Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #10 Feb 16 2021 02:11:25
%S 33,118,119,134,146,226,247,249,287,295,334,335,386,391,393,395,422,
%T 478,493,497,502,519,551,583,589,614,629,634,694,697,721,731,749,755,
%U 789,802,817,843,879,898,955,958,985,989,1003,1037,1079,1114,1154,1159,1177
%N Numbers k that are the products of two distinct primes such that 2*k-1 and 4*k-3 are also products of two distinct primes.
%H Harvey P. Dale, <a href="/A177211/b177211.txt">Table of n, a(n) for n = 1..1000</a>
%e 33 is a term because 33 = 3*11, 2*33 - 1 = 65 = 5*13 and 2*65 - 1 = 4*33 - 3 = 129 = 3*43.
%t f[n_]:=Last/@FactorInteger[n]=={1,1}; lst={};Do[If[f[n]&&f[2*n-1]&&f[4*n-3],AppendTo[lst,n]],{n,0,7!}];lst
%t tdpQ[n_]:=PrimeNu[n]==PrimeOmega[n]==PrimeNu[2n-1]==PrimeOmega[2n-1] == PrimeNu[4n-3]==PrimeOmega[4n-3]==2; Select[Range[1200],tdpQ] (* _Harvey P. Dale_, Nov 15 2020 *)
%Y Cf. A006881, A177210
%K nonn
%O 1,1
%A _Vladimir Joseph Stephan Orlovsky_, May 04 2010
%E Definition clarified by _Harvey P. Dale_, Nov 15 2020