A129926 Semiprimes s such that 3*s - 2 is a prime.

15, 21, 25, 33, 35, 51, 55, 65, 77, 91, 93, 95, 111, 123, 133, 141, 145, 155, 183, 201, 203, 205, 215, 221, 237, 247, 253, 287, 295, 303, 323, 341, 355, 365, 377, 391, 411, 413, 417, 427, 485, 497, 511, 515, 517, 527, 533, 537, 543, 553, 565, 581, 583, 597

Offset: 1

Author

Giovanni Teofilatto, Jun 06 2007

Keywords

• nonn

Comments

Indices of 3-almost prime octagonal numbers.

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Intersection of A153183 and A001358 (semiprimes).

Programs

Maple

isA001358 := proc(n) if numtheory[bigomega](n) = 2 then true ; else false ; end ; end: isA129926 := proc(n) if isA001358(n) then isprime(3*n-2) ; else false ; fi ; end: for n from 1 to 1000 do if isA129926(n) then printf("%d, ",n) ; fi ; od ; # R. J. Mathar, Jun 07 2007

Mathematica

Select[Range[600],PrimeOmega[#]==2&&PrimeQ[3#-2]&] (* James C. McMahon, Feb 02 2025 *)

PARI

list(lim)=my(v=List(),n); forprime(p=2,lim\2, forprime(q=2,min(lim\p,p), n=p*q; if(isprime(3*n-2), listput(v,n)))); Set(v) \\ Charles R Greathouse IV, Jan 31 2017

Extensions

More terms from R. J. Mathar, Jun 07 2007