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 A178418 #18 Sep 08 2022 08:45:54
%S A178418 1,1,2,9,-1,164,-737,5895,-119558,-113489,-39697759,-800750664,
%T A178418 -25602199327,-2344309254991,14264330936074,-14182654502256615,
%U A178418 1282764246790221919,-163799892405003723284,67851128001519788451263
%N A178418 A (-1, 2) Somos-4 sequence associated to the elliptic curve y^2 + 2*x*y + y = x^3 + x^2 + x.
%C A178418 Hankel transform of the sequence with g.f. 1/(1-x^2/(1-2x^2/(1-(9/4)x^2/(1+(2/81)x^2/(1-1476x^2/(1-.... where 0/1, 2/1, 9/4, -2/81, 1476/1,... are the x-coordinates of the multiples of (0, 0).
%H A178418 G. C. Greubel, <a href="/A178418/b178418.txt">Table of n, a(n) for n = 1..125</a> (offset adapted by _Georg Fischer_, Jan 31 2019)
%F A178418 a(n) = (-a(n-1)*a(n-3) + 2*a(n-2)^2)/a(n-4), n>4.
%F A178418 a(n) = -(-1)^n * a(-n), a(n+3)*a(n-2) = -a(n+2)*a(n-1) + 9*a(n+1)*a(n) for all n in Z. - _Michael Somos_, Sep 19 2018
%e A178418 G.f. = x + x^2 + 2*x^3 + 9*x^4 - x^5 + 164*x^6 - 737*x^7 + ... - _Michael Somos_, Sep 19 2018
%t A178418 RecurrenceTable[{a[1]==a[2]==1,a[3]==2,a[4]==9,a[n]==(-a[n-1]a[n-3]+ 2a[n-2]^2)/a[n-4]},a[n],{n,20}] (* _Harvey P. Dale_, Sep 20 2011 *)
%o A178418 (PARI) m=30; v=concat([1,1,2,9], vector(m-4)); for(n=5, m, v[n] = ( -v[n-1]*v[n-3] + 2*v[n-2]^2)/v[n-4]); v \\ _G. C. Greubel_, Sep 18 2018
%o A178418 (Magma) I:=[1,1,2,9]; [n le 4 select I[n] else (-Self(n-1)*Self(n-3) + 2*Self(n-2)^2)/Self(n-4): n in [1..30]]; // _G. C. Greubel_, Sep 18 2018
%K A178418 easy,sign
%O A178418 1,3
%A A178418 _Paul Barry_, May 27 2010
%E A178418 Offset changed to 1 by _Michael Somos_, Sep 19 2018