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 A281964 #29 Dec 19 2017 18:37:37
%S A281964 1,2,4,16,104,624,3648,29184,302976,3029760,29698560,356382720,
%T A281964 5111976960,71567677440,986336870400,15781389926400,289206418636800,
%U A281964 5205715535462400,92506221468057600,1850124429361152000,41285515024760832000,908281330544738304000
%N A281964 Real part of n!*Sum_{k=1..n} i^(k-1)/k, where i is sqrt(-1).
%H A281964 Iain Fox, <a href="/A281964/b281964.txt">Table of n, a(n) for n = 1..450</a> (first 100 terms from Daniel Suteu)
%F A281964 a(n) ~ Pi/4 * n!.
%F A281964 a(1) = 1, a(n+1) = a(n)*(n+1) + n!*cos(Pi*n/2).
%F A281964 E.g.f.: arctan(x)/(1 - x). - _Ilya Gutkovskiy_, Dec 19 2017
%e A281964 For n=5, a(5) = 104, which is the real part of 5!*(1/1 + i/2 - 1/3 - i/4 + 1/5) = 104+30*i.
%o A281964 (PARI) a(n) = real(n!*sum(k=1, n, I^(k-1)/k));
%o A281964 (PARI) first(n) = x='x+O('x^(n+1)); Vec(serlaplace(atan(x)/(1 - x))) \\ _Iain Fox_, Dec 19 2017
%Y A281964 The corresponding imaginary part is A282132.
%Y A281964 Cf. A003881, A024167.
%K A281964 nonn
%O A281964 1,2
%A A281964 _Daniel Suteu_, Feb 06 2017