A129926 - OEIS

%I #18 Feb 11 2025 17:46:23

%S 15,21,25,33,35,51,55,65,77,91,93,95,111,123,133,141,145,155,183,201,

%T 203,205,215,221,237,247,253,287,295,303,323,341,355,365,377,391,411,

%U 413,417,427,485,497,511,515,517,527,533,537,543,553,565,581,583,597

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

%C Indices of 3-almost prime octagonal numbers.

%H Charles R Greathouse IV, <a href="/A129926/b129926.txt">Table of n, a(n) for n = 1..10000</a>

%p 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

%t Select[Range[600],PrimeOmega[#]==2&&PrimeQ[3#-2]&] (* _James C. McMahon_, Feb 02 2025 *)

%o (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

%Y Intersection of A153183 and A001358 (semiprimes).

%K nonn

%O 1,1

%A _Giovanni Teofilatto_, Jun 06 2007

%E More terms from _R. J. Mathar_, Jun 07 2007