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%I #17 Apr 01 2020 09:11:36
%S 1,4,4,7,13,19,31,52,82,133,217,349,565,916,1480,2395,3877,6271,10147,
%T 16420,26566,42985,69553,112537,182089,294628,476716,771343,1248061,
%U 2019403,3267463,5286868,8554330,13841197,22395529,36236725
%N a(n) = a(n-2) + 2*a(n-3) + a(n-4).
%D D. E. Knuth, Art of Computer Programming, Vol. 3, Sect. 5.4.2, Eq. (25).
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,1,2,1).
%F 3 * [Fibonacci(n+2)/2] + 1. - _Ralf Stephan_, Dec 02 2004
%F a(n) = (A099837(n+2)+A022086(n+2))/2. G.f. ( -1-4*x-3*x^2-x^3 ) / ( (1+x+x^2)*(x^2+x-1) ). - _R. J. Mathar_, Mar 21 2011
%p A036605 := proc(n) option remember; if n <= 0 then 1 else A036605(n-2)+2*A036605(n-3)+A036605(n-4); fi; end;
%t LinearRecurrence[{0, 1, 2, 1}, {1, 4, 4, 7}, 36] (* _Jean-François Alcover_, Apr 01 2020 *)
%Y Cf. A004695.
%K nonn,easy
%O 0,2
%A _N. J. A. Sloane_.
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