This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A178065 #27 Feb 02 2025 19:14:42
%S A178065 1,2,2,2,3,4,3,3,3,3,2,2,3,3,4,4,3,4,5,4,4,5,2,3,3,4,3,4,4,5,5,5,5,6,
%T A178065 4,6,5,5,6,5,5,6,2,3,3,4,4,5,3,3,5,5,6,3,4,4,4,4,6,5,6,3,4,4,5,5,5,4,
%U A178065 5,5,6,5,5,6,6,4,6,6,7,6,7,7,3,3,3,4,3,4,6,3,4,5,4,5,5,5,6,4,5,5,7,3,4,4,5
%N A178065 Number of 1's in binary representation of n-th semiprime.
%H A178065 Robert Israel, <a href="/A178065/b178065.txt">Table of n, a(n) for n = 1..10000</a>
%e A178065 a(1)=1 because the first semiprime is 4 = 100_2.
%e A178065 a(2)=2 because the second semiprime is 6 = 110_2.
%p A178065 N:= 10^3: # for semiprimes <= N
%p A178065 P:= select(isprime,[2,seq(i,i=3..N/2,2)]):
%p A178065 SP:= sort(select(`<=`,[seq(seq(P[i]*P[j],j=1..i),i=1..nops(P))],N)):
%p A178065 map(t -> convert(convert(t,base,2),`+`), SP); # _Robert Israel_, Feb 02 2025
%t A178065 s={};Do[If[PrimeOmega[n]==2,AppendTo[s,Total[IntegerDigits[n,2]]]],{n,400}];s (* _James C. McMahon_, Jan 02 2025 *)
%o A178065 (PARI) lista(nn) = {for (n=2, nn, if (bigomega(n)==2, print1(norml2(binary(n)), ", ")););} \\ _Michel Marcus_, Jun 05 2013
%Y A178065 Cf. A001358, A014499, A102029.
%K A178065 nonn,base,easy
%O A178065 1,2
%A A178065 _Juri-Stepan Gerasimov_, May 18 2010
%E A178065 a(56) corrected by _R. J. Mathar_, May 23 2010