OFFSET
0,3
COMMENTS
a(n) counts permutations of length n which embed into the (infinite) increasing oscillating sequence given by 4,1,6,3,8,5,...,2k+2,2k-1,...; these are also the permutations which avoid {321, 2341, 3412, 4123}. - Vincent Vatter, May 23 2008
a(n) is the top left entry of the n-th power of any of the 3X3 matrices [1, 1, 0; 1, 1, 1; 1, 0, 0] or [1, 1, 1; 1, 1, 0; 0, 1, 0] or [1, 1, 1; 0, 0, 1; 1, 0, 1] or [1, 0, 1; 1, 0, 0; 1, 1, 1]. - R. J. Mathar, Feb 03 2014
a(n) is the number of possible tilings of a 2 X n board, using dominoes and L-shaped trominoes. - Michael Tulskikh, Aug 21 2019
a(n) = A190512(n-1) for n>0. - Greg Dresden, Feb 28 2020
REFERENCES
Kenneth Edwards, Michael A. Allen, A new combinatorial interpretation of the Fibonacci numbers squared, Part II, Fib. Q., 58:2 (2020), 169-177.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
R. Brignall, N. Ruskuc and V. Vatter, Simple permutations: decidability and unavoidable substructures, Theoretical Computer Science 391 (2008), 150-163.
Greg Dresden and Michael Tulskikh, Tilings of 2 X n boards with dominos and L-shaped trominos, Washington & Lee University (2021).
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 1053
D. Oudrar, Sur l'énumération de structures discrètes, une approche par la théorie des relations, Thesis (in French), arXiv:1604.05839 [math.CO], 2016.
D. Oudrar and M. Pouzet, Profile and hereditary classes of ordered relational structures, arXiv preprint arXiv:1409.1108 [math.CO], 2014 [The first version of this document erroneously gives the A-number as A005298]
V. Vatter, Small permutation classes, arXiv:0712.4006 [math.CO], 2007-2016.
Index entries for linear recurrences with constant coefficients, signature (2,0,1).
FORMULA
Recurrence: a(0)=1, a(1)=1, a(2)=2; thereafter a(n) = 2*a(n-1)+a(n-3).
a(n) = Sum(1/59*(4+3*_alpha^2+17*_alpha)*_alpha^(-1-n), _alpha = RootOf(-1+2*_Z+_Z^3)).
Let u1 = 2.20556943... denote the real root of x^3-2*x^2-1. There is an explicit constant c1 = 0.460719842... such that for n>0, a(n) = nearest integer to c1*u1^n. - N. J. A. Sloane, Nov 07 2016
a(2n) = a(n)^2 - a(n-1)^2 + (1/2)*(a(n+2) - a(n+1) - a(n))^2. - Greg Dresden and Michael Tulskikh, Aug 20 2019
a(n) = 2^(n-1) + Sum_{i=3..n}(2^(n-i)*a(i-3)). - Greg Dresden, Aug 27 2019
a(n+1) = (Sum_{i >= 0} 2^(n-3i-2)*(4*binomial(n-2i, i) + binomial(n-2i-2, i))). - Michael Tulskikh, Feb 14 2020
MAPLE
spec := [S, {S=Sequence(Prod(Union(Prod(Z, Z, Z), Z), Sequence(Z)))}, unlabeled ]: seq(combstruct[count ](spec, size=n), n=0..20);
MATHEMATICA
CoefficientList[Series[(1 - x)/(1 - 2 x - x^3), {x, 0, 40}], x] (* Vincenzo Librandi, Feb 05 2014 *)
PROG
(PARI) Vec((1-x)/(1-2*x-x^3)+O(x^99)) \\ Charles R Greathouse IV, Nov 20 2011
(Magma) I:=[1, 1, 2]; [n le 3 select I[n] else 2*Self(n-1)+Self(n-3): n in [1..40]]
(Magma) R<x>:=PowerSeriesRing(Integers(), 32); Coefficients(R!( (1 - x)/(1 - 2*x - x^3))); // Marius A. Burtea, Feb 14 2020
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved