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

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

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: A308238 A118865 A118608 * A075034 A363770 A022110

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