A162921 - OEIS

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A162921

Number of reduced words of length n in Coxeter group on 4 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.

1, 4, 12, 36, 102, 288, 816, 2304, 6510, 18396, 51972, 146844, 414894, 1172232, 3312024, 9357768, 26439366, 74701620, 211061436, 596331540, 1684871094, 4760423280, 13450067520, 38001729168, 107369826654, 303361976844

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OFFSET

0,2

COMMENTS

The initial terms coincide with those of A003946, 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..25.

Index entries for linear recurrences with constant coefficients, signature (2, 2, 2, -3).

FORMULA

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

CROSSREFS

Sequence in context: A001411 A095350 A084776 * A237045 A291732 A291264

Adjacent sequences: A162918 A162919 A162920 * A162922 A162923 A162924

KEYWORD

nonn

AUTHOR

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

STATUS

approved