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 A036999 N1692 #18 Jul 02 2023 14:14:28
%S A036999 6,9,12,18,27,42,66,105,168,270,435,702,1134,1833,2964,4794,7755,
%T A036999 12546,20298,32841,53136,85974,139107,225078,364182,589257,953436,
%U A036999 1542690,2496123,4038810,6534930,10573737,17108664,27682398,44791059
%N A036999 Restricted permutations.
%D A036999 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%H A036999 Vincenzo Librandi, <a href="/A036999/b036999.txt">Table of n, a(n) for n = 0..1000</a>
%H A036999 N. S. Mendelsohn, <a href="http://dx.doi.org/10.4153/CMB-1961-005-4">Permutations with confined displacement</a>, Canad. Math. Bull., 4 (1961), 29-38.
%H A036999 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2, 0, -1).
%F A036999 3*A000381.
%F A036999 G.f.: 3*(2-x-2*x^2)/((x^2+x-1)*(x-1)). - _Vincenzo Librandi_, Jul 01 2017
%t A036999 Join[{6}, Table[3 Fibonacci[n+3] + 3, {n, 0, 40}]] (* or *) CoefficientList[Series[3 (2 - x - 2 x^2) / ((x^2 + x - 1) (x - 1)), {x, 0, 33}], x] (* _Vincenzo Librandi_, Jul 01 2017 *)
%o A036999 (Magma) [6] cat [3*Fibonacci(n+3)+3: n in [0..40]]; // _Vincenzo Librandi_, Jul 01 2017
%Y A036999 Equals A022086(n+3) + 3.
%K A036999 nonn
%O A036999 0,1
%A A036999 _N. J. A. Sloane_