A383875 Number of pairs in the Bruhat order of type A_n.

1, 3, 19, 213, 3781, 98407, 3550919

Offset: 0

Author

Dmitry I. Ignatov, May 18 2025

Keywords

• nonn
• more

Comments

The number of ordered pairs in the Bruhat poset of the Weyl group A_n (isomorphic to the symmetric group S_{n+1}).

Examples

			For n=0, the only element is 1 (identity) so a(0)=1.
For n=1 the elements are 1 (identity) and s1. The order relation consists of pairs (1, 1), (1, s1), and (s1, s1). So a(1) = 3.
For n=2 the line (Hasse) diagram is below.
       s1*s2*s1
        /   \
      s2*s1 s1*s2
       |  X  |
       s2    s1
        \   /
          1
The order relation consists of the six reflexive pairs, the eight pairs shown in the diagram as edges, and the five pairs (1, s2*s1), (1, s1*s2), (1, s1*s2*s1), (s1, s1*s2*s1), and (s2, s1*s2*s1). So a(2) = 6+8+5 = 19.
		

References

• A. Bjorner and F. Brenti, Combinatorics of Coxeter Groups, Springer, 2009, 27-64.

Links

• V. V. Deodhar, On Bruhat ordering and weight-lattice ordering for a Weyl group, Indagationes Mathematicae, vol. 81, 1 (1978), 423-435.

Crossrefs

Cf. A000142 (the order size), A002538 (edges in the cover relation), A005130 (the size of Dedekind-MacNeille completion), A384061 (antichains), A384062 (maximal antichains).

Extensions

a(0)=1 prepended by Sara Billey, Jul 02 2025