5,300 research outputs found
Inequalities for the h- and flag h-vectors of geometric lattices
We prove that the order complex of a geometric lattice has a convex ear
decomposition. As a consequence, if D(L) is the order complex of a rank (r+1)
geometric lattice L, then for all i \leq r/2 the h-vector of D(L) satisfies
h(i-1) \leq h(i) and h(i) \leq h(r-i).
We also obtain several inequalities for the flag h-vector of D(L) by
analyzing the weak Bruhat order of the symmetric group. As an application, we
obtain a zonotopal cd-analogue of the Dowling-Wilson characterization of
geometric lattices which minimize Whitney numbers of the second kind. In
addition, we are able to give a combinatorial flag h-vector proof of h(i-1)
\leq h(i) when i \leq (2/7)(r + 5/2).Comment: 15 pages, 2 figures. Typos fixed; most notably in Table 1. A note was
added regarding a solution to problem 4.
g-elements, finite buildings and higher Cohen-Macaulay connectivity
The main result is a proof that the g-vector of a simplicial complex with a
convex ear decomposition is an M-vector. This is a generalization of similar
results for matroid complexes. We also show that a finite building has a convex
ear decomposition. This leads to connections between higher Cohen-Macaulay
connectivity and increasing h-vectors.Comment: To appear in JCT A. 20 page
Inhomogeneous minima of mixed signature lattices
We establish an explicit upper bound for the Euclidean minimum of a number
field which depends, in a precise manner, only on its discriminant and the
number of real and complex embeddings. Such bounds were shown to exist by
Davenport and Swinnerton-Dyer. In the case of totally real fields, an optimal
bound was conjectured by Minkowski and it is proved for fields of small degree.
In this note we develop methods of McMullen in the case of mixed signature in
order to get explicit bounds for the Euclidean minimum.Comment: To appear in the Journal of Number Theor
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