6,317 research outputs found
Recurrence and transience for suspension flows
We study the thermodynamic formalism for suspension flows over countable
Markov shifts with roof functions not necessarily bounded away from zero. We
establish conditions to ensure the existence and uniqueness of equilibrium
measures for regular potentials. We define the notions of recurrence and
transience of a potential in this setting. We define the "renewal flow", which
is a symbolic model for a class of flows with diverse recurrence features. We
study the corresponding thermodynamic formalism, establishing conditions for
the existence of equilibrium measures and phase transitions. Applications are
given to suspension flows defined over interval maps having parabolic fixed
points.Comment: In this version of the paper some typos have been corrected and some
references updated. Note that the former title of this paper was "Parabolic
suspension flows
Transience and multifractal analysis
We study dimension theory for dissipative dynamical systems, proving a
conditional variational principle for the quotients of Birkhoff averages
restricted to the recurrent part of the system. On the other hand, we show that
when the whole system is considered (and not just its recurrent part) the
conditional variational principle does not necessarily hold. Moreover, we
exhibit the first example of a topologically transitive map having
discontinuous Lyapunov spectrum. The mechanism producing all these pathological
features on the multifractal spectra is transience, that is, the non-recurrent
part of the dynamics.Comment: Some updates following referee suggestion
Comparing Powers of Edge Ideals
Given a nontrivial homogeneous ideal , a
problem of great recent interest has been the comparison of the th ordinary
power of and the th symbolic power .
This comparison has been undertaken directly via an exploration of which
exponents and guarantee the subset containment
and asymptotically via a computation of the resurgence , a number for
which any guarantees .
Recently, a third quantity, the symbolic defect, was introduced; as
, the symbolic defect is the minimal number of generators
required to add to in order to get .
We consider these various means of comparison when is the edge ideal of
certain graphs by describing an ideal for which .
When is the edge ideal of an odd cycle, our description of the structure
of yields solutions to both the direct and asymptotic containment
questions, as well as a partial computation of the sequence of symbolic
defects.Comment: Version 2: Revised based on referee suggestions. Lemma 5.12 was added
to clarify the proof of Theorem 5.13. To appear in the Journal of Algebra and
its Applications. Version 1: 20 pages. This project was supported by Dordt
College's undergraduate research program in summer 201
Cross-middleware Interoperability in Distributed Concurrent Engineering
Secure, distributed collaboration between different organizations is a key challenge in Grid computing today. The GDCD project has produced a Grid-based demonstrator Virtual Collaborative Facility (VCF) for the European Space Agency. The purpose of this work is to show the potential of Grid technology to support fully distributed concurrent design, while addressing practical considerations including network security, interoperability, and integration of legacy applications. The VCF allows domain engineers to use the concurrent design methodology in a distributed fashion to perform studies for future space missions. To demonstrate the interoperability and integration capabilities of Grid computing in concurrent design, we developed prototype VCF components based on ESA’s current Excel-based Concurrent Design Facility (a non-distributed environment), using a STEP-compliant database that stores design parameters. The database was exposed as a secure GRIA 5.1 Grid service, whilst a .NET/WSE3.0-based library was developed to enable secure communication between the Excel client and STEP database
Expansion of -Schur functions for maximal -rectangles within the affine nilCoxeter algebra
We give several explicit combinatorial formulas for the expansion of k-Schur
functions indexed by maximal rectangles in terms of the standard basis of the
affine nilCoxeter algebra. Using our result, we also show a commutation
relation of k-Schur functions corresponding to rectangles with the generators
of the affine nilCoxeter algebra.Comment: to appear in Journal of Combinatorics, 28 page
Refining the Gatekeeping Metaphor for Local Television News
A book review of Refining the Gatekeeping Metaphor for Local Television News by Dan Berkowitz
Understanding the Jobs-Affordable Housing Balance in the Richmond Region
The mismatch between location of jobs and housing has a significant impact on the efficiency and quality of life within metropolitan areas. A well-planned region strives to be a “community of short distances.” A wide range of housing choices located close to employment centers could shorten commuting distances and substantially reduce government outlays for transportation facilities, reduce household transportation expenses, and increase feasibility of pedestrian movement. These needs are particularly important to families earning modest wages.
CURA, with support from The Community Foundation Serving Richmond and Central Virginia and the Richmond Association of Realtors, has analyzed the spatial pattern of lower-wage jobs and lower-cost housing within the Richmond Metropolitan Statistical Area (MSA). The analysis reveals how low-cost housing and modest-wage jobs in the Richmond region are not well-balanced. Few areas in which modest-wage jobs cluster have comparable levels of low-cost housing. The established suburban areas north, west, and south of Richmond’s urban center have a large number of retail and service jobs that pay modest wages, yet these areas provide few affordable-dwelling units for these wage earners.
The second part of this study addressed a major obstacle to the construction of new, affordable-housing units: fears. Many new, affordable dwelling units, by financial necessity, will be built at higher densities and smaller size to reduce cost. Homeowners in nearby neighborhoods often oppose construction of these units over fear of reduced property values, higher crime, and other factors. Six higher-density, 3 lower-cost housing projects were studied for their impact on the nearby middle-income neighborhoods. Documentation of home sale prices, assessment values, and crime rates before and after construction of the more affordable dwelling units did not reveal any notable long-term impact on crime rates, property values, or property sales
Is what you see what you get? representations, metaphors and tools in mathematics didactics
This paper is exploratory in character. The aim is to investigate ways in which it is possible to use the theoretical concepts of representations, tools and metaphors to try to understand what learners of mathematics ‘see’ during classroom interactions (in their widest sense) and what they might get from such interactions. Through an analysis of a brief classroom episode, the suggestion is made that what learners see may not be the same as what they get. From each of several theoretical perspectives utilised in this paper, what learners ‘get’ appears to be something extra. According to our analysis, this something ‘extra’ is likely to depend on the form of technology being used and the representations and metaphors that are available to both teacher and learner
Calculation of disease dynamics in a population of households
Early mathematical representations of infectious disease dynamics assumed a single, large, homogeneously mixing population. Over the past decade there has been growing interest in models consisting of multiple smaller subpopulations (households, workplaces, schools, communities), with the natural assumption of strong homogeneous mixing within each subpopulation, and weaker transmission between subpopulations. Here we consider a model of SIRS (susceptible-infectious-recovered-susceptible) infection dynamics in a very large (assumed infinite) population of households, with the simplifying assumption that each household is of the same size (although all methods may be extended to a population with a heterogeneous distribution of household sizes). For this households model we present efficient methods for studying several quantities of epidemiological interest: (i) the threshold for invasion; (ii) the early growth rate; (iii) the household offspring distribution; (iv) the endemic prevalence of infection; and (v) the transient dynamics of the process. We utilize these methods to explore a wide region of parameter space appropriate for human infectious diseases. We then extend these results to consider the effects of more realistic gamma-distributed infectious periods. We discuss how all these results differ from standard homogeneous-mixing models and assess the implications for the invasion, transmission and persistence of infection. The computational efficiency of the methodology presented here will hopefully aid in the parameterisation of structured models and in the evaluation of appropriate responses for future disease outbreaks
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