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 A334724 #8 May 09 2020 07:22:46 %S A334724 1,2,3,6,20,20,105,210,504,504,4620,4620,51480,51480,9009,18018, %T A334724 272272,272272,23279256,23279256,21162960,21162960,446185740, %U A334724 446185740,2059318800,2059318800,5736673800,5736673800,22181805360,22181805360,644658718275,1289317436550,1213475234400 %N A334724 Denominator of the sum of all elements of the n X n matrix M with M[i,j] = (-1)^(i+j)*i/j for i,j = 1..n. %e A334724 The absolute values of the first few fractions are 1, 1/2, 5/3, 7/6, 47/20, 37/20, 319/105, 533/210, 1879/504, ... = A120301/A334724. %o A334724 (PARI) a(n) = denominator(sum(j=1, n, sum(i=1, n, (-1)^(i+j)*i/j))); \\ _Michel Marcus_, May 09 2020 %Y A334724 Cf. A120301 (absolute values of numerators). %K A334724 nonn,frac %O A334724 1,2 %A A334724 _Petros Hadjicostas_, May 09 2020 %E A334724 More terms from _Michel Marcus_, May 09 2020