This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A161899 #10 Mar 17 2025 22:16:05
%S A161899 1,21,230,1750,10394,51338,219373,833129,2867249,9073845,26706681,
%T A161899 73769301,192639951,478480891,1136148412,2590157556,5690334091,
%U A161899 12085148911,24881271591,49780020675,96990931345,184385864445,342595978935,623097898035,1110823234484,1943507590140
%N A161899 Number of reduced words of length n in the Weyl group B_21.
%C A161899 Computed with MAGMA using commands similar to those used to compute A161409.
%D A161899 J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See under Poincaré polynomial.
%D A161899 N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6. (The group is defined in Planche II.)
%H A161899 Andrew Howroyd, <a href="/A161899/b161899.txt">Table of n, a(n) for n = 0..441</a>
%F A161899 G.f. for B_m is the polynomial Prod_{k=1..m}(1-x^(2k))/(1-x). Only finitely many terms are nonzero. This is a row of the triangle in A128084.
%Y A161899 Row n=21 of A128084.
%K A161899 nonn,fini,full
%O A161899 0,2
%A A161899 _John Cannon_ and _N. J. A. Sloane_, Nov 30 2009