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