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 A271920 #16 Mar 26 2018 06:06:38
%S A271920 1,5,10,11,22,187,935,1955,391,11339,45356,1334,2668,27347,601634,
%T A271920 614713,6147130,162898945,11847196,12051458,24102916,30128645,
%U A271920 512186965,7273054903,7273054903,80003603933,400018019665,809792576395,9526971487,77081860213,1772882784899,188604551585,188604551585
%N A271920 Denominator of Product_{j=1..n-1} ((3*j+1)/(3*j+2)).
%H A271920 J. de Gier, <a href="https://arxiv.org/abs/math/0211285">Loops, matchings and alternating-sign matrices</a>, arXiv:math.CO/0211285, 2002-2003.
%e A271920 1, 4/5, 7/10, 7/11, 13/22, 104/187, 494/935, 988/1955, 190/391, 5320/11339, 20615/45356, 589/1334, 1147/2668, 11470/27347, ...
%p A271920 f:=proc(n) local j;
%p A271920 mul(((3*j+1)/(3*j+2)),j=1..n-1); end;
%p A271920 t1:=[seq(f(n),n=1..50)];
%p A271920 map(numer,t1);
%p A271920 map(denom,t1);
%t A271920 Table[Product[(3*j+1)/(3*j+2), {j, 1, n-1}] // Denominator, {n, 1, 33}] (* _Jean-François Alcover_, Mar 25 2018 *)
%o A271920 (PARI) a(n) = denominator(prod(j=1, n-1, (3*j+1)/(3*j+2))); \\ _Michel Marcus_, Mar 25 2018
%Y A271920 Cf. A271919 (numerators).
%Y A271920 Other sequences of fractions from de Gier paper: A271921, A271922, A271923, A271924, A271925, A271926.
%K A271920 nonn,frac
%O A271920 1,2
%A A271920 _N. J. A. Sloane_, May 04 2016