A163145 Number of reduced words of length n in Coxeter group on 21 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.

1, 21, 420, 8400, 167790, 3351600, 66948210, 1337288400, 26712295890, 533577313500, 10658190898110, 212897044846500, 4252612111884990, 84945799915397400, 1696789816099808010, 33893325895893882600, 677018172425014524090, 13523417772619230573300

Offset

0,2

Comments

The initial terms coincide with those of A170740, 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..17.

Index entries for linear recurrences with constant coefficients, signature (19,19,19,-190).

Formula

G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(190*t^4 - 19*t^3 - 19*t^2 - 19*t + 1).

Mathematica

coxG[{4, 190, -19}] (* The coxG program is at A169452 *) (* Harvey P. Dale, Mar 25 2019 *)

Prog

(PARI) Vec((t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(190*t^4 - 19*t^3 - 19*t^2 - 19*t + 1) + O(t^20)) \\ Jinyuan Wang, Mar 23 2020

Crossrefs

Sequence in context: A083043 A162807 A097833 * A163503 A163977 A164634

Adjacent sequences: A163142 A163143 A163144 * A163146 A163147 A163148

Keyword

nonn

Author

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

Extensions

More terms from Jinyuan Wang, Mar 23 2020

Status

approved