A177221 Numbers k that are the products of two distinct primes such that 2*k + 1 is also the product of two distinct primes.

10, 34, 38, 46, 55, 57, 77, 91, 93, 106, 118, 123, 129, 133, 143, 145, 159, 161, 177, 185, 201, 203, 205, 206, 213, 218, 226, 235, 259, 267, 291, 295, 298, 305, 314, 327, 334, 335, 339, 358, 365, 377, 381, 394, 395, 403, 407, 415, 417, 446, 447, 458, 466, 469

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

10 is in the sequence because 10 = 2*5 and 2*10+1 = 21 = 3*7.

Maple

isA006881:= proc(n) local F;
  F:= ifactors(n)[2];
  nops(F)=2 and F[1, 2]+F[2, 2]=2
end proc:
filter:= n -> andmap(isA006881, [n, 2*n+1]); select(filter, [$1..1000]); # Robert Israel, Nov 09 2017

Mathematica

f[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; Do[If[f[n]&&f[2*n+1], AppendTo[lst, n]], {n, 0, 3*6!}]; lst
Select[Range[500], PrimeNu[#]==PrimeOmega[#]==PrimeNu[2#+1] == PrimeOmega[ 2#+1] == 2&] (* Harvey P. Dale, Feb 22 2018 *)

Crossrefs

Cf. A006881, A111153, A177210, A177211, A177212, A177213, A177214, A177215, A177216, A177217, A177220.

Sequence in context: A140866 A219848 A320565 * A045087 A324746 A119086

Adjacent sequences: A177218 A177219 A177220 * A177222 A177223 A177224

Keyword

nonn

Author

Vladimir Joseph Stephan Orlovsky, May 05 2010

Status

approved