Art M. Duval

We study the vector spaces and integer lattices of cuts and flows associated with an arbitrary finite CW complex, and their relationships to group invariants including the critical group of a complex. Our results extend to higher dimension the theory of cuts and flows in graphs, most notably the work of Bacher, de la Harpe and Nagnibeda. We construct explicit bases for the cut and flow spaces, interpret their coefficients topologically, and give sufficient conditions for them to be integral bases of the cut and flow lattices. Second, we determine the precise relationships between the discriminant groups of the cut and flow lattices and the higher critical and cocritical groups with error terms corresponding to torsion (co)homology. As an application, we generalize a result of Kotani and Sunada to give bounds for the complexity, girth, and connectivity of a complex in terms of Hermite's constant.

We study the vector spaces and integer lattices of cuts and flows of an arbitrary finite CW complex, and their relationships to its critical group and related invariants. Our results extend the theory of cuts and flows in graphs, in particular the work of Bacher, de la Harpe and Nagnibeda. We construct explicit bases for the cut and flow spaces, interpret their coefficients topologically, and describe sufficient conditions for them to be integral bases of the cut and flow lattices. Second, we determine the precise relationships between the discriminant groups of the cut and flow lattices and the higher critical and cocritical groups; these are expressed as short exact sequences with error terms corresponding to torsion (co)homology. As an application, we generalize a result of Kotani and Sunada to give bounds for the complexity, girth, and connectivity of a complex in terms of Hermite's constant. Nous étudions les espaces vectoriels et les réseaux entiers des coupures et flots d’un CW-complexe arbitraire fini, et leur relations avec son groupe critical et invariants similaires. Nos résultats développent la théorie des coupures et flots dans les graphes, en particulier le travail de Bacher, de la Harpe et Nagnibeda. Nous construisons des bases explicites pour les espaces des coupures et des flots, donnons une description topologique de leurs coefficients, et décrivons conditions suffisants pour qu’ils soient des bases entières des réseaux des coupures et des flots.De plus, nous déterminons les relations précises entre les groupes discriminantes des réseaux, et les groupes critical et cocritical; ces relations prennent la forme des suites exactes courtes, avec termes correspondant à la torsion (co)homologie. Comme application, nous généralisons un résultat de Kotani et Sunada sur bornes pour la complexité, la circonférence, et la connectivité d’un CW-complexe en termes de la constante d’Hermite.

The Laplacian spectral recursion, satisfied by matroid complexes and shifted complexes, expresses the eigenvalues of the combinatorial Laplacian of a simplicial complex in terms of its deletion and contraction with respect to vertex e, and the relative simplicial pair of the deletion modulo the contraction. We generalize this recursion to relative simplicial pairs, which we interpret as intervals in the Boolean algebra. The deletion modulo contraction term is replaced by the result of removing from the interval all pairs of faces in the interval that differ only by vertex e. We show that shifted pairs and some matroid pairs satisfy this recursion. We also show that the class of intervals satisfying this recursion is closed under a wide variety of operations, including duality and taking skeleta.

A recursion due to Kook expresses the Laplacian eigenvalues of a matroid M in terms of the eigenvalues of its deletion M-e and contraction M/e by a fixed element e, and an error term. We show that this error term is given simply by the Laplacian eigenvalues of the pair (M-e, M/e). We further show that by suitably generalizing deletion and contraction to arbitrary simplicial complexes, the Laplacian eigenvalues of shifted simplicial complexes satisfy this exact same recursion. We show that the class of simplicial complexes satisfying this recursion is closed under a wide variety of natural operations, and that several specializations of this recursion reduce to basic recursions for natural invariants. We also find a simple formula for the Laplacian eigenvalues of an arbitrary pair of shifted complexes in terms of a kind of generalized degree sequence.

A recursion due to Kook expresses the Laplacian eigenvalues of a matroid M in terms of the eigenvalues of its deletion M-e and contraction M/e by a fixed element e , and an error term. We show that this error term is given simply by the Laplacian eigenvalues of the pair (M-e, M/e) . We further show that by suitably generalizing deletion and contraction to arbitrary simplicial complexes, the Laplacian eigenvalues of shifted simplicial complexes satisfy this exact same recursion. We show that the class of simplicial complexes satisfying this recursion is closed under a wide variety of natural operations, and that several specializations of this recursion reduce to basic recursions for natural invariants. We also find a simple formula for the Laplacian eigenvalues of an arbitrary pair of shifted complexes in terms of a kind of generalized degree sequence.