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 A138524 #25 Apr 07 2022 09:20:21
%S A138524 2,26,746,41066,3669866,482671466,87660962666,21010450850666,
%T A138524 6423384156578666,2439325392333218666,1126440053169940898666,
%U A138524 621574841786409380258666,403913035968392044964258666,305292257647682252546468258666
%N A138524 a(n) = Sum_{k=1..n} (2*k)!.
%H A138524 G. C. Greubel, <a href="/A138524/b138524.txt">Table of n, a(n) for n = 1..224</a>
%F A138524 A007623(a(n)) = A163662(n). - _Amiram Eldar_, Apr 07 2022
%t A138524 Table[Sum[(2*i)!, {i,n}], {n,15}] (* _Stefan Steinerberger_, Mar 25 2008 *)
%t A138524 Accumulate[(2*Range[20])!] (* _Harvey P. Dale_, Nov 12 2016 *)
%o A138524 (PARI) for(n=1,25, print1(sum(k=1,n, (2*k)!), ", ")) \\ _G. C. Greubel_, Sep 29 2017
%o A138524 (Python)
%o A138524 from math import factorial
%o A138524 def a(n): return sum(factorial(2*k) for k in range(1, n+1))
%o A138524 print([a(n) for n in range(1, 16)]) # _Michael S. Branicky_, Feb 26 2021
%Y A138524 Cf. A007489, A007623, A138523, A138525, A163662.
%K A138524 easy,nonn
%O A138524 1,1
%A A138524 _Leroy Quet_, Mar 23 2008
%E A138524 More terms from _Stefan Steinerberger_ and _Robert G. Wilson v_, Mar 25 2008