4,351 research outputs found
Toric degenerations of Grassmannians and Schubert varieties from matching field tableaux
We study the combinatorics of Gr\"obner degenerations of Grassmannians and
the Schubert varieties inside them. We provide a family of binomial ideals
whose combinatorics is governed by tableaux induced by matching fields in the
sense of Sturmfels and Zelevinsky. We prove that these ideals are all
quadratically generated and they yield a SAGBI basis of the Pl\"ucker algebra.
This leads to a new family of toric degenerations of Grassmannians. Moreover,
we apply our results to construct a family of Gr\"obner degenerations of
Schubert varieties inside Grassmannians. We provide a complete characterization
of toric ideals among these degenerations in terms of the combinatorics of
matching fields, permutations, and semi-standard tableaux
Toric degenerations of flag varieties from matching field tableaux
We present families of tableaux which interpolate between the classical
semi-standard Young tableaux and matching field tableaux. Algebraically, this
corresponds to SAGBI bases of Pl\"ucker algebras. We show that each such family
of tableaux leads to a toric ideal, that can be realized as initial of the
Pl\"ucker ideal, hence a toric degeneration for the flag variety
Standard monomial theory and toric degenerations of Schubert varieties from matching field tableaux
We study Gr\"obner degenerations of Schubert varieties inside flag varieties.
We consider toric degenerations of flag varieties induced by matching fields
and semi-standard Young tableaux. We describe an analogue of matching field
ideals for Schubert varieties inside the flag variety and give a complete
characterization of toric ideals among them. We use a combinatorial approach to
standard monomial theory to show that block diagonal matching fields give rise
to toric degenerations. Our methods and results use the combinatorics of
permutations associated to Schubert varieties, matching fields and their
corresponding tableaux
The expression of stlA in Photorhabdus luminescens is controlled by nutrient limitation
Photorhabdus is a genus of Gram-negative entomopathogenic bacteria that also maintain a mutualistic association with nematodes from the family Heterorhabditis. Photorhabdus has an extensive secondary metabolism that is required for the interaction between the bacteria and the nematode. A major component of this secondary metabolism is a stilbene molecule, called ST. The first step in ST biosynthesis is the non-oxidative deamination of phenylalanine resulting in the production of cinnamic acid. This reaction is catalyzed by phenylalanine-ammonium lyase, an enzyme encoded by the stlA gene. In this study we show, using a stlA-gfp transcriptional fusion, that the expression of stlA is regulated by nutrient limitation through a regulatory network that involves at least 3 regulators. We show that TyrR, a LysR-type transcriptional regulator that regulates gene expression in response to aromatic amino acids in E. coli, is absolutely required for stlA expression. We also show that stlA expression is modulated by σS and Lrp, regulators that are implicated in the regulation of the response to nutrient limitation in other bacteria. This work is the first that describes pathway-specific regulation of secondary metabolism in Photorhabdus and, therefore, our study provides an initial insight into the complex regulatory network that controls secondary metabolism, and therefore mutualism, in this model organism
Conditional independence ideals with hidden variables
We study a class of determinantal ideals that are related to conditional
independence (CI) statements with hidden variables. Such CI statements
correspond to determinantal conditions on a matrix whose entries are
probabilities of events involving the observed random variables. We focus on an
example that generalizes the CI ideals of the intersection axiom. In this
example, the minimal primes are again determinantal ideals, which is not true
in general.Comment: 20 pages, 1 figure, 4 table
The equivariant Ehrhart theory of polytopes with order-two symmetries
We study the equivariant Ehrhart theory of families of polytopes that are
invariant under a non-trivial action of the group with order two. We study
families of polytopes whose equivariant -polynomial both succeed and fail
to be effective, in particular, the symmetric edge polytopes of cycles and the
rational cross-polytope. The latter provides a counterexample to the
effectiveness conjecture if the requirement that the vertices of the polytope
have integral coordinates is loosened to allow rational coordinates. Moreover,
we exhibit such a counterexample whose Ehrhart function has period one and
coincides with the Ehrhart function of a lattice polytope.Comment: 16 page
A note on the Cops & Robber game on graphs embedded in non-orientable surfaces
The Cops and Robber game is played on undirected finite graphs. A number of
cops and one robber are positioned on vertices and take turns in sliding along
edges. The cops win if they can catch the robber. The minimum number of cops
needed to win on a graph is called its cop number. It is known that the cop
number of a graph embedded on a surface of genus is at most ,
if is orientable (Schroeder 2004), and at most , otherwise
(Nowakowski & Schroeder 1997).
We improve the bounds for non-orientable surfaces by reduction to the
orientable case using covering spaces.
As corollaries, using Schroeder's results, we obtain the following: the
maximum cop number of graphs embeddable in the projective plane is 3; the cop
number of graphs embeddable in the Klein Bottle is at most 4, and an upper
bound is for all other .Comment: 5 pages, 1 figur
Bifunctional catalytic electrode
The present invention relates to an oxygen electrode for a unitized regenerative hydrogen-oxygen fuel cell and the unitized regenerative fuel cell having the oxygen electrode. The oxygen electrode contains components electrocatalytically active for the evolution of oxygen from water and the reduction of oxygen to water, and has a structure that supports the flow of both water and gases between the catalytically active surface and a flow field or electrode chamber for bulk flow of the fluids. The electrode has an electrocatalyst layer and a diffusion backing layer interspersed with hydrophilic and hydrophobic regions. The diffusion backing layer consists of a metal core having gas diffusion structures bonded to the metal core
Conditional probabilities via line arrangements and point configurations
We study the connection between probability distributions satisfying certain
conditional independence (CI) constraints, and point and line arrangements in
incidence geometry. To a family of CI statements, we associate a polynomial
ideal whose algebraic invariants are encoded in a hypergraph. The primary
decompositions of these ideals give a characterisation of the distributions
satisfying the original CI statements. Classically, these ideals are generated
by 2-minors of a matrix of variables, however, in the presence of hidden
variables, they contain higher degree minors. This leads to the study of the
structure of determinantal hypergraph ideals whose decompositions can be
understood in terms of point and line configurations in the projective space.Comment: 24 pages, 5 figure
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