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 A334721 #10 May 09 2020 00:35:03
%S A334721 1,1,2,3,12,10,60,105,280,252,2520,2310,27720,25740,24024,9009,144144,
%T A334721 136136,2450448,11639628,11085360,10581480,232792560,223092870,
%U A334721 1070845776,1029659400,2974571600,2868336900,11473347600,11090902680,332727080400,644658718275,625123605600
%N A334721 Denominator of the product of n and the n-th harmonic alternating number, Sum_{k=1..n} (-1)^(k+1)/k.
%C A334721 For n = 1 to 15, we have a(n) = A002944, but a(16) = 9009 <> 45045 = A002944(16).
%e A334721 The first few fractions are 1, 1, 5/2, 7/3, 47/12, 37/10, 319/60, 533/105, 1879/280, ... = A119787/A334721.
%o A334721 (PARI) a(n) = denominator(n*sum(k=1, n, (-1)^(k+1)/k)); \\ _Michel Marcus_, May 09 2020
%Y A334721 Cf. A002944, A119787 (numerators).
%K A334721 nonn,frac
%O A334721 1,3
%A A334721 _Petros Hadjicostas_, May 08 2020