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

Author

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

Keywords

• nonn

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

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

Programs

Mathematica

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

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

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).

Extensions

More terms from Jinyuan Wang, Mar 23 2020