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 #17 Feb 21 2017 03:37:04
%S 8,12,16,18,20,24,28,30,32,36,40,42,44,48,50,52,56,60,64,66,68,70,72,
%T 76,78,80,84,88,92,96,98,100,102,104,110,112,114,116,120,124,128,130,
%U 132,136,138,140,144,148,152,154,156,160,164,168,170,172,174,176
%N Semiprimes times powers of 2.
%C Semiprimes times powers of 2 greater than 2^0 = 1.
%H Charles R Greathouse IV, <a href="/A192991/b192991.txt">Table of n, a(n) for n = 1..10000</a>
%F {A001358(i) * 2^j, j>0}.
%e 240 = 6th semiprime * 2^4 = 3 * 5 * 16.
%t upto=176;Select[Union[Flatten[2^Range[Ceiling[Log[2,upto]]] #&/@ Select[ Range[upto],PrimeOmega[#]==2&]]],#<=upto&](* _Harvey P. Dale_, Jul 19 2011 *)
%o (PARI) is(n)=my(e=valuation(n,2)); (e>2 && n>>e==1) || (e>1 && isprime(n>>e)) || (e>0 && bigomega(n>>e)==2) \\ _Charles R Greathouse IV_, Feb 21 2017
%Y Cf. A001358, A192990, A192992.
%K nonn,easy
%O 1,1
%A _Jonathan Vos Post_, Jul 13 2011