# Publications

- Polarization and Gorenstein liaison (with S. Faridi, P. Klein and J. Rajchgot)

Given a monomial ideal I and a vertex decomposition of the Stanley-Reisner complex of its polarization P(I), we give conditions that allow for the lifting of an associated basic double G-link of P(I) to a basic double G-link of I itself. We then introduce and study polarization of a Grobner basis of an arbitrary homogeneous ideal and give a relationship between geometric vertex decomposition of a polarization and elementary G-biliaison that is analogous to our result on vertex decomposition and basic double G-linkage.

- Products and powers of principal symmetric ideals (with Polymath) J. Algebra and Its Applications

Principal symmetric ideals are ideals generated by the orbit of a single polynomial under permutations of variables in a polynomial ring. In this paper we give obstructions for an ideal to be a principal symmetric ideal and we determine when a product of two principal symmetric ideals is principal symmetric and when all the powers of a principal symmetric ideal are again principal symmetric ideals.

- Betti numbers for connected sums of graded Gorenstein artinian algebras (with N. Altafi, R. Di Gennaro, F. Galetto, S. Grate, R. Miro-Roig, U. Nagel, J. Watanabe) Transactions AMS

We determine the graded Betti numbers for connected sums of graded Artinian Gorenstein algebras. We relate the connected sum construction to the doubling construction, which also produces Gorenstein rings. Specifically, we show that a connected sum of doublings is the doubling of a fiber product ring.

- The minimal free resolution of a general principal symmetric ideal (with M. Harada and L. Sega)

We introduce the class of principal symmetric ideals, which are ideals generated by the orbit of a single polynomial under the action of the symmetric group. Fixing the degree of the generating polynomial, this class of ideals is parametrized by points in a suitable projective space. We show that the minimal free resolution of a principal symmetric ideal is constant on a nonempty Zariski open subset of this projective space and we determine this resolution explicitly.

- Duality for asymptotic invariants of graded families (with M. DiPasquale and T. Nguyen) Advances in Math. 430 (2023), paper no. 109208.

The starting point of this paper is a duality for sequences of natural numbers which interchanges superadditive and subadditive sequences. We show how this duality has natural algebraic consequences in conjunction with the theory of Macaulay inverse systems and how it underpins the reciprocity between two geometric invariants: the Seshadri constant and the asymptotic regularity of a finite set of points in projective space.

- The combinatorial structure of symmetric strongly shifted ideals (with A. Costantini)

Symmetric shifted ideals are a class of monomial ideals which come equipped with an action of the symmetric group and share many of the properties of the well-studied strongly stable ideal from combinatorial commutative algebra. We study algebraic and combinatorial properties of these ideals: behavior under operations, primary decomposition, Rees algebra, and uncover interesting connections to the theory of discrete polymatroids, convex geometry, and permutohedral toric varieties.

- Lorentzian polynomial, higher Hessian and the Hodge-Riemann property of graded Artinian Gorenstein algebras (with P. Macias Marques, C. McDaniel, J. Watanabe)

We classify Artinian Gorenstein algebras in codimension two that have the Hodge-Riemann property in terms of higher Hessian matrices and total positivity of certain Toeplitz matrices.

- Polynomial growth of Betti sequences over local rings (with L. Avramov and Y. Zheng) Collectanea Math

We study sequences of Betti numbers of modules over complete intersection rings. It is known that the subsequences with even, respectively odd index I are given eventually by some polynomial in i. We study conditions under which the two polynomials agree and more generally give bounds on the degree of their difference.

- Lefschetz properties of some codimension three Artinian Gorenstein algebras (with N. Abdallah, N. Altafi, A. Iarrobino, J. Yameogo) J. Algebra 625 (2023), 28-45.

We show that every standard graded codimension three Artinian Gorenstein algebra A having low maximum value of the Hilbert function, at most six, satisfies the strong Lefschetz property provided that the characteristic is zero. When the characteristic is greater than the socle degree of A, we show that A is almost strong Lefschetz.

- Axial constants and sectional regularity of homogeneous ideals (with Polymath) Proceedings Amer. Math. Soc. 151 (2023), no. 4, 1363-1378

We introduce a notion of sectional regularity for a homogeneous ideal I which measures the regularity of its generic sections with respect to linear spaces of various dimensions. This is related to axial constants defined as the intercepts on the coordinate axes of the set of exponents of monomials in the reverse lexicographic generic initial ideal of I.

