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A162879

Number of reduced words of length n in Coxeter group on 42 generators S_i with relations (S_i)^2 = (S_i S_j)^3 = I.

1, 42, 1722, 69741, 2824080, 114340800, 4629407580, 187434189600, 7588784431200, 307252630616400, 12439960566432000, 503665724648352000, 20392280251485912000, 825637071380896320000, 33428168171083640640000

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OFFSET

0,2

COMMENTS

The initial terms coincide with those of A170761, although the two sequences are eventually different.

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

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..500

Index entries for linear recurrences with constant coefficients, signature (40, 40, -820).

FORMULA

G.f.: (t^3 + 2*t^2 + 2*t + 1)/(820*t^3 - 40*t^2 - 40*t + 1).

a(n) = 40*a(n-1) + 40*a(n-2) - 820*a(n-3), n > 0. - Muniru A Asiru, Oct 24 2018

G.f.: (1+x)*(1-x^3)/(1 - 41*x + 860*x^3 - 820*x^4). - G. C. Greubel, Apr 27 2019

MAPLE

seq(coeff(series((x^3+2*x^2+2*x+1)/(820*x^3-40*x^2-40*x+1), x, n+1), x, n), n = 0 .. 20); # Muniru A Asiru, Oct 24 2018

MATHEMATICA

CoefficientList[Series[(t^3+2*t^2+2*t+1)/(820*t^3-40*t^2-40*t+1), {t, 0, 20}], t] (* Wesley Ivan Hurt, Apr 12 2017 *)

Join[{1}, LinearRecurrence[{40, 40, -820}, {42, 1722, 69741}, 20]] (* Vincenzo Librandi, Apr 14 2017 *)

coxG[{3, 820, -40}] (* The coxG program is at A169452 *) (* G. C. Greubel, Apr 27 2019 *)

PROG

(Magma) I:=[1, 42, 1722, 69741]; [n le 4 select I[n] else 40*Self(n-1) +40*Self(n-2)-820*Self(n-3): n in [1..20]]; // Vincenzo Librandi, Apr 14 2017

(Magma) R<t>:=PowerSeriesRing(Integers(), 20); Coefficients(R!((t^3 +2*t^2+2*t+1)/(820*t^3-40*t^2-40*t+1))); // G. C. Greubel, Oct 24 2018

(PARI) my(t='t+O('t^20)); Vec((t^3+2*t^2+2*t+1)/(820*t^3-40*t^2-40*t+1)) \\ G. C. Greubel, Oct 24 2018

(GAP) a:=[42, 1722, 69741];; for n in [4..20] do a[n]:=40*a[n-1]+40*a[n-2] -820*a[n-3]; od; Concatenation([1], a); # Muniru A Asiru, Oct 24 2018

(SageMath) ((1+x)*(1-x^3)/(1 -41*x +860*x^3 -820*x^4)).series(x, 20).coefficients(x, sparse=False) # G. C. Greubel, Apr 27 2019

CROSSREFS

Cf. A154638, A170761.

Sequence in context: A215545 A004997 A120815 * A163225 A163743 A164112

Adjacent sequences: A162876 A162877 A162878 * A162880 A162881 A162882

KEYWORD

nonn

AUTHOR

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

STATUS

approved

URL: https://oeis.org/A162879

⇱ A162879 - OEIS