119,476 research outputs found
The tropical double description method
We develop a tropical analogue of the classical double description method
allowing one to compute an internal representation (in terms of vertices) of a
polyhedron defined externally (by inequalities). The heart of the tropical
algorithm is a characterization of the extreme points of a polyhedron in terms
of a system of constraints which define it. We show that checking the
extremality of a point reduces to checking whether there is only one minimal
strongly connected component in an hypergraph. The latter problem can be solved
in almost linear time, which allows us to eliminate quickly redundant
generators. We report extensive tests (including benchmarks from an application
to static analysis) showing that the method outperforms experimentally the
previous ones by orders of magnitude. The present tools also lead to worst case
bounds which improve the ones provided by previous methods.Comment: 12 pages, prepared for the Proceedings of the Symposium on
Theoretical Aspects of Computer Science, 2010, Nancy, Franc
Computing the vertices of tropical polyhedra using directed hypergraphs
We establish a characterization of the vertices of a tropical polyhedron
defined as the intersection of finitely many half-spaces. We show that a point
is a vertex if, and only if, a directed hypergraph, constructed from the
subdifferentials of the active constraints at this point, admits a unique
strongly connected component that is maximal with respect to the reachability
relation (all the other strongly connected components have access to it). This
property can be checked in almost linear-time. This allows us to develop a
tropical analogue of the classical double description method, which computes a
minimal internal representation (in terms of vertices) of a polyhedron defined
externally (by half-spaces or hyperplanes). We provide theoretical worst case
complexity bounds and report extensive experimental tests performed using the
library TPLib, showing that this method outperforms the other existing
approaches.Comment: 29 pages (A4), 10 figures, 1 table; v2: Improved algorithm in section
5 (using directed hypergraphs), detailed appendix; v3: major revision of the
article (adding tropical hyperplanes, alternative method by arrangements,
etc); v4: minor revisio
A-Tint: A polymake extension for algorithmic tropical intersection theory
In this paper we study algorithmic aspects of tropical intersection theory.
We analyse how divisors and intersection products on tropical cycles can
actually be computed using polyhedral geometry. The main focus of this paper is
the study of moduli spaces, where the underlying combinatorics of the varieties
involved allow a much more efficient way of computing certain tropical cycles.
The algorithms discussed here have been implemented in an extension for
polymake, a software for polyhedral computations.Comment: 32 pages, 5 figures, 4 tables. Second version: Revised version, to be
published in European Journal of Combinatoric
Tropical polar cones, hypergraph transversals, and mean payoff games
We discuss the tropical analogues of several basic questions of convex
duality. In particular, the polar of a tropical polyhedral cone represents the
set of linear inequalities that its elements satisfy. We characterize the
extreme rays of the polar in terms of certain minimal set covers which may be
thought of as weighted generalizations of minimal transversals in hypergraphs.
We also give a tropical analogue of Farkas lemma, which allows one to check
whether a linear inequality is implied by a finite family of linear
inequalities. Here, the certificate is a strategy of a mean payoff game. We
discuss examples, showing that the number of extreme rays of the polar of the
tropical cyclic polyhedral cone is polynomially bounded, and that there is no
unique minimal system of inequalities defining a given tropical polyhedral
cone.Comment: 27 pages, 6 figures, revised versio
Influence of Doubled CO2 on Ozone via Changes in the Brewer–Dobson Circulation
In this short note, the effect of enhanced circulation due to doubling CO2 on ozone is investigated. The difference of Brewer–Dobson circulation (BDC) between the doubled CO2 and control run from an idealized atmospheric general circulation model is added to the BDC climatology derived from National Centers for Environmental Prediction—Department of Energy Reanalysis 2 (NCEP2) from 1979 to 2002. Then it is used to drive the California Institute of Technology/Jet Propulsion Laboratory (Caltech/JPL) two-dimensional chemistry and transport model. The results reveal that the total ozone increases by 7 and 3.5 Dobson units (DU) in the high latitudes of the Northern and Southern Hemispheres, respectively, and decreases by 4 DU in the Tropics as a result of the increase in BDC associated with doubled CO2. If the change of eddy mixing coefficients after doubling CO2 is also considered, the total ozone will increase by 6.5 and 3 DU in the high latitudes of the Northern and Southern Hemispheres after combining both effects from the change in BDC and eddy mixing coefficients
Studies on Barbula consanguinea (Thw. & Mitt.) Jaeg. sensu Eddy : a pan-tropical species
Barbula consanguinea (Thw. & Mitt.) Jaeg. sensu Eddy is considered as a pan-tropical species. This taxon is reported new for several central African countries, the Arabian peninsula, and Middle America. This variable species is shortly discussed and compared with (most) related taxa. Hymenostylium crispulum Broth. & Par. and Barbula obscura Sull. (= Barbula wrightii Sauerb.) are considered as synonyms of Barbula consanguinea
A seed-diffusion model for tropical tree diversity patterns
Diversity patterns of tree species in a tropical forest community are
approached by a simple lattice model and investigated by Monte Carlo
simulations using a backtracking method. Our spatially explicit neutral model
is based on a simple statistical physics process, namely the diffusion of
seeds. The model has three parameters: the speciation rate, the size of the
meta-community in which the studied tree-community is embedded, and the average
surviving time of the seeds. By extensive computer simulations we aim the
reproduction of relevant statistical measures derived from the experimental
data of the Barro Colorado Island tree census in year 1995. The first two
parameters of the model are fixed to known values, characteristic of the
studied community, thus obtaining a model with only one freely adjustable
parameter. As a result of this, the average number of species in the considered
territory, the relative species abundance distribution, the species-area
relationship and the spatial auto-correlation function of the individuals in
abundant species are simultaneously fitted with only one parameter which is the
average surviving time of the seeds.Comment: 12 pages, 5 figure
Logarithmic Gromov-Witten invariants
The goal of this paper is to give a general theory of logarithmic
Gromov-Witten invariants. This gives a vast generalization of the theory of
relative Gromov-Witten invariants introduced by Li-Ruan, Ionel-Parker, and Jun
Li, and completes a program first proposed by the second named author in 2002.
One considers target spaces X carrying a log structure. Domains of stable log
curves are log smooth curves. Algebraicity of the stack of such stable log maps
is proven, subject only to the hypothesis that the log structure on X is fine,
saturated, and Zariski. A notion of basic stable log map is introduced; all
stable log maps are pull-backs of basic stable log maps via base-change. With
certain additional hypotheses, the stack of basic stable log maps is proven to
be proper. Under these hypotheses and the additional hypothesis that X is log
smooth, one obtains a theory of log Gromov-Witten invariants.Comment: 58 pages, 5 figure
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