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 A174170 #9 Sep 08 2022 08:45:51
%S A174170 1,-2,1,13,-23,-242,-1559,16273,-145799,-4222034,3164281,3257872813,
%T A174170 94135157041,-7612225739042,563989131103537,69655619036668993,
%U A174170 4504341812743290961,-2245876068496447419362,-169454688152728033203119
%N A174170 A (1,3) Somos-4 sequence.
%C A174170 Hankel transform of A174169.
%H A174170 G. C. Greubel, <a href="/A174170/b174170.txt">Table of n, a(n) for n = 0..122</a>
%F A174170 a(n) = (a(n-1)*a(n-3) + 3*a(n-2)^2)/a(n-4).
%F A174170 a(n) = a(-2-n) = (-3*a(n-1)*a(n-4) - 8*a(n-2)*a(n-3))/a(n-5) for all n in Z. - _Michael Somos_, May 04 2020
%t A174170 RecurrenceTable[{a[n] == (a[n-1]*a[n-3] +3*a[n-2]^2)/a[n-4], a[0] == 1, a[1] == -2, a[2] == 1, a[3] == 13}, a, {n, 0, 30}] (* _G. C. Greubel_, Sep 18 2018 *)
%o A174170 (PARI) m=30; v=concat([1,-2,1,13], vector(m-4)); for(n=5, m, v[n] = ( v[n-1]*v[n-3] +3*v[n-2]^2)/v[n-4]); v \\ _G. C. Greubel_, Sep 18 2018
%o A174170 (Magma) I:=[1,-2,1,13]; [n le 4 select I[n] else (Self(n-1)*Self(n-3) + 3*Self(n-2)^2)/Self(n-4): n in [1..30]]; // _G. C. Greubel_, Sep 18 2018
%K A174170 easy,sign
%O A174170 0,2
%A A174170 _Paul Barry_, Mar 10 2010