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 A237545 #23 Jan 08 2018 01:58:32 %S A237545 1,81,2187,59049,177147,1594323,14348907,43046721,1162261467, %T A237545 3486784401,31381059609,22876792454961,68630377364883,205891132094649, %U A237545 16677181699666569,150094635296999121,36472996377170786403,328256967394537077627,8862938119652501095929,79766443076872509863361 %N A237545 Odious powers of 3. %C A237545 Intersection of A000069 and A000244. %C A237545 Exponents of a(n): A223024. %C A237545 It seems that this sequence includes about half of the powers of 3. For example, a(50171) = 3^99999. - _Charles R Greathouse IV_, Mar 05 2014 %H A237545 Robert Israel, <a href="/A237545/b237545.txt">Table of n, a(n) for n = 1..1058</a> %p A237545 select(t -> convert(convert(t,base,2),`+`)::odd, [seq(3^i,i=0..100)]); # _Robert Israel_, Oct 10 2016 %t A237545 Select[3^Range[32],OddQ[First[DigitCount[#,2] ] ]&] (* _Wouter Meeussen_, Feb 09 2014 *) %o A237545 (PARI) for(n=0,100,if(hammingweight(t=3^n)%2,print1(t", "))) \\ _Charles R Greathouse IV_, Mar 05 2014 %Y A237545 Cf. A000069 (odious numbers), A000244 (powers of 3), A223024. %K A237545 nonn,easy %O A237545 1,2 %A A237545 _Ilya Lopatin_ and _Juri-Stepan Gerasimov_, Feb 09 2014 %E A237545 Corrected and more terms added by _Robert Israel_, Oct 10 2016