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A176241 Numbers n such that n-th semiprime-n is prime 3

%I

%S 1,20,21,32,33,34,35,44,45,54,56,66,67,86,90,106,107,108,122,130,134,

%T 140,141,147,154,155,156,169,170,172,182,189,190,210,211,212,220,224,

%U 225,226,242,248,249,256,279,280,281,312,316,318,321,324,329,338,344

%N Numbers n such that n-th semiprime-n is prime

%C Or, numbers n such that A175708(n) is prime. [Zak Seidov, Apr 12 2011]

%H Harvey P. Dale, <a href="/A176241/b176241.txt">Table of n, a(n) for n = 1..1000</a>

%e a(1)=1 because A001358(1)-1=4-1=3=prime.

%t Module[{nn=1500,sems},sems=Select[Range[nn],PrimeOmega[#]==2&];Select[ Thread[ {sems,Range[Length[sems]]}],PrimeQ[#[[1]]-#[[2]]]&]][[All,2]] (* _Harvey P. Dale_, Oct 14 2018 *)

%Y Cf. A175708, A176240. [Zak Seidov, Apr 12 2011]

%K nonn

%O 1,2

%A _Juri-Stepan Gerasimov_, Apr 12 2010

%E More terms from _R. J. Mathar_, Apr 13 2010

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Last modified April 18 22:08 EDT 2019. Contains 322237 sequences. (Running on oeis4.)