Extreme point characterizations for infinite network flow problems

We study capacitated network flow problems with demands defined on a countably infinite collection of nodes having finite degree. This class of network flow models includes, for example, all infinite horizon deterministic dynamic programs with finite action sets, because these are equivalent to the problem of finding a shortest path in an infinite directed network. We derive necessary and sufficient conditions for flows to be extreme points of the set of feasible flows. Under an additional regularity condition met by all such problems with integer data, we show that a feasible solution is an extreme point if and only if it contains neither a cycle nor a doubly-infinite path consisting of free arcs (an arc is free if its flow is strictly between its upper and lower bounds). We employ this result to show that the extreme points can be characterized by specifying a basis. Moreover, we establish the integrality of extreme point flows whenever node demands and arc capacities are integer valued. We illustrate our results with an application to an infinite horizon economic lot-sizing problem. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 48(4), 209–222 2006Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/55830/1/20134_ftp.pd

Response of Glass and Liquid Phases in the Vortex lattice to an external AC magnetic field at different frequencies

We individuated a method to distinguish a glass phase from a highly viscous liquid phase in a lattice of vortices, established in type-two superconductors. Our analysis is based on the study of the temperature dependence of numerically obtained 1st and 3rd harmonics curves of the AC magnetic susceptibility, by changing the frequency of the applied AC magnetic field. The harmonics are obtained by integrating the non-linear diffusion equation for the magnetic field, with different voltage-current characteristics, corresponding to the two different phases. This method could be applied to the analysis of experimental curves in order to determine if the detected magnetic response of superconducting samples may be interpreted in terms of vortex glass or Kim-Anderson model.Comment: 6 pages, 5 figures, to be published on Physica

A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology

A relativistic modified gravity (MOG) theory leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters of galaxies data and cosmology.Comment: 16 pages. Latex file. Talk given at the International Workshop "From Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton, Virginia, USA. To be published in Int. J. Mod. Phys D. Equation correcte

Students, Universities and Employers: Why We All Win When We Promote Social Justice through SoTL

This essay discusses the benefits of promoting social justice through the scholarship of teaching and learning. We start by highlighting the prevalence of injustices and discrimination (racism, sexism, homophobia, heteronormativity) experienced on college campuses in the US. We go on to highlight the (1) outcomes associated with social justice centered teaching and learning activities and (2) ways this approach makes students more valuable to potential employers upon graduation. We conclude with a call for campus wide promotion of social justice as both micro (classroom) and macro (university) level interventions are needed to truly create equitable learning spaces

Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}

Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions, we study the effect of iron disproportionation on the magnetic ordering, and describe the first-order magnetic transition occurring in the perovskite Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of the exchange integrals for the antiferromagntic and ferromagnetic interactions, in agreement with previous works on kindered compounds. Spin-wave theory yields a magnon spectrum with a gapless antiferromagnetic mode together with two gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to J. Mag. Mag. Ma

Flux Creep and Flux Jumping

We consider the flux jump instability of the Bean's critical state arising in the flux creep regime in type-II superconductors. We find the flux jump field, $B_j$, that determines the superconducting state stability criterion. We calculate the dependence of $B_j$ on the external magnetic field ramp rate, $\dot B_e$. We demonstrate that under the conditions typical for most of the magnetization experiments the slope of the current-voltage curve in the flux creep regime determines the stability of the Bean's critical state, {\it i.e.}, the value of $B_j$. We show that a flux jump can be preceded by the magneto-thermal oscillations and find the frequency of these oscillations as a function of $\dot B_e$.Comment: 7 pages, ReVTeX, 2 figures attached as postscript file

Structural Adjustment in Nicaragua: The Impact on Workers in the Agricultural and Manufacturing Sectors

The focus of this study is to analyze how Nicaraguan workers in the manufacturing and agricultural sectors were affected by structural adjustment programs. I analyze three objective well-being indicators: health, education and material well being. Previous research on Nicaragua has investigated how structural adjustment has affected the agricultural and manufacturing sectors separately. This study aims to seek out how the two sectors are affected differently or similarly by structural adjustment. The data analyzed for this research are from a survey conducted by Measure Demographic and Health Survey (DHS). Measure DHS conducts nationally representative, population-based surveys in developing countries. My dependent variables include: literacy, highest education achieved, ownership of radio, ownership of telephone, number of rooms for sleeping, material used to construct the floor, and immunization for Diphtheria, Polio and Measles. Three departments were analyzed to represent the agricultural and manufacturing sectors. Managua represents the manufacturing sector, Chinandega represents the agricultural sector and the RAAS, because of its autonomous nature is used as a control variable. This study utilizes two representative household surveys 1997(11528 households) and 2001 (11328 households). Data analysis revealed that the manufacturing sector made more improvements than the agricultural sector in education and material-well being, and the agricultural sector made more improvements than the manufacturing sector in health. Overall, Nicaraguan workers in the agricultural sector were worse off as a result of structural adjustment than workers in the manufacturing sector

In Print

Faculty authors explore Spanish vanguard poetry, fatherhood, multi-modal learning, and more

Magnetization Relaxation via Quantum and Classical Vortex Motion in a Bose Glass Superconductor

I show that in Bose Glass superconductor with high $j_c$ and at low $T$ the magnetization relaxation (S), dominated by quantum tunneling, is $\propto{\sqrt j_c}$, which crosses over to the conventional classical rate $\propto T/j_c$ at higher $T$ and lower $j_c$, with the crossover $T^*\sim j_c^{3/2}$. I argue that due to interactions between flux lines there exist three relaxation regimes, depending on whether $BB_\phi$, corresponding to Strongly-pinned Bose Glass (SBG) with large $j_{c2}$, Mott Insulator (MI) with vanishing S, and Weakly-pinned Bose Glass (WBG) characterized by small $j_{c1}$. I discuss the effects of interactions on $j_c$ and focus attention on the recent experiment which is consistently described by the theory.Comment: 4 pages, self-unpacking uuencoded compressed postscript file with figures already inside text; to appear in Phys. Rev. Lett.(1995