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A161513 Number of reduced words of length n in the Weyl group A_20. 0
1, 20, 209, 1519, 8624, 40734, 166495, 604902, 1991637, 6029727, 16976806, 44853151, 112016669, 266039599, 603919164, 1315952428, 2762629983, 5605337934, 11022366544, 21056764914, 39163212165, 71048746505 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Computed with MAGMA using commands similar to those used to compute A161409.

REFERENCES

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.)

LINKS

Table of n, a(n) for n=0..21.

FORMULA

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.

CROSSREFS

Sequence in context: A135179 A220939 A231059 * A162359 A161880 A074668

Adjacent sequences:  A161510 A161511 A161512 * A161514 A161515 A161516

KEYWORD

nonn

AUTHOR

John Cannon and N. J. A. Sloane, Nov 30 2009

STATUS

approved

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Last modified February 25 09:44 EST 2020. Contains 332221 sequences. (Running on oeis4.)