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A162360 Number of reduced words of length n in the Weyl group D_21.

%I A162360 #18 Mar 17 2025 22:37:17
%S A162360 1,21,230,1750,10394,51338,219373,833129,2867249,9073845,26706681,
%T A162360 73769301,192639951,478480891,1136148412,2590157556,5690334091,
%U A162360 12085148911,24881271591,49780020675,96990931345,184385864444,342595978914,623097897805,1110823232734
%N A162360 Number of reduced words of length n in the Weyl group D_21.
%D A162360 N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6. (The group is defined in Planche IV.)
%D A162360 J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See under Poincaré polynomial.
%H A162360 Andrew Howroyd, <a href="/A162360/b162360.txt">Table of n, a(n) for n = 0..420</a>
%H A162360 <a href="/index/Gre#GROWTH">Index entries for growth series for groups</a>
%F A162360 The growth series for D_k is the polynomial f(k)*Product_{i=1..k-1} f(2*i), where f(m) = (1-x^m)/(1-x) [Corrected by _N. J. A. Sloane_, Aug 07 2021]. This is a row of the triangle in A162206.
%p A162360 # Growth series for D_k, truncated to terms of order M. - _N. J. A. Sloane_, Aug 07 2021
%p A162360 f := proc(m::integer) (1-x^m)/(1-x) ; end proc:
%p A162360 g := proc(k,M) local a,i; global f;
%p A162360 a:=f(k)*mul(f(2*i),i=1..k-1);
%p A162360 seriestolist(series(a,x,M+1));
%p A162360 end proc;
%t A162360 f[m_] := (1-x^m)/(1-x);
%t A162360 With[{k = 21}, CoefficientList[f[k]*Product[f[2i], {i, 1, k-1}] + O[x]^(k+1), x]] (* _Jean-François Alcover_, Feb 15 2023, after Maple code *)
%Y A162360 Row 21 of A162206.
%Y A162360 Growth series for groups D_n, n = 3,...,50: A161435, A162207, A162208, A162209, A162210, A162211, A162212, A162248, A162288, A162297, A162300, A162301, A162321, A162327, A162328, A162346, A162347, A162359, A162360, A162364, A162365, A162366, A162367, A162368, A162369, A162370, A162376, A162377, A162378, A162379, A162380, A162381, A162384, A162388, A162389, A162392, A162399, A162402, A162403, A162411, A162412, A162413, A162418, A162452, A162456, A162461, A162469, A162492.
%K A162360 nonn,fini,full
%O A162360 0,2
%A A162360 _John Cannon_ and _N. J. A. Sloane_, Dec 01 2009