OFFSET
2,2
COMMENTS
We will denote this number by a(n,n). It is also the value of the Tutte polynomial T_{G}(0,1) for G=K_{n,n}.
The formula given for a(s,t) is valid for all s>1 and t>0. Also note that a(s,t)=a(t,s) because K_{s,t}=K_{t,s}. For small values of s we have the following formulas: a(2,t)=t-1, a(3,t)=2^{t-2}(t-1)(3t-4), a(4,t)=3^{t-3}(t-1)(16t^2-41t+27), a(5,t)=4^{t-4}(t-1)(125t^3-376t^2+378t-133)
REFERENCES
W. Kook, Möbius coinvariant of complete multipartite graphs, preprint, 2002
I. Novik, A. Postnikov and B. Sturmfels: Syzygies of oriented matroids, Duke Math. J. 111 (2002), no. 2, 287-317
FORMULA
a(s, t)= sum_{i=0..s-2} (-1)^{i}*binomial(s-1,i)*w(s-1-i, t), where s, t>1 and an e.g.f. for w(a, b) is given by exp( sum_{i, j>0}i^{j-1}j^{i-1}(j-1)x^{i}y^{j}/i!j!).
EXAMPLE
a(2,2)=1. Since K_{2,2} is a cycle with four edges, the matroid complex of cycle matroid for K_{2,2} is the 2-skeleton of standard 3-simplex. Therefore the unsigned reduced Euler characteristic for this complex is |-1+4-6+4|=1
CROSSREFS
KEYWORD
nonn
AUTHOR
W. Kook and L. Thoma (andrewk(AT)math.uri.edu), Aug 20 2002
STATUS
approved