About
Me
I am an associate professor
of mathematics at the University of Nebraska-Lincoln. My research focuses on
commutative algebra with a geometric and computational flavor.
I have been awarded the
2018 Harold & Esther
Edgerton Junior Faculty Award for creative research,
extraordinary teaching abilities, and academic promise.
I co-organize a conference
on Unexpected and Asymptotic
Properties of Algebraic Varieties in Lincoln, NE on August
11-13, 2023.
I co-organize the workshop Women in Commutative Algebra II at CIRM Trento on October
16-20, 2023.
In the news: When life
gives you lemons, make mathematicians, an article in the Notices
of the AMS about the Polymath program - a large scale online REU I mentor.
My Students
Graduate students:
- Ben Drabkin
- PhD
2020
- Andrew Conner - MA 2020
- Erica
Hopkins (co-advised
with Mark Walker) - PhD 2021
- Michael
DeBellevue (co-advised with Mark Walker) - PhD 2022
- Shah Roshan Zamir
- Ben
Carse Noltig award
(2021-2022)
- Juliann
Geraci - Don
Miller Outstanding GTA award (2022-2023)
Undergraduate
students:
- Joey
Becker – Chair’s prize for best
graduating math major (2015)
- Diana
(Xuehua) Zhong - graduate student at North
Carolina State University (since 2018)
Undergraduate
Activities
Videos
from Selected Talks
- Reflection arrangements, syzygies, and
the containment problem (video)
Fellowship of the Ring
national
commutative algebra seminar
organized by MSRI, Jul 2020
- Connected
sums of graded artinian Gorenstein
algebras and the Lefschetz properties (video) BIRS workshop New
Trends in Syzygies in Banff, June 2018
- Combinatorial methods for symbolic powers (video) CMO Workshop on Symbolic
and Ordinary Powers of Ideals in Oaxaca, May 2017
- A hands-on approach to tensor product surfaces of
bidegree (2,1) (video) CMO Workshop on
Computational Algebra and geometric Modeling in Oaxaca, Aug 2016
Research and Publications
My research is in commutative algebra, with an interest
in computational algebra, homological methods and connections to algebraic
geometry.
1.
Duality for asymptotic invariants
of graded families (with
M. DiPasquale and T. Nguyen)
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.
2.
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.
3.
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.
4.
Polynomial growth of Betti sequences over local rings (with L. Avramov and Y.
Zheng)
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.
5. 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.
6. 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.
7. 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.
8.
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.
9.
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.
10. 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.
11. 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.
12.
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.
13. 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.
14. 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.
15. 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.
16.
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.
17. Betti
numbers of symmetric shifted ideals (with J. Biermann, H. de Alba, F. Galetto,
S. Murai, U. Nagel, A. O’Keefe, T. Römer) 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.
18. 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.
19. General minimum distance
functions and algebraic invariants of Geramita
ideals (with S. Cooper, Ș.
Tohăneanu, 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 Tohăneanu-Van Tuyl and Eisenbud-Green-Harris.
20. 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.
21. 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.
22.
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.
23. 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.
24. 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.
25. 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.
26. A homological
criterion for the failure of containment of the symbolic cube in the square
of some ideals of points in P^2 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.
27. 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.
28. 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-Gasińska ) J. Algebra 443 (2015), 383-394
It had been expected for several years that
I^(Nr−N+1)⊆ I^r should hold 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.
29. 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.
30. 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.
31. 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.
32. 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.
33. 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.
34. 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.
35. 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).
36. 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.
37. 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.
38. 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.
39. 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.
40. 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.
Software
I have
authored or co-authored several packages for the computer algebra program Macaulay2:
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