Michael Falk

Arrangements and cohomology

To a matroidM is associated a graded commutative algebraA=A(M), the Orlik-Solomon algebra ofM. Motivated by its role in the construction of generalized hypergeometric functions, we study the cohomologyH* (A, dω) ofA(M) with coboundary mapdω given by multiplication by a fixed element ω ofA1. Using a description of decomposable relations inA, we construct new examples of “resonant” values of ω, and give a precise calculation ofH1 (A, dω) as a function of ω. We describe the setR1(A={ω|H1(A(M,dω)≠}, and use it as a tool in the classification of Orlik-Solomon algebras, with applications to the topology of complex hyperplane complements. We show thatR1(A) is a complete invariant of the quadratic closure ofA, and show under various hypotheses that one can reconstruct the matroidM, or at least its Tutte polynomial, from the varietyR1(A). We demonstrate with several examples thatR1 is easily calculable, may contain nonlocal components, and that combinatorial properties ofR1(A) are often sufficient to distinguish nonisomorphic rank three Orlik-Solomon algebras.

Homotopy types of line arrangements

SummaryWe prove that the complement of a real affine line arrangement inC2 is homotopy equivalent to the canonical 2-complex associated with Randells presentation of the fundamental group. This provides a much smaller model for the homotopy type of the complement of a real affine 2- or central 3-arrangement than the Salvetti complex and its cousins. As an application we prove that these exist (infinitely many) pairs of central arrangements inC3 with different underlying matroids whose complements are homotopy equivalent. We also show that two real 3-arrangements whose oriented matroids are connected by a sequence of flips are homotopy equivalent.

The minimal model of the complement of an arrangement of hyperplanes

In this paper the methods of rational homotopy theory are applied to a family of examples from singularity theory. Let A be a finite collection of hyperplanes in Cl, and let M = Cl-UHEA H. We say A is a rational K(w, 1) arrangement if the rational completion of M is aspherical. For these arrangements an identity (the LCS formula) is established relating the lower central series of 7r1 (M) to the cohomology of M. This identity was established by group-theoretic means for the class of fiber-type arrangements in previous work. We reproduce this result by showing that the class of rational K(w, 1) arrangements contains all fiber-type arrangements. This class includes the reflection arrangements of types Al and Bl. There is much interest in arrangements for which M is a K(w, 1) space. The methods developed here do not apply directly because M is rarely a nilpotent space. We give examples of K(w, 1) arrangements which are not rational K(w, 1) for which the LCS formula fails, and K(w, 1) arrangements which are not rational K(w, 1) where the LCS formula holds. It remains an open question whether rational K(w, 1) arrangements are necessarily K(w, 1). l. Introduction. An arrangement of hyperplanes is a finite collection of Clinear subspaces of dimension (1-l) in C1. To such an arrangement A is associated an open 21-manifold, the complement M = Cl-U{HIH E A}. The connections between the topology of M and the combinatorial geometry of A are the source of much current research in this area. The most successful investigations concern the cohomology of M [13, 15], whereas the most difficult unsolved problems involve the homotopy groups of M [5]. In this paper we study the link between cohomology and homotopy provided by Sullivans theory of minimal models [14]. In [6], a numerical relationship was established between 1(M) and H*(M) for the class of fiber-type arrangements. This LCS formula reads as follows: rI ( 1-tn ) Y7n (M) = PM (-t) n>l where the (>n(M) are the ranks of successive quotients in the lower central series of 1(M)) and PM(t) is the Poincare polynomial of H*(M). Because the sequence (>n(M) is related to the l-minimal model y of M, we conjecture in [5] that the LdS formula holds precisely when R determines H* (M). With the methods developed in this paper we can resolve this conjecture. Specifically, we show (Corollary 3.8) that the LCS formula holds when H*(5°) is isomorphic to H*(M). Arrangements satisfying the latter condition are called rational K(7r, l). Corollary 3.8 may be Received by the editors June 1, 1987. 1980 Mathematicts Subject Clatstsification (1985 Revitsion). Primary 55P62, 57M05; Secondary 06C10. (r)1988 American Mathematical Society 0002-9947/88

