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 A084253 #9 Feb 16 2025 08:32:49
%S A084253 1,3,5,21,108,660,4680,37800,342720,3447360,38102400,459043200,
%T A084253 5987520000,84064780800,1264085222400,20268952704000,345226033152000,
%U A084253 6224529991680000,118443913555968000,2372079457972224000
%N A084253 a(n) is the denominator of the coefficient of z^(2n-1) in the Maclaurin expansion of Sqrt[Pi]Erfi[z].
%C A084253 Numerators are unity for n>2.
%C A084253 Same as A007680/2 for n>2.
%H A084253 G. C. Greubel, <a href="/A084253/b084253.txt">Table of n, a(n) for n = 1..445</a>
%H A084253 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Erfi.html">Erfi</a>
%F A084253 a(n) = (2*n-1)*(n-1)!/2 for n>2.
%t A084253 Join[{1, 3}, Table[(2*n - 1)*n!/(2*n), {n,3,50}]] (* or *) Denominator[ CoefficientList[Series[Sqrt[Pi]*Erf[t], {t, 0, 10}], t]][[2 ;; ;; 2]] (* _G. C. Greubel_, Jan 12 2017 *)
%o A084253 (PARI) concat([1,3], for(n=3, 50, print1((2*n-1)*n!/(2*n), ", "))) \\ _G. C. Greubel_, Jan 12 2017
%Y A084253 Cf. A007680.
%K A084253 nonn,easy
%O A084253 1,2
%A A084253 _Eric W. Weisstein_, May 23 2003