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 A239967 #12 Sep 08 2022 08:46:07 %S A239967 1,1,2,2,3,4,8,26,196,5064,992336,5025184408,4986671393704544, %T A239967 25058943335463698842360448, %U A239967 124960715887419957580506993886784668778624,3131383498581835095655557937198087383978439131276209799741457913600 %N A239967 a(n) = a(n-1)*a(n-2) - a(n-3)*a(n-4), starting with 1,1,2,2. %o A239967 (PARI) al(n)=local(v=vector(n));v[1]=v[2]=1;v[3]=v[4]=2;for(i=5,n,v[i]=v[i-1]*v[i-2]-v[i-3]*v[i-4]);v %o A239967 (Magma) I:=[1,1,2,2]; [n le 4 select I[n] else Self(n-1)*Self(n-2)-Self(n-3)*Self(n-4): n in [1..16]]; // _Vincenzo Librandi_, Mar 30 2014 %Y A239967 Cf. A111235. %K A239967 nonn %O A239967 1,3 %A A239967 _Franklin T. Adams-Watters_, Mar 29 2014