A122183 - OEIS

%I #9 Jul 11 2015 11:27:58

%S 4,6,9,11,14,15,17,19,20,26,27,34,39,41,43,56,59,61,62,64,76,83,85,86,

%T 96,101,102,109,111,112,119,124,138,140,144,147,149,150,154,161,166,

%U 168,170,171,175,192,198,203,216,219,222,224,225,235,236,239,240,246,251

%N Primes p_i by index i for which the period length of 1/p_i is a semiprime.

%C Semiprime analog of A072859 based on A002371.

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

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

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

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

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

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

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

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

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

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

%K base,easy,nonn

%O 1,1

%A _Jonathan Vos Post_, May 10 2007

%E Edited, corrected and extended by _Robert G. Wilson v_, May 22 2007