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 A078530 #10 Dec 15 2023 19:17:56
%S A078530 0,3,1,1,1,1,2,3,9,27,81,729,0,59049,-531441,14348907,-387420489,
%T A078530 10460353203,-564859072962,22876792454961,-1853020188851841,
%U A078530 150094635296999121,-12157665459056928801,2954312706550833698643,0
%N A078530 Bilinear recursive sequence.
%F A078530 a(n) * a(n-8) = 81 * (a(n-2)*a(n-6) - 2*a(n-4)^2).
%F A078530 0 = a(n) * a(n-5) + 3 * a(n-1) * a(n-4) - 9 * a(n-2)*a(n-3).
%F A078530 a(12*n) = 0.
%F A078530 a(2*n+1) = a(-2*n+7) = a(4*n+2)/(81^(n-1)*(a(2*n-1)*a(2*n+2)^2 - a(2*n+3)*a(2*n)^2)) for all n in Z. - _Michael Somos_, Dec 10 2023
%F A078530 a(n+12) = -(-27)^(n+2) * a(n) for all n in Z. - _Michael Somos_, Dec 11 2023
%t A078530 a[ n_] := With[{m = Mod[n, 12]}, Sign[m] * 2^Boole[m==6] * (-1)^(Mod[Floor[n/12], 2]*(n-1)) * 3^(Boole[m==0] + Floor[(n-4)^2/8])]; (* _Michael Somos_, Dec 10 2023 *)
%o A078530 (PARI) {a(n) = sign(n%12) * (1 + (n%12==6)) * (-1)^(n\12%2 * (n-1)) * 3^((n%12==0) + (n-4)^2\8)};
%Y A078530 Cf. A078529, A006720, A006721.
%K A078530 sign,easy
%O A078530 0,2
%A A078530 _Michael Somos_, Nov 25 2002