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 A184121 #10 Sep 08 2022 08:45:55
%S A184121 1,-1,-7,-31,-23,2239,39889,603521,-27466831,-2445382081,
%T A184121 -204734091383,7545563759329,7265319037194073,2502938339020429439,
%U A184121 404477135616551491361,-932929530161758614571519
%N A184121 A (4,-3) Somos-4 sequence.
%C A184121 Hankel transform of A184120.
%H A184121 G. C. Greubel, <a href="/A184121/b184121.txt">Table of n, a(n) for n = 0..75</a>
%F A184121 a(n) = (4*a(n-1)*a(n-3) - 3*a(n-2)^2)/a(n-4).
%t A184121 RecurrenceTable[{a[0]==1,a[1]==-1,a[2]==-7,a[3]==-31,a[n]==(4a[n-1] a[n-3]- 3a[n-2]^2)/a[n-4]},a,{n,20}] (* _Harvey P. Dale_, Apr 03 2016 *)
%o A184121 (Magma) I:=[-23, 2239, 39889, 603521]; [1, -1, -7, -31] cat [n le 4 select I[n] else (4*Self(n-1)*Self(n-3) - 3*Self(n-2)^2 )/Self(n-4): n in [1..30]]; // _G. C. Greubel_, Aug 14 2018
%K A184121 sign,easy
%O A184121 0,3
%A A184121 _Paul Barry_, Jan 09 2011