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A007851
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Number of elements w of the Weyl group D(n) such that the roots sent negative by w span an Abelian subalgebra of the Lie algebra.
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1
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1, 4, 14, 48, 167, 593, 2144, 7864, 29171, 109173, 411501, 1560089, 5943199, 22732739, 87253604, 335897864, 1296447899, 5015206349, 19439895089, 75487384829, 293595204239, 1143532045499, 4459774977449, 17413705988873
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OFFSET
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1,2
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LINKS
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Boothby, T.; Burkert, J.; Eichwald, M.; Ernst, D. C.; Green, R. M.; Macauley, M. On the cyclically fully commutative elements of Coxeter groups, J. Algebr. Comb. 36, No. 1, 123-148 (2012), Table 1 FC Type D.
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FORMULA
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a(n) = (n+3)*C(n)/2 - 1, where C(n) is a Catalan number (see A000108).
D-finite with recurrence: -(n+1)*(3*n^2+n-12)*a(n) +(15*n^3+14*n^2-85*n+36)*a(n-1) -2*(2*n-3)*(3*n^2+7*n-8)*a(n-2)=0. - R. J. Mathar, Jun 11 2019
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MATHEMATICA
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Table[(n+3) CatalanNumber[n]/2-1, {n, 30}] (* Harvey P. Dale, Oct 06 2017 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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C. Kenneth Fan [ ckfan(AT)MIT.EDU ]
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STATUS
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approved
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