- Cohomological blow ups of graded artinian Gorenstein algebras along surjective maps (with A. Iarrobino, P. Macias Marques, C. McDaniel, J. Watanabe) International Math. Research Notices IMRN (2023), no.7, 5816-5886.

We introduce the cohomological blow up of a graded Artinian Gorenstein algebra along a surjective map, which we term BUG (Blow Up Gorenstein) for short. This is intended to translate to an algebraic context the cohomology ring of a blow up of a projective manifold along a projective submanifold.

- Rees algebras of filtrations of covering polyhedra and integral closure of powers of monomial ideals (with G. Grisalde, R. Villarreal) Research in the Mathematical Sciences 9 (2022), no.1, paper 13

The aims of this work are to study Rees algebras of filtrations of monomial ideals associated to covering polyhedra of rational matrices with non-negative entries and non-zero columns using combinatorial optimization and integer programming, and to study powers of monomial ideals and their integral closures using irreducible decompositions and polyhedral geometry.

- Computing real powers of monomial ideals (with Polymath) J. Symbolic Comput. 116 (2023) 39-57

While it is customary for the exponentiation operation on ideals to consider natural powers, we extend this notion to powers where the exponent is a positive real number. Real powers of a monomial ideal generalize the integral closure operation and highlight many interesting connections to the theory of convex polytopes.

- Consequences of the packing problem (with Polymath) J. Algebraic Combinatorics 54 (2021), no.4 1095-1117

We study several consequences of the packing problem, a conjecture from combinatorial optimization, using algebraic invariants of square-free monomial ideals. While the packing problem is currently unresolved, we successfully settle the validity of its consequences.

- Convex bodies and asymptotic invariants for powers of monomial ideals (with Polymath) J. Pure Applied Algebra 226 (2022), no.10

Continuing a well-established tradition of associating convex bodies to monomial ideals, we initiate a program to construct asymptotic Newton polyhedra from decompositions of monomial ideals. This is achieved by forming a graded family of ideals based on a given decomposition. Based on irreducible decompositions, we introduce a novel family of irreducible powers which generalizes the notions of ordinary and symbolic powers.

- Symbolic Rees algebras (with E. Grifo) a chapter in the volume Commutative Algebra, Springer, 2021

We survey old and new approaches to the study of symbolic powers of ideals. Our focus is on the symbolic Rees algebra of an ideal, viewed both as a tool to investigate its symbolic powers and as a source of challenging problems in its own right. We provide an invitation to this area of investigation by stating several open questions.

- Canonical resolutions over Koszul algebras (with E. Faber, M. Juhnke-Kubitzke, H. Lindo, C. Miller, R. R.G.) Women in Commutative Algebra - Proceedings of the WICA Conference, Springer, 2021

We generalize Buchsbaum and Eisenbud’s resolutions for the powers of the maximal ideal of a polynomial ring to resolve powers of the homogeneous maximal ideal over graded Koszul algebras.

- Quadratic Gorenstein algebras with many surprising properties (with J. McCullough) Arch. Math. (Basel) 115 (2020), no. 5, 509-521

Using the method of idealization, we produce examples of graded Artinian Gorenstein algebras that are not Koszul, do not satisfy the subadditivity property for degree of syzygies and fail to satisfy the Lefschetz property.

- Singular loci of reflection arrangements and the containment problem (with B. Drabkin) Math. Zeitschrift 299 (2021), no. 1-2, 867-895

We paper provide insights into the role of symmetry in studying polynomial functions vanishing to high order on an algebraic variety. The varieties we study are singular loci of hyperplane arrangements in projective space, with emphasis on arrangements arising from complex reflection groups. We provide minimal sets of equations for the radical ideals defining these singular loci and study containments between the ordinary and symbolic powers of these ideals.

- Implicitization of tensor product surfaces via virtual projective resolutions (with E. Duarte) Mathematics of Computation 89 (2020), no. 326, 3023-3056

We derive the implicit equations for certain parametric surfaces in three-dimensional projective space termed tensor product surfaces. Our method computes the implicit equation for such a surface based on the knowledge of the syzygies of the base point locus of the parametrization by means of constructing an explicit virtual projective resolution.

