9,555 research outputs found
The Schottky problem in genus five
In this paper, we present a solution to the Schottky problem in the spirit of
Schottky and Jung for genus five curves. To do so, we exploit natural incidence
structures on the fibers of several maps to reduce all questions to statements
about the Prym map for genus six curves. This allows us to find all components
of the big Schottky locus and thus, to show that the small Schottky locus
introduced by Donagi is irreducible.Comment: 20 page
A Combinatorial Grassmannian Representation of the Magic Three-Qubit Veldkamp Line
It is demonstrated that the magic three-qubit Veldkamp line occurs naturally
within the Veldkamp space of combinatorial Grassmannian of type ,
. The lines of the ambient symplectic polar space are
those lines of whose cores feature an odd number of
points of . After introducing basic properties of three different types
of points and six distinct types of lines of , we
explicitly show the combinatorial Grassmannian composition of the magic
Veldkamp line; we first give representatives of points and lines of its core
generalized quadrangle GQ, and then additional points and lines of a
specific elliptic quadric (5,2), a hyperbolic quadric
(5,2) and a quadratic cone (4,2) that
are centered on the GQ. In particular, each point of
(5,2) is represented by a Pasch configuration and its
complementary line, the (Schl\"afli) double-six of points in
(5,2) comprise six Cayley-Salmon configurations and six
Desargues configurations with their complementary points, and the remaining
Cayley-Salmon configuration stands for the vertex of
(4,2).Comment: 6 pages, 2 figure
Loops, matchings and alternating-sign matrices
The appearance of numbers enumerating alternating sign matrices in stationary
states of certain stochastic processes is reviewed. New conjectures concerning
nest distribution functions are presented as well as a bijection between
certain classes of alternating sign matrices and lozenge tilings of hexagons
with cut off corners.Comment: LaTeX, 26 pages, 44 figures, extended version of a talk given at the
14th International Conference on Formal Power Series and Algebraic
Combinatorics (Melbourne 2002); Version2: Changed title, expanded some
sections and included more picture
Bounds on the maximum multiplicity of some common geometric graphs
We obtain new lower and upper bounds for the maximum multiplicity of some
weighted and, respectively, non-weighted common geometric graphs drawn on n
points in the plane in general position (with no three points collinear):
perfect matchings, spanning trees, spanning cycles (tours), and triangulations.
(i) We present a new lower bound construction for the maximum number of
triangulations a set of n points in general position can have. In particular,
we show that a generalized double chain formed by two almost convex chains
admits {\Omega}(8.65^n) different triangulations. This improves the bound
{\Omega}(8.48^n) achieved by the double zig-zag chain configuration studied by
Aichholzer et al.
(ii) We present a new lower bound of {\Omega}(12.00^n) for the number of
non-crossing spanning trees of the double chain composed of two convex chains.
The previous bound, {\Omega}(10.42^n), stood unchanged for more than 10 years.
(iii) Using a recent upper bound of 30^n for the number of triangulations,
due to Sharir and Sheffer, we show that n points in the plane in general
position admit at most O(68.62^n) non-crossing spanning cycles.
(iv) We derive lower bounds for the number of maximum and minimum weighted
geometric graphs (matchings, spanning trees, and tours). We show that the
number of shortest non-crossing tours can be exponential in n. Likewise, we
show that both the number of longest non-crossing tours and the number of
longest non-crossing perfect matchings can be exponential in n. Moreover, we
show that there are sets of n points in convex position with an exponential
number of longest non-crossing spanning trees. For points in convex position we
obtain tight bounds for the number of longest and shortest tours. We give a
combinatorial characterization of the longest tours, which leads to an O(nlog
n) time algorithm for computing them
Boundary Partitions in Trees and Dimers
Given a finite planar graph, a grove is a spanning forest in which every
component tree contains one or more of a specified set of vertices (called
nodes) on the outer face. For the uniform measure on groves, we compute the
probabilities of the different possible node connections in a grove. These
probabilities only depend on boundary measurements of the graph and not on the
actual graph structure, i.e., the probabilities can be expressed as functions
of the pairwise electrical resistances between the nodes, or equivalently, as
functions of the Dirichlet-to-Neumann operator (or response matrix) on the
nodes. These formulae can be likened to generalizations (for spanning forests)
of Cardy's percolation crossing probabilities, and generalize Kirchhoff's
formula for the electrical resistance. Remarkably, when appropriately
normalized, the connection probabilities are in fact integer-coefficient
polynomials in the matrix entries, where the coefficients have a natural
algebraic interpretation and can be computed combinatorially. A similar
phenomenon holds in the so-called double-dimer model: connection probabilities
of boundary nodes are polynomial functions of certain boundary measurements,
and as formal polynomials, they are specializations of the grove polynomials.
Upon taking scaling limits, we show that the double-dimer connection
probabilities coincide with those of the contour lines in the Gaussian free
field with certain natural boundary conditions. These results have direct
application to connection probabilities for multiple-strand SLE_2, SLE_8, and
SLE_4.Comment: 46 pages, 12 figures. v4 has additional diagrams and other minor
change
On symplectic semifield spreads of PG(5,q2), q odd
We prove that there exist exactly three non-equivalent symplectic semifield spreads of PG ( 5 , q2), for q2> 2 .38odd, whose associated semifield has center containing Fq. Equivalently, we classify, up to isotopy, commutative semifields of order q6, for q2> 2 .38odd, with middle nucleus containing q2Fq2and center containing q Fq
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