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A080721 Triangle of binomial(n,k)*(binomial(n+k,k)-binomial(n+k-2,k-1)). 4
1, 1, 1, 1, 4, 4, 1, 9, 21, 14, 1, 16, 66, 100, 50, 1, 25, 160, 410, 455, 182, 1, 36, 330, 1260, 2310, 2016, 672, 1, 49, 609, 3220, 8610, 12222, 8778, 2508, 1, 64, 1036, 7224, 26250, 53592, 61908, 37752, 9438, 1, 81, 1656, 14700, 69300, 189882, 312312, 303732, 160875, 35750, 1 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

For n>1 and 0 <= k <= n, a(n,k) is the number of compatible k-sets of cluster variables in Fomin and Zelevinsky's 'cluster algebra' of finite type D_n.

Triangle of f-vectors of the simplicial complexes dual to the generalized associahedra of type D_n (n >= 2). See A145903 for the corresponding triangle of h-vectors. For the triangles of f-vectors of type A and type B associahedra see A033282 and A063007 respectively. [Peter Bala, Oct 28 2008]

LINKS

Table of n, a(n) for n=0..55.

S. Fomin and A. Zelevinsky, Cluster algebras I: Foundations, J. Amer. Math. Soc. 15 (2002), no. 2, 497-529.

S. Fomin and A. Zelevinsky, Y-systems and generalized associahedra, Ann. of Math. (2) 158 (2003), no. 3, 977-1018.

S. Fomin, N. Reading, Root systems and generalized associahedra, Lecture notes for IAS/Park-City 2004. [From Peter Bala, Oct 28 2008]

EXAMPLE

Contribution from Peter Bala, Oct 28 2008: (Start)

Triangle begins

n\k|..0....1....2....3....4....5

================================

0..|..1

1..|..1....1

2..|..1....4....4

3..|..1....9...21...14

4..|..1...16...66..100...50

5..|..1...25..160..410..455..182

...

(End)

MAPLE

A080721 := proc(n, k)

    binomial(n, k)*(binomial(n+k, k)-binomial(n+k-2, k-1))

end proc: # R. J. Mathar, Mar 22 2013

MATHEMATICA

Flatten[Table[Binomial[n, k](Binomial[n+k, k]-Binomial[Abs[n+k-2], k-1]), {n, 0, 10}, {k, 0, n}]] (* Harvey P. Dale, Feb 20 2013 *)

PROG

(PARI)

T(n, k)=binomial(n, k)*(binomial(n+k, k)-binomial(n+k-2, k-1))

for (n=0, 10, for (k=0, n, print1(T(n, k), ", ")));

/* Joerg Arndt, Feb 21 2013 */

CROSSREFS

Cf. A033282, A063007.

A051924 (main diagonal), A145903( h-vectors type D associahedra). [From Peter Bala, Oct 28 2008]

Sequence in context: A016706 A138679 A179399 * A123588 A289710 A243594

Adjacent sequences:  A080718 A080719 A080720 * A080722 A080723 A080724

KEYWORD

easy,nonn,tabl

AUTHOR

Paul Boddington, Mar 07 2003

STATUS

approved

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Last modified August 18 13:12 EDT 2019. Contains 326100 sequences. (Running on oeis4.)