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 A229726 #12 Mar 07 2020 07:00:38
%S A229726 1,3,15,105,315,3465,45045,9009,153153,2909907,14549535,334639305,
%T A229726 1673196525,5019589575,145568097675,4512611027925,4512611027925,
%U A229726 4512611027925,166966608033225,166966608033225,6845630929362225,294362129962575675,294362129962575675,13835020108241056725,96845140757687397075,96845140757687397075,5132792460157432044975
%N A229726 Denominator of Sum_{k=1..2n+1} 2^k/k.
%C A229726 Numerator is A108866.
%H A229726 Hugo Pfoertner, <a href="/A229726/b229726.txt">Table of n, a(n) for n = 0..500</a>
%e A229726 2, 20/3, 256/15, 4832/105, 42496/315, 1467392/3465, 62836736/45045, 42790912/9009, 2535587840/153153, 170851041280/2909907, 3070050172928/14549535, 256460544016384/334639305, 4697678227177472/1673196525, 52001663422038016/5019589575, ...
%o A229726 (PARI) for(n=0,26,print1(denominator(sum(k=1,2*n+1,2^k/k)),", ")) \\ _Hugo Pfoertner_, Mar 07 2020
%Y A229726 Cf. A229727, A108866.
%K A229726 nonn,frac
%O A229726 0,2
%A A229726 _N. J. A. Sloane_, Sep 28 2013