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 A171422 #7 Sep 08 2022 08:45:50
%S A171422 1,-1,-9,-41,-241,-271,27329,651521,18425809,399122991,-24055184809,
%T A171422 -3943757411849,-498726356978849,-74609317288333151,
%U A171422 2771780972000481601,8459493968101503667649,5139175203350482789079649
%N A171422 A (4,-5) Somos-4 sequence.
%C A171422 Hankel transform of A143013.
%H A171422 G. C. Greubel, <a href="/A171422/b171422.txt">Table of n, a(n) for n = 0..109</a>
%F A171422 a(n) = (4*a(n-1)*a(n-3) - 5*a(n-2)^2)/a(n-4), n>=3.
%t A171422 RecurrenceTable[{a[n] == (4*a[n-1]*a[n-3] -5*a[n-2]^2)/a[n-4], a[0] == 1, a[1] == -1, a[2] == -9, a[3] == -41}, a, {n, 0, 30}] (* _G. C. Greubel_, Sep 18 2018 *)
%o A171422 (PARI) m=30; v=concat([1,-1,-9,-41], vector(m-4)); for(n=5, m, v[n] = ( 4*v[n-1]*v[n-3] -5*v[n-2]^2)/v[n-4]); v \\ _G. C. Greubel_, Sep 18 2018
%o A171422 (Magma) I:=[1,-1,-9,-41]; [n le 4 select I[n] else (4*Self(n-1)*Self(n-3) -5*Self(n-2)^2)/Self(n-4): n in [1..30]]; // _G. C. Greubel_, Sep 18 2018
%K A171422 easy,sign
%O A171422 0,3
%A A171422 _Paul Barry_, Dec 08 2009