0,3

Number of unlabeled (2+2)-free posets.

Also number of upper triangular matrices with nonnegative integer entries and without zero rows or columns such that sum of all entries is equal to n. - Vladeta Jovovic (vladeta(AT)eunet.rs), Mar 10 2008

B. Bollobas and O. Riordan, Linearized chord diagrams and an upper bound for Vassiliev invariants. J. Knot Theory Ramifications 9 (2000), no. 7, 847-853.

P. E. Haxell, J. J. McDonald and S. K. Thomason, Counting interval orders, Order, 4 (1987), 269-272.

J. A. Reeds and P. C. Fishburn, Counting split interval orders, Order, Vol. 18, No. 2, 2001, pp. 129-135.

D. Zagier, Vassiliev invariants and a strange identity related to the Dedekind eta-function, Topology 40(5) (2001), 945-960.

Mireille Bousquet-Melou, Anders Claesson, Mark Dukes, Sergey Kitaev, (2+2)-free posets, ascent sequences and pattern avoiding permutations, arXiv:0806.0666 [From Mark Dukes (dukes(AT)hi.is), May 14 2009]

D. Bar-Natan, Bibliography of Vassiliev Invariants

Julie Christophe, Jean-Paul Doignon and Samuel Fiorini, Counting Biorders, J. Integer Seqs., Vol. 6, 2003.

A. Stoimenow, Enumeration of chord diagrams and an upper bound for Vassiliev invariants, J. Knot Theory Ramifications, 7 (1998), no. 1, 93-114.

Mireille Bousquet-Melou, Anders Claesson, Mark Dukes, Sergey Kitaev, (2+2)-free posets, ascent sequences and pattern avoiding permutations. arxiv:0806.0666 [From Mark Dukes (dukes(AT)hi.is), May 12 2009]

Coefficients in expansion of Sum_{k=0..inf} Prod_{j=1..k} (1-x^j) about x=1 give (-1)^n*a(n) - R. W. Gosper Aug 08, 2001

(PARI) {a(n)=polcoeff(sum(i=0, n, prod(j=1, i, 1-(1-x)^j, 1+x*O(x^n))), n)} /* Michael Somos Jul 21 2006 */

Cf. A059685, A035378.

Cf. A079144 for the labeled analogue, A059685, A035378.

Cf. A138265.

Sequence in context: A007548 A120567 A125280 this_sequence A006966 A056841 A107112

Adjacent sequences: A022490 A022491 A022492 this_sequence A022494 A022495 A022496

nonn,nice

Alexander Stoimenow (stoimeno(AT)math.toronto.edu)