A143673 Number of antichains in the poset of Dyck paths ordered by inclusion.

2, 2, 3, 7, 42, 2361, 37620704

Offset

0,1

Comments

Also the number of order ideals (down-sets) for this poset.

This is the breakdown by size of (or number of elements in) the antichains beginning with antichains of size 0 and increasing:

n=0: 1, 1;

n=1: 1, 1;

n=2: 1, 2;

n=3: 1, 5, 1;

n=4: 1, 14, 21, 6;

n=5: 1, 42, 309, 793, 810, 348, 56, 2;

n=6: 1, 132, 4059, 54706, 390885, 1648100, 4380095, 7682096, 9172750, 7585779, 4370731, 1749626, 481189, 89055, 10676, 785, 38, 1;

Note that the number of maximum antichains (for each n) is given by the rightmost entry in each of these rows.

References

R. P. Stanley, Enumerative Combinatorics 1, Cambridge University Press, New York, 1997.

Links

Table of n, a(n) for n=0..6.

J. Woodcock, Properties of the poset of Dyck paths ordered by inclusion

Example

For n = 3 there are 7 antichains. Assume that the five elements in the D_3 poset are depicted using a Hasse diagram and labeled A through E from bottom to top. Then the 7 antichains are: { }, {A}, {B}, {C}, {D}, {E}, {B,C}.

Crossrefs

Cf. A143672. Number of maximal antichains A143674.

Sequence in context: A052449 A053413 A232542 * A089543 A139073 A329955

Adjacent sequences: A143670 A143671 A143672 * A143674 A143675 A143676

Keyword

nonn,more

Author

Jennifer Woodcock (jennifer.woodcock(AT)ugdsb.on.ca), Aug 28 2008

Extensions

a(6) from Alois P. Heinz, Jul 28 2011

Status

approved