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A162920 Number of reduced words of length n in Coxeter group on 3 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I. 0
1, 3, 6, 12, 21, 36, 63, 108, 186, 321, 552, 951, 1638, 2820, 4857, 8364, 14403, 24804, 42714, 73557, 126672, 218139, 375654, 646908, 1114029, 1918452, 3303735, 5689308, 9797466, 16872057, 29055096, 50035311, 86164998, 148383348 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

The initial terms coincide with those of A003945, although the two sequences are eventually different.

Computed with MAGMA using commands similar to those used to compute A154638.

LINKS

Table of n, a(n) for n=0..33.

Index entries for linear recurrences with constant coefficients, signature (1, 1, 1, -1).

FORMULA

G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(t^4 - t^3 - t^2 - t + 1)

CROSSREFS

Sequence in context: A234248 A294387 A128128 * A247662 A215005 A006330

Adjacent sequences:  A162917 A162918 A162919 * A162921 A162922 A162923

KEYWORD

nonn

AUTHOR

John Cannon and N. J. A. Sloane, Dec 03 2009

STATUS

approved

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Last modified November 22 18:55 EST 2019. Contains 329410 sequences. (Running on oeis4.)