Equidivisible pseudovarieties of semigroups

We give a complete characterization of pseudovarieties of semigroups whose finitely generated relatively free profinite semigroups are equidivisible. Besides the pseudovarieties of completely simple semigroups, they are precisely the pseudovarieties that are closed under Mal'cev product on the left by the pseudovariety of locally trivial semigroups. A further characterization which turns out to be instrumental is as the non-completely simple pseudovarieties that are closed under two-sided Karnofsky-Rhodes expansion

Joint Distributed Access Point Selection and Power Allocation in Cognitive Radio Networks

Spectrum management has been identified as a crucial step towards enabling the technology of the cognitive radio network (CRN). Most of the current works dealing with spectrum management in the CRN focus on a single task of the problem, e.g., spectrum sensing, spectrum decision, spectrum sharing or spectrum mobility. In this work, we argue that for certain network configurations, jointly performing several tasks of the spectrum management improves the spectrum efficiency. Specifically, we study the uplink resource management problem in a CRN where there exist multiple cognitive users (CUs) and access points (APs), with each AP operates on a set of non-overlapping channels. The CUs, in order to maximize their uplink transmission rates, have to associate to a suitable AP (spectrum decision), and to share the channels belong to this AP with other CUs (spectrum sharing). These tasks are clearly interdependent, and the problem of how they should be carried out efficiently and distributedly is still open in the literature. In this work we formulate this joint spectrum decision and spectrum sharing problem into a non-cooperative game, in which the feasible strategy of a player contains a discrete variable and a continuous vector. The structure of the game is hence very different from most non-cooperative spectrum management game proposed in the literature. We provide characterization of the Nash Equilibrium (NE) of this game, and present a set of novel algorithms that allow the CUs to distributively and efficiently select the suitable AP and share the channels with other CUs. Finally, we study the properties of the proposed algorithms as well as their performance via extensive simulations.Comment: Accepted by Infocom 2011; Infocom 2011, The 30th IEEE International Conference on Computer Communication

Dynamical Contents of Unconventional Supersymmetry

The Dirac Hamiltonian formalism is applied to a system in $(2+1)$-dimensions consisting of a Dirac field $\psi$ minimally coupled to Chern-Simons $U(1)$ and $SO(2,1)$ connections, $A$ and $\omega$, respectively. This theory is connected to a supersymmetric Chern-Simons form in which the gravitino has been projected out (unconventional supersymmetry) and, in the case of a flat background, corresponds to the low energy limit of graphene. The separation between first-class and second-class constraints is performed explicitly, and both the field equations and gauge symmetries of the Lagrangian formalism are fully recovered. The degrees of freedom of the theory in generic sectors shows that the propagating states correspond to fermionic modes in the background determined by the geometry of the graphene sheet and the nondynamical electromagnetic field. This is shown for the following canonical sectors: i) a conformally invariant generic description where the spinor field and the dreibein are locally rescaled; ii) a specific configuration for the Dirac fermion consistent with its spin, where Weyl symmetry is exchanged by time reparametrizations; iii) the vacuum sector $\psi=0$, which is of interest for perturbation theory. For the latter the analysis is adapted to the case of manifolds with boundary, and the corresponding Dirac brackets together with the centrally extended charge algebra are found. Finally, the $SU(2)$ generalization of the gauge group is briefly treated, yielding analogous conclusions for the degrees of freedom.Comment: 17 pages. Accepted version for publication in JHE

