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 A121869 #12 Sep 08 2022 08:45:27
%S A121869 -1,1,0,-5,-15,104,1827,7893,-207000,-5646249,-47897675,1479282600,
%T A121869 74711288407,1396956334921,-21032523700672,-2719998717430365,
%U A121869 -104158663871982343,-715846242343471272,189941380201812700699,14820744271258596866013,507768838531742620183176
%N A121869 Monthly Problem 10791, first expression.
%H A121869 G. C. Greubel, <a href="/A121869/b121869.txt">Table of n, a(n) for n = 0..330</a>
%H A121869 A. Fekete and G. Martin, <a href="https://www.jstor.org/stable/2695545">Problem 10791: Squared Series Yielding Integers</a>, Amer. Math. Monthly, 108 (No. 2, 2001), 177-178.
%F A121869 a(n) = A024429(n)^2 - A024430(n)^2.
%p A121869 with(combinat): seq(-bell(n)*BellB(n, -1), n = 0..25); # _G. C. Greubel_, Oct 08 2019
%t A121869 Table[-BellB[n]*BellB[n, -1], {n,0,25}] (* _G. C. Greubel_, Oct 08 2019 *)
%o A121869 (PARI) a(n) = (-1)*sum(k=0,n, stirling(n,k,2))*sum(k=0,n, (-1)^k*stirling(n,k,2));
%o A121869 vector(25, n, a(n-1)) \\ _G. C. Greubel_, Oct 08 2019
%o A121869 (Magma) a:= func< n | (-1)*(&+[StirlingSecond(n,k): k in [0..n]])*(&+[ (-1)^k*StirlingSecond(n,k): k in [0..n]]) >;
%o A121869 [a(n): n in [0..25]]; // _G. C. Greubel_, Oct 08 2019
%o A121869 (Sage) [ -sum(stirling_number2(n, k) for k in (0..n))*sum((-1)^k* stirling_number2(n,k) for k in (0..n)) for n in (0..25)] # _G. C. Greubel_, Oct 08 2019
%o A121869 (GAP) List([0..25], n-> (-1)*Sum([0..n], k-> Stirling2(n,k)) *Sum([0..n], k-> (-1)^k*Stirling2(n,k)) ); # _G. C. Greubel_, Oct 08 2019
%Y A121869 Cf. A121870, A024429, A024430, A121867, A121868.
%K A121869 sign
%O A121869 0,4
%A A121869 _N. J. A. Sloane_, Sep 05 2006