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 A369490 #14 Mar 04 2024 00:55:09
%S A369490 -1,17,11,113,179,857,1931,7073,18659,61097,173051,539633,1577939,
%T A369490 4815737,14283371,43177793,128878019,387944777,1161212891,3488881553,
%U A369490 10456158899,31389448217,94126401611,282463090913,847221500579,2542000046057
%N A369490 a(n) = 3^(n+1) + 2*(-2)^(n+1).
%H A369490 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1, 6).
%F A369490 a(n) = a(n-1) + 6*a(n-2); a(0) = -1, a(1) = 17.
%F A369490 G.f.: (18*x-1)/((1+2*x)*(1-3*x)).
%F A369490 a(2*n) = A003063(2*n+2).
%F A369490 a(2*n+1) = A085279(2*n+3).
%F A369490 a(n) = 18*A015441(n) - A015441(n+1).
%t A369490 LinearRecurrence[{1,6},{-1,17},26] (* _James C. McMahon_, Jan 30 2024 *)
%o A369490 (Python)
%o A369490 def A369490(n): return 3**(n+1)+(1<<n+2 if n&1 else -(1<<n+2)) # _Chai Wah Wu_, Feb 25 2024
%Y A369490 Cf. A000079, A000244, A003063, A015441, A085279.
%K A369490 sign,easy
%O A369490 0,2
%A A369490 _Philippe Deléham_, Jan 24 2024