No Arabic abstract
We introduce a new algebra associated with a hyperplane arrangement $mathcal{A}$, called the Solomon-Terao algebra $mbox{ST}(mathcal{A},eta)$, where $eta$ is a homogeneous polynomial. It is shown by Solomon and Terao that $mbox{ST}(mathcal{A},eta)$ is Artinian when $eta$ is generic. This algebra can be considered as a generalization of coinvariant algebras in the setting of hyperplane arrangements. The class of Solomon-Terao algebras contains cohomology rings of regular nilpotent Hessenberg varieties. We show that $mbox{ST}(mathcal{A},eta)$ is a complete intersection if and only if $mathcal{A}$ is free. We also give a factorization formula of the Hilbert polynomials when $mathcal{A}$ is free, and pose several related questions, problems and conjectures.
We study the combinatorics of hyperplane arrangements over arbitrary fields. Specifically, we determine in which situation an arrangement and its reduction modulo a prime number have isomorphic lattices via the use of minimal strong $sigma$-Grobner bases. Moreover, we prove that the Teraos conjecture over finite fields implies the conjecture over the rationals.
There is a trinity relationship between hyperplane arrangements, matroids and convex polytopes. We expand it as resolving the complexity issue expected by Mnevs universality theorem and conduct combinatorializing so the theory over fields becomes realization of our combinatorial theory. A main theorem is that for n less than or equal to 9 a specific and general enough kind of matroid tilings in the hypersimplex Delta(3,n) extend to matroid subdivisions of Delta(3,n) with the bound n=9 sharp. As a straightforward application to realizable cases, we solve an open problem in algebraic geometry proposed in 2008.
We study the combinatorics of tropical hyperplane arrangements, and their relationship to (classical) hyperplane face monoids. We show that the refinement operation on the faces of a tropical hyperplane arrangement, introduced by Ardila and Develin in their definition of a tropical oriented matroid, induces an action of the hyperplane face monoid of the classical braid arrangement on the arrangement, and hence on a number of interesting related structures. Along the way, we introduce a new characterization of the types (in the sense of Develin and Sturmfels) of points with respect to a tropical hyperplane arrangement, in terms of partial bijections which attain permanents of submatrices of a matrix which naturally encodes the arrangement.
We study the hyperplane arrangements associated, via the minimal model programme, to symplectic quotient singularities. We show that this hyperplane arrangement equals the arrangement of CM-hyperplanes coming from the representation theory of restricted rational Cherednik algebras. We explain some of the interesting consequences of this identification for the representation theory of restricted rational Cherednik algebras. We also show that the Calogero-Moser space is smooth if and only if the Calogero-Moser families are trivial. We describe the arrangements of CM-hyperplanes associated to several exceptional complex reflection groups, some of which are free.
We study the symmetric subquotient decomposition of the associated graded algebras $A^*$ of a non-homogeneous commutative Artinian Gorenstein (AG) algebra $A$. This decomposition arises from the stratification of $A^*$ by a sequence of ideals $A^*=C_A(0)supset C_A(1)supsetcdots$ whose successive quotients $Q(a)=C(a)/C(a+1)$ are reflexive $A^*$ modules. These were introduced by the first author, and have been used more recently by several groups, especially those interested in short Gorenstein algebras, and in the scheme length (cactus rank) of forms. For us a Gorenstein sequence is an integer sequence $H$ occurring as the Hilbert function for an AG algebra $A$, that is not necessarily homogeneous. Such a Hilbert function $H(A)$ is the sum of symmetric non-negative sequences $H_A(a)=H(Q_A(a))$, each having center of symmetry $(j-a)/2$ where $j$ is the socle degree of $A$: we call these the symmetry conditions, and the decomposition $mathcal{D}(A)=(H_A(0),H_A(1),ldots)$ the symmetric decomposition of $H(A)$. We here study which sequences may occur as the summands $H_A(a)$: in particular we construct in a systematic way examples of AG algebras $A$ for which $H_A(a)$ can have interior zeroes, as $H_A(a)=(0,s,0,ldots,0,s,0)$. We also study the symmetric decomposition sets $mathcal{D}(A)$, and in particular determine which sequences $H_A(a)$ can be non-zero when the dual generator is linear in a subset of the variables. Several groups have studied exotic summands of the Macaulay dual generator $F$. Studying these, we recall a normal form for the Macaulay dual generator of an AG algebra that has no exotic summands. We apply this to Gorenstein algebras that are connected sums. We give throughout many examples and counterexamples, and conclude with some open questions about symmetric decomposition.