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 A161880 #9 Mar 17 2025 18:31:51
%S A161880 1,20,209,1520,8644,40944,168035,613756,2034120,6206596,17632836,
%T A161880 47062620,118870650,285840940,657667521,1454009144,3100176535,
%U A161880 6394814820,12796122680,24898749084,47210910670,87394933100,158210114490,280501919100,487725336449,832684355656
%N A161880 Number of reduced words of length n in the Weyl group B_20.
%C A161880 Computed with MAGMA using commands similar to those used to compute A161409.
%D A161880 J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See under Poincaré polynomial.
%D A161880 N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6. (The group is defined in Planche II.)
%H A161880 Andrew Howroyd, <a href="/A161880/b161880.txt">Table of n, a(n) for n = 0..400</a>
%F A161880 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 A161880 Row n=20 of A128084.
%K A161880 nonn,fini,full
%O A161880 0,2
%A A161880 _John Cannon_ and _N. J. A. Sloane_, Nov 30 2009