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 A321521 #9 Nov 14 2018 14:02:20
%S A321521 1,1,7,21,121,715,5041,40293,362887,3628681,39916801,479000895,
%T A321521 6227020801,87178286161,1307674368127,20922789847653,355687428096001,
%U A321521 6402373705365835,121645100408832001,2432902008173011101,51090942171709445047,1124000727777567763201
%N A321521 a(n) = Sum_{d|n} (-1)^(n/d+1)*d!.
%F A321521 G.f.: Sum_{k>=1} k!*x^k/(1 + x^k).
%F A321521 a(n) ~ n!. - _Vaclav Kotesovec_, Nov 13 2018
%t A321521 Table[Sum[(-1)^(n/d + 1) d!, {d, Divisors[n]}], {n, 22}]
%t A321521 nmax = 22; Rest[CoefficientList[Series[Sum[k! x^k/(1 + x^k), {k, 1, nmax}], {x, 0, nmax}], x]]
%o A321521 (PARI) a(n) = sumdiv(n, d, (-1)^(n/d+1)*d!); \\ _Michel Marcus_, Nov 12 2018
%Y A321521 Cf. A000142, A062363, A321522.
%K A321521 nonn
%O A321521 1,3
%A A321521 _Ilya Gutkovskiy_, Nov 12 2018