and matroids

Abstract not found

Abstract We define tests of boolean functions which distinguish between linear (or quadratic)polynomials, and functions which are very far, in an appropriate sense, from these polynomials. The tests have optimal or nearly optimal trade-offs between soundness and thenumber of queries. In particular, we show that functions with small Gowers uniformity norms behave "ran-domly " with respect to hypergraph linearity tests. A central step in our analysis of quadraticity tests is the proof of an inverse theorem forthe third Gowers uniformity norm of boolean functions. The last result has also a coding theory application. It is possible to estimate efficientlythe distance from the second-order Reed-Muller code on inputs lying far beyond its listdecoding radius.

There are exactly seven excluded minors for the class of GF(4)--representable matroids. 1 Introduction We prove the following theorem. Theorem 1.1 A matroid M is GF(4)--representable if and only if M has no minor isomorphic to any of U 2;6 , U 4;6 , P 6 , F \Gamma 7 , F \Gamma 7 , P 8 , and P 00 8 . The definitions for these matroids, with a summary of their interesting properties, can be found in the Appendix. Other than P 00 8 , they were all known to be excluded minors for GF(4)-- representability (see Oxley [13,15]). The matroid P 00 8 is obtained by relaxing the unique pair of disjoint circuit--hyperplanes of P 8 . Ever since Whitney's introductory paper [24] on matroid theory, researchers have sought ways to distinguish the representable matroids. For any field F, the class of F--representable matroids is closed under taking minors. Thus, it is natural to characterize the minor--minimal matroids that are not F--representable; we refer to such matroids as excluded ...

The discrete polymatroid is a multiset analogue of the matroid. Based on the polyhedral theory on integral polymatroids developed in late 1960’s and in early 1970’s, in the present paper the combinatorics and algebra on discrete polymatroids will be studied.

We consider the concept of triangulation of an oriented matroid. We provide a definition which generalizes the previous ones by Billera-- Munson and by Anderson and which specializes to the usual notion of triangulation (or simplicial fan) in the realizable case. Then we study the relation existing between triangulations of an oriented matroid M and extensions of its dual M , via the so-called lifting triangulations. We show that this duality behaves particularly well in the class of Lawrence matroid polytopes. In particular, that the extension space conjecture for realizable oriented matroids posed by Sturmfels and Ziegler is equivalent to the restriction to Lawrence polytopes of the Generalized Baues problem for subdivisions of polytopes. We finish showing examples and a combinatorial characterization of lifting triangulations. Introduction Matroids (see [23]) and oriented matroids (see [8]) are axiomatic abstract models for combinatorial geometry over general fields and ordere...

Abstract. We present a new algorithm that can output the rankdecomposition of width at most k of a graph if such exists. For that we use an algorithm that, for an input matroid represented over a fixed finite field, outputs its branch-decomposition of width at most k if such exists. This algorithm works also for partitioned matroids. Both these algorithms are fixed-parameter tractable, that is, they run in time O(n 3) for each fixed value of k where n is the number of vertices / elements of the input. (The previous best algorithm for construction of a branchdecomposition or a rank-decomposition of optimal width due to Oum and Seymour [Testing branch-width. J. Combin. Theory Ser. B, 97(3) (2007) 385–393] is not fixed-parameter tractable.)

We verify a conjecture of P. Seymour (Europ. J. Combinatorics 2, p. 289) regarding circuits of a binary matroid. A circuit cover of a integer-weighted matroid (M; p) is a list of circuits of M such that each element e is in exactly p(e) circuits from the list. We characterize those binary matroids for which two obvious necessary conditions for a weighting (M; p) to have a circuit cover are also sufficient. Keywords: matroids, circuit covers, cycle covers, Hilbert base, sums of circuits, bond covers, cut covers AMS Classifications: 05B35, ( 05C38, 05C70, 90C10 ) Support from the National Sciences and Engineering Research Council is gratefully acknowledged. 1 Introduction In this paper, we verify a conjecture of P. Seymour [13, (16.5)] which regards covering the elements of a binary matroid with circuits. We give a forbidden-minor characterization of those matroids which have a certain "Circuit Cover Property". The special case regarding graphic matroids, solved in [1], has had a nu...

Abstract. This is a foundational paper in tropical linear algebra, which is linear algebra over the min-plus semiring. We introduce and compare three natural definitions of the rank of a matrix, called the Barvinok rank, the Kapranov rank and the tropical rank. We demonstrate how these notions arise naturally in polyhedral and algebraic geometry, and we show that they differ in general. Realizability of matroids plays a crucial role here. Connections to optimization are also discussed. 1.

Abstract. This paper introduces two new decomposition techniques which are related to the classical notion of shellability of simplicial complexes, and uses the existence of these decompositions to deduce certain numerical properties for an associated enumerative invariant. First, we introduce the notion of M-shellability, which is a generalization to pure posets of the property of shellability of simplicial complexes, and derive inequalities that the rank-numbers of M-shellable posets must satisfy. We also introduce a decomposition property for simplicial complexes called a convex ear-decomposition, and, using results of Kalai and Stanley on h-vectors of simplicial polytopes, we show that h-vectors of pure rank-d simplicial complexes that have this property satisfy h0 ≤ h1 ≤ ·· · ≤ h [d/2] and hi ≤ hd−i for 0 ≤ i ≤ [d/2]. We then show that the abstract simplicial complex formed by the collection of independent sets of a matroid (or matroid complex) admits a special type of convex eardecomposition called a PS ear-decomposition. This enables us to construct an associated M-shellable poset, whose set of rank-numbers is the h-vector of the matroid complex. This results in a combinatorial proof of a conjecture of Hibi [17] that the h-vector of a matroid complex satisfies the above two sets of inequalities. 1.