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A293006 Expansion of 2*x^2*(x+1) / (2*x^3-3*x+1). 4
0, 0, 2, 8, 24, 68, 188, 516, 1412, 3860, 10548, 28820, 78740, 215124, 587732, 1605716, 4386900, 11985236, 32744276, 89459028, 244406612, 667731284, 1824275796, 4984014164, 13616579924, 37201188180, 101635536212, 277673448788, 758617970004, 2072582837588 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Number of associative, quasitrivial, and order-preserving binary operations on the n-element set {1,...,n} that have annihilator elements.

LINKS

Robert Israel, Table of n, a(n) for n = 0..2289

M. Couceiro, J. Devillet, and J.-L. Marichal, Quasitrivial semigroups: characterizations and enumerations, arXiv:1709.09162 [math.RA] (2017).

Index entries for linear recurrences with constant coefficients, signature (3,0,-2).

FORMULA

a(n) = 2*A293005(n-1), a(0) = 0.

From Colin Barker, Sep 28 2017: (Start)

a(n) = (-8 + (1-sqrt(3))^(1+n) + (1+sqrt(3))^(1+n)) / 6 for n>0.

a(n) = 3*a(n-1) - 2*a(n-2) for n>3.

(End)

MAPLE

f:= gfun:-rectoproc({a(n) = 3*a(n-1) - 2*a(n-3), a(0)=0, a(1)=0, a(2)=2, a(3)=8}, a(n), remember):

map(f, [$0..100]); # Robert Israel, Sep 28 2017

MATHEMATICA

Join[{0}, LinearRecurrence[{3, 0, -2}, {0, 2, 8}, 30]] (* Jean-Fran├žois Alcover, Sep 19 2018 *)

PROG

(PARI) concat(vector(2), Vec(2*x^2*(1 + x) / ((1 - x)*(1 - 2*x - 2*x^2)) + O(x^30))) \\ Colin Barker, Sep 28 2017

CROSSREFS

Cf. A002605, A028860, A293005, A293007.

Sequence in context: A229136 A261452 A225524 * A018045 A050242 A045697

Adjacent sequences:  A293003 A293004 A293005 * A293007 A293008 A293009

KEYWORD

nonn,easy

AUTHOR

J. Devillet, Sep 28 2017

STATUS

approved

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Last modified April 24 19:49 EDT 2019. Contains 322446 sequences. (Running on oeis4.)