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 A265376 #18 Mar 05 2022 00:19:42
%S A265376 1,-1,-3,6,30,-120,-840,5040,45360,-362880,-3991680,39916800,
%T A265376 518918400,-6227020800,-93405312000,1307674368000,22230464256000,
%U A265376 -355687428096000,-6758061133824000,121645100408832000,2554547108585472000,-51090942171709440000,-1175091669949317120000
%N A265376 a(1) = 1 and a(n) = Sum_{i=1..n-1} (-1)^i*i*a(i).
%C A265376 1/abs(a(n)) + 1/abs(a(n+1)) = 1/(n-1)!, n = 3,5,7,... hence Sum_{n>1} 1/abs(a(n)) = cosh(1). - _Peter McNair_, Mar 04 2022
%F A265376 For n>1, a(n) = (-1)^floor(n/2) * A001710(n) / floor(n/2). - _Vaclav Kotesovec_, Jan 26 2016
%t A265376 a[1]=1; a[n_] := a[n] = Sum[(-1)^i*i*a[i], {i, 1, n - 1}]; Array[a,33]
%Y A265376 Cf. A004442, A001710 (b(n)=Sum_{i=1..n-1} i*b(i)).
%K A265376 sign
%O A265376 1,3
%A A265376 _José María Grau Ribas_, Dec 07 2015