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A176241 Numbers n such that n-th semiprime-n is prime 3
1, 20, 21, 32, 33, 34, 35, 44, 45, 54, 56, 66, 67, 86, 90, 106, 107, 108, 122, 130, 134, 140, 141, 147, 154, 155, 156, 169, 170, 172, 182, 189, 190, 210, 211, 212, 220, 224, 225, 226, 242, 248, 249, 256, 279, 280, 281, 312, 316, 318, 321, 324, 329, 338, 344 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

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

MATHEMATICA

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 *)

CROSSREFS

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

Sequence in context: A014368 A118865 A118608 * A075034 A022110 A041808

Adjacent sequences:  A176238 A176239 A176240 * A176242 A176243 A176244

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Apr 12 2010

EXTENSIONS

More terms from R. J. Mathar, Apr 13 2010

STATUS

approved

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Last modified March 18 22:11 EDT 2019. Contains 321305 sequences. (Running on oeis4.)