Combinatorial and Algebraic Structure in Orlik-Solomon Algebras

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On the algebra associated with a geometric lattice

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Orlik-Solomon Algebras and Tutte Polynomials

The Orlik?Solomon algebra A(G) of a matroid G is the free exterior algebra on the points, modulo the ideal generated by the circuit boundaries. On one hand, this algebra is a homotopy invariant of the complement of any complex hyperplane arrangement realizing G. On the other hand, some features of the matroid G are reflected in the algebraic structure of A(G). In this mostly expository article, we describe recent developments in the construction of algebraic invariants of A(G). We develop a categorical framework for the statement and proof of recently discovered isomorphism theorems which suggests a possible setting for classification theorems. Several specific open problems are formulated.

A note on discriminantal arrangements

Abstract Let L be a geometric lattice. Following P. Orlik and L. Solomon, Combinatorics and topology of complements of hyperplanes, Invent. math.56 (1980), 167–189, we associate with L a graded commutative algebra A(L). In this paper we introduce a new invariant ψ of the algebra A(L) which suffices to distinguish algebras for which all other known invariants coincide. This result is applied to the study of arrangements of complex hyperplanes, with L being the intersection lattice. In this case A(L) is isomorphic to the cohomology algebra of the associated hyperplane complement. The goal is to find examples of arrangements with non-isomorphic lattices but homotopy equivalent complements. The invariant introduced here effectively narrows the list of candidates. Nevertheless, we exhibit combinatorially inequivalent arrangements for which all known invariants, including ψ, coincide.

Critical points and resonance of hyperplane arrangements

The OS algebra A of a matroid M is a graded algebra related to the Whitney homology of the lattice of flats of M. In case M is the underlying matroid of a hyperplane arrangement A in ℂr , A is isomorphic to the cohomology algebra of the complement ℂr ∖∪A. Few examples are known of pairs of arrangements with non-isomorphic matroids but isomorphic OS algebras. In all known examples, the Tutte polynomials are identical, and the complements are homotopy equivalent but not homeomorphic.We construct, for any given simple matroid M0, a pair of infinite families of matroids Mn and Mn′, n ≥ 1, each containing M0 as a submatroid, in which corresponding pairs have isomorphic OS algebras. If the seed matroid M0 is connected, then Mn and Mn′ have different Tutte polynomials. As a consequence of the construction, we obtain, for any m, m different matroids with isomorphic OS algebras. Suppose one is given a pair of central complex hyperplane arrangements A0 and A1 . Let S denote the arrangement consisting of the hyperplane {0} in ∪1 . We define the parallel connection P(A0, A1), an arrangement realizing the parallel connection of the underlying matroids, and show that the direct sums A0 ⊕ A1 and S ⊕ P (A0, A1) have diffeomorphic complements.

A geometric duality for order complexes and hyperplane complements

Let v O be a fixed affine arrangement of n hyperplanes in general position in Kk . Let U(n, k) denote the set of general position arrangements whose elements are parallel translates of the hyperplanes of v o . Then U(n, k) is the complement of a central arrangement R (n, k). These are the well-known discriminantal arrangements introduced by Y. I. Manin and V. V. Schechtman. In this note we give an explicit description of R (n, k) in terms of the original arrangement v o . In terms of the underlying matroids, _q (n, k) realizes an adjoint of the dual of the matroid associated with v o . Using this description we show that, contrary to the conventional wisdom, neither the intersection lattice of S7 (n, k) nor the topology of U(n, k) is independent of the original arrangement v o .

Pure braid groups are not residually free

If �� is a master function corresponding to a hyperplane arrangement A and a collection of weights �, we investigate the relationship between the critical set of ��, the variety defined by the vanishing of the one-form !� = dlog ��, and the resonance of �. For arrangements satisfying certain conditions, we show that if � is resonant in dimension p, then the critical set of �� has codimension at most p. These include all free arrangements and all rank 3 arrangements.