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A222865 Weakly graded (3+1)-free partially ordered sets (posets) on n labeled vertices. 3
1, 1, 3, 19, 195, 2551, 41343, 826939, 20616795, 658486351, 28264985223, 1725711709459, 155998194920835, 21019550046219271, 4162663551546902223, 1192847436856343300779, 489879387071459457083115, 286844271719979335180726911, 238844671940165660117456403543 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Here "weakly graded" means that there is a rank function rk from the vertices to the integers such that whenever x covers y we have rk(x) = rk(y) + 1.  Alternate terminology includes "graded" and "ranked."  A poset is said to be (3+1)-free if it does not contain four vertices  a, b, c, d such that a < b < c and d is incomparable to the other three.

LINKS

Table of n, a(n) for n=0..18.

J. B. Lewis and Y. X. Zhang, Enumeration of Graded (3+1)-Avoiding Posets, To appear, J. Combinatorial Theory, Series A.

FORMULA

G.F. is W(e^x, Psi(x)) where W(x, y) = (1 - x)y/x + (2x^3 + (x^3 - 2x^2)y)/(2x^2 + x + (x^2 - 2x - 1)y) and Psi(x) is the GF for A047863.

MATHEMATICA

m = maxExponent = 19;

Psi[x_] = Sum[E^(2^n x) x^n/n!, {n, 0, m}] + O[x]^m;

W[x_, y_] = (1-x)y/x + (2x^3 + (x^3 - 2x^2)y)/(2x^2 + x + (x^2-2x-1) y);

CoefficientList[W[E^x, Psi[x]] + O[x]^m, x] Range[0, m-1]! (* Jean-Fran├žois Alcover, Dec 11 2018 *)

CROSSREFS

For weakly graded (3+1)-free posets by height, see A222866.  For strongly graded (3+1)-free posets, see A222863.  For all weakly graded posets, see A001833.  For all (3+1)-free posets, see A079145.

Sequence in context: A001517 A080893 A028854 * A108292 A053554 A048172

Adjacent sequences:  A222862 A222863 A222864 * A222866 A222867 A222868

KEYWORD

nonn

AUTHOR

Joel B. Lewis, Mar 07 2013

STATUS

approved

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Last modified February 17 15:19 EST 2019. Contains 320220 sequences. (Running on oeis4.)