- Betti numbers of symmetric shifted ideals (with J. Biermann, H. de Alba, F. Galetto, S. Murai, U. Nagel, A. O’Keefe, T. Romer) J. Algebra 560 (2020), 312?342

We introduce a new class of monomial ideals which we call symmetric shifted ideals. Symmetric shifted ideals are fixed by the natural action of the symmetric group and, within the class of monomial ideals fixed by this action, they can be considered as an analogue of stable monomial ideals within the class of monomial ideals. We show that a symmetric shifted ideal has linear quotients and compute its (equivariant) graded Betti numbers. As an application of this result, we obtain several consequences for graded Betti numbers of symbolic powers of defining ideals of star configurations.

- Connected sums of graded artinian Gorenstein algebras and Lefschetz properties (with A. Iarrobino and C. McDaniel) J. Pure Applied Algebra, 226 (2022), no. 1, 106787

A connected sum construction for local rings was introduced in a paper by H. Ananthnarayan, L. Avramov, and W.F. Moore. In the graded artinian Gorenstein case, this can be viewed as an algebraic analogue of the topological construction of the same name. We give two alternative description of this algebraic connected sum: the first uses algebraic analogues of Thom classes of vector bundles and Gysin homomorphisms, the second is in terms of Macaulay dual generators. We also investigate the extent to which the connected sum construction preserves the weak or strong Lefschetz property.

- General minimum distance functions and algebraic invariants of Geramita ideals (with S. Cooper, S. Tohaneanu, M. Vaz Pinto and R. Villarreal), Advances in Applied Mathematics 112 (2020) 101940

Motivated by notions from coding theory, we study the generalized minimum distance (GMD) function of a graded ideal I in a polynomial ring over an arbitrary field using commutative algebraic methods. It is shown that the GMD function is non-decreasing as a function of its first argument and non-increasing as a function of the second argument. For vanishing ideals over finite fields, we show that the GMD function is in fact strictly decreasing as a function of the second argument until it stabilizes. We also study algebraic invariants of Geramita ideals. Those ideals are graded, unmixed, 1-dimensional and their associated primes are generated by linear forms. We also examine GMD functions of complete intersections and show some special cases of two conjectures of Toh?neanu-Van Tuyl and Eisenbud-Green-Harris.

- Frieze varieties: A characterization of the finite-tame-wild trichotomy for acyclic quivers (with K. Li, L. Li, M. Mills and R. Schiffler) Advances in Math. 367 (2020), 107130

We introduce a new class of algebraic varieties which we call frieze varieties. Each frieze variety is determined by an acyclic quiver. The frieze variety is defined in an elementary recursive way by constructing a set of points in affine space. From a more conceptual viewpoint, the coordinates of these points are specializations of cluster variables in the cluster algebra associated to the quiver. We give a new characterization of the finite-tame-wild trichotomy for acyclic quivers in terms of their frieze varieties by showing that an acyclic quiver is representation finite, tame, or wild, respectively, if and only if the dimension of its frieze variety is 0,1, or ?2, respectively.

- Computations involving symbolic powers (with B. Drabkin, E. Grifo, B. Stone) and the Macaulay2 package SymbolicPowers J. Software for Algebra and Geometry vol. 9, no.1 (2019), 71?80

Symbolic powers are a classical commutative algebra topic that relates to primary decomposition, consisting, in some circumstances, of the functions that vanish up to a certain order on a given variety. However, these are notoriously difficult to compute, and there are seemingly simple questions related to symbolic powers that remain open even over polynomial rings. In this paper, we describe a Macaulay2 software package that allows for computations of symbolic powers of ideals and which can be used to study the equality and containment problems, among others.

- Negative curves on symmetric blowups of the projective plane, resurgences and Waldschmidt constants (with Th. Bauer, S. Di Rocco, B. Harbourne, J. Huizenga, T. Szemberg) International Math. Research Notices IMRN 2019, no. 24, 7459-7514

In this paper we study the surface X obtained by blowing up the projective plane in the singular points of one of two highly symmetric line configurations known as the Klein configuration and the Wiman configuration respectively. We study invariant curves on X in detail, with a particular emphasis on curves of negative self-intersection. We use the representation theory of the stabilizers of the singular points to discover several invariant curves of negative self-intersection on X, and use these curves to study Nagata-type questions for linear series on X.

- Symbolic powers of codimension two Cohen-Macaulay ideals (with S. Cooper, G. Fatabbi, E. Guardo, A. Lorenzini, J. Migliore, U. Nagel, J. Szpond, A. Van Tuyl) Comm. Algebra 48 (2020), no.11, 4663-4680

Under the additional hypothesis that X is a local complete intersection, we classify when I(X)^(m) = I(X)^m for all m >= 1. The key tool to prove this classification is the ability to construct a graded minimal free resolution of I^m under these hypotheses. Among our applications are significantly simplified proofs for known results about symbolic powers of ideals of points in P^1 x P^1.

