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A052980 G.f.: (1-x)/(1-2*x-x^3). 15
1, 1, 2, 5, 11, 24, 53, 117, 258, 569, 1255, 2768, 6105, 13465, 29698, 65501, 144467, 318632, 702765, 1549997, 3418626, 7540017, 16630031, 36678688, 80897393, 178424817, 393528322, 867954037, 1914332891, 4222194104, 9312342245 (list; graph; refs; listen; history; text; internal format)
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

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..200

R. Brignall, N. Ruskuc and V. Vatter, Simple permutations: decidability and unavoidable substructures, Theoretical Computer Science 391 (2008), 150-163.

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, 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).

Sum(1/59*(4+3*_alpha^2+17*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^3))

a(n) = A008998(n) - A008998(n-1). - R. J. Mathar, Feb 04 2014

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

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]]

CROSSREFS

See A110513 for another version.

Column k=2 of A219987.

Sequence in context: A286945 A111297 A077864 * A190512 A110513 A018115

Adjacent sequences:  A052977 A052978 A052979 * A052981 A052982 A052983

KEYWORD

easy,nonn

AUTHOR

encyclopedia(AT)pommard.inria.fr, Jan 25 2000

STATUS

approved

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Last modified September 25 13:09 EDT 2017. Contains 292485 sequences.