PUBLIC INVESTMENT IN TRANSPORTATION INFRASTRUCTURES AND INDUSTRY PERFORMANCE IN PORTUGAL

The objective of this paper is to evaluate the effects at the industry level of public investment in transportation infrastructures in Portugal. The empirical results are based on VAR/ECM models for the Portuguese economy and for eighteen industries covering the whole spectrum of economic activity in the country. These models consider private-sector output, employment and investment as well as public investment. Empirical results at the aggregate level indicate that public investment has a positive effect on both private inputs as well as on private output and that it affects labor productivity positively. These aggregate results, however, hide a wide variety of industry-level effects. In absolute terms, the industries that benefit the most from public investment are Construction, Trade, Transportation, Finance, Real Estate, and Services. In turn, relative to their size, the industries that benefit the most are Mining, Non-Metal Products, Metal Products, Construction, Restaurants, Transportation, and Finance, and, therefore, public investment tends to shift the industry mix toward these industries. Accordingly, our empirical results suggest that although public investment has been a powerful instrument to enhance the long-term economic performance in Portugal it does so in a way that is rather unbalanced across industries.Infrastructures, Industry Performance, Portugal

Stocks, Flows and Valuation Effects of Foreign Assets and Liabilities: Do They Matter?

Large holdings of foreign assets and liabilities, along with increasing relevance of valuation effects—capital gains or losses—have characterized global financial integration. In this paper, we assess empirically the implications of stocks, flows and valuation adjustments in external crises (current-account reversals, sudden stops and currency crises), sovereign credit ratings and the longrun real exchange rates (RER), in both industrial and developing economies. We find support for the view that foreign assets and liabilities are rather distinctive external holdings with different implications in the occurrence of external crisis. Valuation adjustments have an impact on crises, although quantitatively not very large. Portfolio liabilities (particularly equity) increase the probability of current-account reversals and currency crises, while the likelihood of sudden stops increases with the stock of foreign direct investment (FDI) assets. In the case of sovereign credit ratings, we find a noteworthy effect of the stock and flows of FDI liabilities on improving sovereign ratings. Finally, as for the RER, gross assets and liabilities appear equally important, but components of external holdings have considerably different effects. While the cumulative current account is associated with real depreciation, the valuation effect is strongly linked with real currency appreciations in developing economies.

On the group of a rational maximal bifix code

We give necessary and sufficient conditions for the group of a rational maximal bifix code $Z$ to be isomorphic with the $F$-group of $Z\cap F$, when $F$ is recurrent and $Z\cap F$ is rational. The case where $F$ is uniformly recurrent, which is known to imply the finiteness of $Z\cap F$, receives special attention. The proofs are done by exploring the connections with the structure of the free profinite monoid over the alphabet of $F$

Joint Access Point Selection and Power Allocation for Uplink Wireless Networks

We consider the distributed uplink resource allocation problem in a multi-carrier wireless network with multiple access points (APs). Each mobile user can optimize its own transmission rate by selecting a suitable AP and by controlling its transmit power. Our objective is to devise suitable algorithms by which mobile users can jointly perform these tasks in a distributed manner. Our approach relies on a game theoretic formulation of the joint power control and AP selection problem. In the proposed game, each user is a player with an associated strategy containing a discrete variable (the AP selection decision) and a continuous vector (the power allocation among multiple channels). We provide characterizations of the Nash Equilibrium of the proposed game, and present a set of novel algorithms that allow the users to efficiently optimize their rates. Finally, we study the properties of the proposed algorithms as well as their performance via extensive simulations.Comment: Revised and Resubmitted to IEEE Transactions on Signal Processin

Zero Mode Effect Generalization for the Electromagnetic Current in the Light Front

We consider in this work the electromagnetic current for a system composed by two charged bosons and show that it has a structure of many bodies even in the impulse approximation, when described in the light front time $x^+$. In terms of the two-body component for the bound state, the current contains two-body operators. We discuss the process of pair creation from the interacting photon and interpret it as a zero mode contribution to the current and its consequences for the components of currents in the light-front.Comment: 13 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:hep-th/050911

Semántica leibniziana polivalente

Este es un resumen de un desarrollo conjetural de una semántica leibniziana que admite finitos valores de verdad para casi todas las verdades y falsedades de razón e infinitos valores de verdad para alguna verdad y falsedad de razón y todas las verdades y falsedades de hecho.draf