- The Waldschmidt constant for squarefree monomial ideals (with C. Bocci, S. Cooper, E. Guardo, B. Harbourne, M. Janssen, U. Nagel, A. Van Tuyl, T. Vu) J. Algebraic Combinatorics 44 (2016) no. 4, 875-904

Given a squarefree monomial ideal I, we show that the Waldschmidt constant of I can be expressed as the optimal solution to a linear program constructed from the primary decomposition of I. By applying results from fractional graph theory, we can then express the Waldschmidt constant in terms of the fractional chromatic number of a hypergraph also constructed from the primary decomposition of I. Moreover, we prove a Chudnovsky-like lower bound on this constant, thus verifying a conjecture of Cooper-Embree-Ha-Hoefel for monomial ideals in the squarefree case.

- Ordinary and symbolic algebras of Fermat point configurations (with U. Nagel) J. Algebra 468 (2016), 80-102

Fermat ideals define planar point configurations that are closely related to the base locus of a specific pencil of curves. We describe the minimal generators and free resolutions of the ordinary powers and many of the symbolic powers of these ideals. We show that the symbolic Rees algebras of Fermat ideals are Noetherian.

- [A homological criterion for the failure of containment of the symbolic cube in the square of some ideals of points in P^2] (http://arxiv.org/abs/1410.0312) J. Pure Applied Algebra 219 (2015) no.11, 4857-4871

We establish a criterion for the failure of the containment of the symbolic cube in the square for 3-generated ideals I defining reduced sets of points in P^2. Our criterion arises from studying the minimal free resolutions of the powers of I, specifically the minimal free resolutions for I^2 and I^3. We apply this criterion to two point configurations that have recently arisen as counterexamples to a question of B. Harbourne and C. Huneke: the Fermat configuration and the Klein configuration.

- Determinants of incidence and Hessian matrices arising from the vector space lattice (with S. Nasseh and J. Watanabe) J. Commut. Algebra 11 (2019) no. 1, 131-154

We give explicit formulas for the determinants of the incidence and Hessian matrices arising from the interaction between the rank 1 and rank n-1 level sets of the subspace lattice of an n-dimensional finite vector space. Our exploration is motivated by the fact that both of these matrices arise naturally in the study of the combinatorial and algebraic Lefschetz properties.

- Resurgences for ideals of special point configurations in P^N coming from hyperplane arrangements (with M. Dumnicki, B. Harbourne, U. Nagel, T. Szemberg and H. Tutaj-Gasinska ) J. Algebra 443 (2015), 383-394

It had been expected for several years that I^(Nr-N+1) should be contained in I^r for the ideal I of any finite set of points in P^N for all r>0, but in the last year various counterexamples have now been constructed, all involving point sets coming from hyperplane arrangements. In the present work, we compute their resurgences and obtain in particular the first examples where the resurgence and the asymptotic resurgence are not equal.

- A tight bound on the projective dimension of four quadrics (with C. Huneke, P. Mantero and J. McCullough) with accompanying Macaulay 2 code.

J. Pure Appl. Algebra 222 (2018) no. 9, 2524-2551 Motivated by a question posed by Mike Stillman, we show that the projective dimension of an ideal generated by four quadric forms in a polynomial ring has projective dimension at most 6.

- A multiplicity bound for graded rings and a criterion for the Cohen-Macaulay property (with C. Huneke, P. Mantero and J. McCullough) Proc. Amer. Math. Soc. 143 (2015) no.6, 2365-2377

We prove an upper bound for the multiplicity of R/I, where I is a homogeneous ideal of the form I=J+(F) and J is a Cohen-Macaulay ideal. The bound is given in terms of invariants of R/J and the degree of F. We show that ideals achieving this bound have high depth and deduce a numerical criterion for the Cohen-Macaulay property. Applications to quasi-Gorenstein rings and almost complete intersection ideals are given.

- Containment counterexamples for ideals of various configurations of points in P^N (with B. Harbourne) J. Pure Appl. Algebra 219 (2015), 1062-1072

We provide counterexamples to a conjecture of Harbourne and Huneke regarding containments between regular powers and symbolic powers of ideals of points in projective space P^N. We show that the conjecture fails in every prime characteristic p>2 when N=2 and we provide additional counterexamples for higher dimensional projective spaces.

