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A161499
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Number of reduced words of length n in the Weyl group A_18.
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0
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1, 18, 170, 1121, 5795, 25005, 93651, 312646, 948328, 2651355, 6908989, 16929266, 39287173, 86859412, 183856635, 374153699, 734644291, 1396030396, 2574320659, 4617369760, 8072048882, 13779087108, 23004239550, 37616173304, 60323245445
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OFFSET
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0,2
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COMMENTS
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Computed with MAGMA using commands similar to those used to compute A161409.
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REFERENCES
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J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See under Poincaré polynomial.
N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6. (The group is defined in Planche I.)
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LINKS
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Table of n, a(n) for n=0..24.
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FORMULA
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G.f. for A_m is the polynomial Prod_{k=1..m}(1-x^(k+1))/(1-x). Only finitely many terms are nonzero. This is a row of the triangle in A008302.
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CROSSREFS
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Sequence in context: A125381 A126539 A213802 * A162346 A161878 A139618
Adjacent sequences: A161496 A161497 A161498 * A161500 A161501 A161502
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KEYWORD
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nonn
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AUTHOR
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John Cannon and N. J. A. Sloane, Nov 30 2009
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STATUS
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approved
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