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 A261871 #35 Dec 15 2017 14:28:44
%S A261871 3,7,9,15,21,27,31,33,35,39,45,49,51,57,63,69,75,77,81,87,91,93,99,
%T A261871 105,111,117,119,123,127,129,133,135,141,147,153,155,159,161,165,171,
%U A261871 175,177,183,189,195,201,203,207,213,217,219,225,231,237,243,245,249,255,259,261,267,273,279,285,287,291,297,301
%N A261871 Numbers of the form (2*j-1)*(2^k-1); j>=1, k>=2.
%C A261871 Odd numbers complementary to A185208.
%C A261871 Lim_{n->inf.} a(n)/n > 6/(1 + Sum_{j>=1} (2/(2^(2j+1)-1))) ~ 4.375745.
%H A261871 Charles R Greathouse IV, <a href="/A261871/b261871.txt">Table of n, a(n) for n = 1..10000</a>
%F A261871 2n < a(n) < 5n. For n > 51, 4.3n < a(n) < 4.5n. - _Charles R Greathouse IV_, Sep 05 2015
%t A261871 lmt = 310; Take[ Union@ Flatten@ Table[ (2j - 1)(2^k - 1), {j, lmt/4}, {k, 2, 1 + Log2[ lmt/(2j)] }], 68] (* _Michael De Vlieger_, Sep 04 2015 *) (* and modified by _Robert G. Wilson v_, Sep 05 2015 *)
%o A261871 (PARI) list(lim)=my(v=List(),t); for(k=2,logint(lim\1+1,2), t=2^k-1; forstep(j=1,lim\t,2, listput(v,t*j))); Set(v) \\ _Charles R Greathouse IV_, Sep 05 2015
%Y A261871 Cf. A185208.
%Y A261871 Note that A191131, A261524, A261871, and A282572 are very similar and easily confused with each other.
%K A261871 nonn,easy
%O A261871 1,1
%A A261871 _Bob Selcoe_, Sep 04 2015