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 A227830 #26 Mar 07 2020 12:25:43
%S A227830 1,2,12,1,720,1,30240,1,1209600,1,47900160,1,1307674368000,1,
%T A227830 74724249600,1,10670622842880000,1,5109094217170944000,1,
%U A227830 802857662698291200000,1,14101100039391805440000,1,1693824136731743669452800000,1,186134520519971831808000000,1,37893265687455865519472640000000,1,759790291646040068357842010112000000,1
%N A227830 Denominators of coefficients in expansion of x/(exp(x)-1).
%D A227830 M. Kauers and P. Paule, The Concrete Tetrahedron, Springer 2011, p. 23.
%F A227830 Recurrence: R(0) = 1 and R(n) = - Sum_{k=0..n-1} R(k)/(n-k+1)! for n>=1. Then a(n) = denominator(R(n)). - _Peter Luschny_, Jul 30 2015
%e A227830 1, -1/2, 1/12, 0, -1/720, 0, 1/30240, 0, -1/1209600, 0, 1/47900160, 0, -691/1307674368000, 0, 1/74724249600, 0, ...
%t A227830 Denominator[ CoefficientList[ Series[x/(1 - E^-x), {x, 0, 26}], x]] (* _Robert G. Wilson v_, Dec 29 2016 *)
%o A227830 (Sage)
%o A227830 @cached_function
%o A227830 def R(n): return -sum(R(k)/factorial(n-k+1) for k in (0..n-1)) if n>0 else 1
%o A227830 print([R(n).denominator() for n in (0..31)]) # _Peter Luschny_, Jul 30 2015
%Y A227830 For numerators see A120082.
%Y A227830 Cf. A060054, A060055.
%K A227830 nonn,frac
%O A227830 0,2
%A A227830 _N. J. A. Sloane_, Aug 01 2013