contains 322329 sequences. (Running on oeis4.)