- The projective dimension of codimension two algebra presented by quadrics (with C. Huneke, P. Mantero and J. McCullough) J. Algebra 393 (2013), 170-186

We prove a sharp upper bound for the projective dimension of ideals of height two generated by quadrics in a polynomial ring with arbitrary large number of variables.

- Multiple structures with arbitrarily large projective dimension on linear subspaces (with C. Huneke, P. Mantero and J. McCullough) J. Algebra 447 (2016), 183-205

We show that no finite characterization of multiple structures on linear spaces is possible by constructing structures with arbitrarily large projective dimension. Our methods build upon a family of ideals with large projective dimension using linkage. The result is in stark contrast to Manolache’s characterization of Cohen-Macaulay multiple structures in codimension 2 and multiplicity at most 4 and also to Engheta’s characterization of unmixed ideals of height 2 and multiplicity 2.

Computations in intersection rings of flag bundles (with D. Grayson and M. Stillman) This paper arose out of an observation that was made while I was working on the Symmetric polynomials package for Macaulay2. Intersection rings of flag varieties and of isotropic flag varieties are generated by Chern classes of the tautological bundles modulo the relations coming from multiplicativity of total Chern classes. In this paper we describe the Groebner bases of the ideals of relations and give applications to computation of intersections, as implemented in Macaulay2.

Bounding projective dimension (with J. McCullough) a chapter in the book Commutative Algebra, Springer-Verlag London, 2013

The celebrated Hilbert Syzygy Theorem states that the projective dimension of any ideal in a polynomial ring on n variables is at most n-1. This paper surveys recent progress on Stillman?s question, asking whether the degrees of a set of homogeneous polynomials suffice in order to bound the projective dimension of the ideal they generate, without prior knowledge of the ambient polynomial ring (hence without using the number of variables).

- Syzygies and singularities of tensor product surfaces of bidegree (2,1) (with H. Schenck and J. Validashti), Math. Comp. 83 (2014), 1337-1372

We study the associated ideal of a bigraded parametrization of a surface in P^3 from the standpoint of commutative algebra, proving that there are exactly six numerical types of possible bigraded minimal free resolution. These resolutions play a key role in determining the implicit equation of the image, via work of Buse-Jouanolou, Buse-Chardin, Botbol and Botbol-Dickenstein-Dohm on the approximation complex. In particular this allows us to completely describe the implicit equation and singular locus of the image.

- Syzygy theorems via comparison of order ideals on a hypersurface (with P. A. Griffith) J. Pure Appl. Algebra 216 (2012), no. 2, 468-479

We introduce a weak order ideal property that suffices for establishing the Evans-Griffith Syzygy Theorem. We study this weak order ideal property in settings that allow for comparison between homological algebra over a local ring R versus a hypersurface ring R/(x^n). Consequently, we solve some relevant cases of the Evans-Griffith syzygy conjecture over local rings of unramified mixed characteristic p, with the case of syzygies of prime ideals of Cohen-Macaulay local rings of unramified mixed characteristic being noted.

- Inverse systems, fat points and the weak Lefschetz property (with B. Harbourne and H. Schenck) J. Lond. Math. Soc. (2) 84 (2011), no. 3, 712-730

We use the inverse system dictionary to connect ideals generated by powers of linear forms to ideals of fat points and show that failure of WLP for powers of linear forms in at least four variables is connected to the geometry of the associated fat point scheme.

- Ideals with Large(r) Projective Dimension and Regularity (with J. Beder, J. McCullough, L. Nunez-Betancourt, B. Snapp, B. Stone) J. Symbolic Comput. 46 (2011), no. 10, 1105-1113

This paper is an outcome of the Mathematical Research Communities program. I am grateful to AMS and the organizers for this wonderful opportunity. We define a family of homogeneous ideals with large projective dimension and regularity relative to the number of generators and their common degree. This family subsumes and improves upon constructions given by Caviglia and McCullough In particular, we describe a family of homogeneous ideals with three generators of degree d in arbitrary characteristic whose projective dimension grows asymptotically an exponential function in d. Here is the Macaulay 2 code mentioned in the paper.

- The Weak Lefschetz Property and powers of linear forms in K[x, y, z] (with H. Schenck) Proc. Amer. Math. Soc. 138 (2010), no. 7, 2335-2339

We show that any artinian quotient of K[x, y, z] by an ideal I generated by powers of linear forms has the Weak Lefschetz Property.