Recursos de colección

Estudo Geral - Universidade de Coimbra (19.630 recursos)

ESTUDO GERAL é a designação do repositório digital da produção científica da Universidade de Coimbra, com o objectivo de divulgar conteúdos digitais de natureza científica de autores ligados à Universidade de Coimbra. A sua criação insere-se no movimento de Acesso Livre à Literatura científica (Open Access), ao qual o Conselho de Reitores das Universidades Portuguesas aderiu em 2006 e que a Universidade de Coimbra subscreveu. À semelhança de outras grandes universidades nacionais e internacionais, a UC tem o maior interesse em aumentar a sua presença na rede informática mundial, sendo cada vez mais - e também por essa via - um emissor de conhecimento e cultura.

FCTUC Matemática - Vários

Mostrando recursos 1 - 20 de 213

  1. On categories with semidirect products

    Martins-Ferreira, Nelson; Sobral, Manuela
    We characterize pointed categories having semidirect products in the sense of D. Bourn and G. Janelidze ([3]) providing necessary and sufficient conditions for a pointed category to admit semidirect products and interpreting these conditions in terms of protomodularity and exactness of certain split chains.

  2. Presentations of Schützenberger groups of minimal subshifts

    Almeida, Jorge; Costa, Alfredo
    In previous work, the first author established a natural bijection between minimal subshifts and maximal regular J -classes of free profinite semigroups. In this paper, the Sch¨utzenberger groups of such J -classes are investigated in particular in respect to a conjecture proposed by the first author concerning their profinite presentation. The conjecture is established for several types of minimal subshifts associated with substitutions. The Sch¨utzenberger subgroup of the J -class corresponding to the Prouhet-Thue-Morse subshift is shown to admit a somewhat simpler presentation, from which it follows that it satisfies the conjecture, that it has rank three, and that it is non-free relatively to any pseudovariety of groups.

  3. Comprehensive factorization and universal I-central extensions in the Mal'cev context

    Bourn, Dominique; Rodelo, Diana
    We show that, under suitable left exact conditions on a re ection functor I, the construction of the associated universal I-central extension is reduced to the comprehensive factorization of a speci c internal functor. This observation produces some existence conditions which hold in particular for any re ection from a Mal'cev variety to any Birkho subvariety

  4. On the well-posedness of a two-phase minimization problem

    Urbano, José Miguel; Vorotnikov, Dmitry
    We prove a series of results concerning the emptiness and non-emptiness of a certain set of Sobolev functions related to the well-posedness of a two-phase minimization problem, involving both the p(x)-norm and the in nity norm. The results, although interesting in their own right, hold the promise of a wider applicability since they can be relevant in the context of other problems where minimization of the p-energy in a part of the domain is coupled with the more local minimization of the L1-norm on another region

  5. Perfect category-graded algebras

    Santana, Ana Paula; Yudin, Ivan
    In a perfect category every object has a minimal projective resolution. We give a sufficient condition for the category of modules over a category-graded algebra to be perfect

  6. Optimal control and quasi-velocities

    Abrunheiro, Lígia; Camarinha, Margarida; Cariñena, José F.; Clemente-Gallardo, Jesús; Martínez, Eduardo; Santos, Patricia
    In this paper we study optimal control problems for nonholonomic systems defined on Lie algebroids by using quasi-velocities. We consider both kinemactic, i.e. systems whose cost functional depends only on position and velocities, and dynamic optimal control problems, i.e. systems whose cost functional depends also on accelarations. Formulating the problem directly at the level of Lie algebroids turns out to be the correct framework to explain in detail similar results appeared recently [48]. We also provide several examples to illustrate our construction

  7. On the semiclassical character of orthogonal polynomials satisfying structure relations

    Branquinho, A.; Rebocho, M. N.

  8. Rings of real functions in Pointfree Topology

    Gutiérrez García, Javier; Picado, Jorge
    This paper deals with the algebra F(L) of real functions of a frame L and its subclasses LSC(L) and USC(L) of, respectively, lower and upper semicontinuous real functions. It is well-known that F(L) is a lattice-ordered ring; this paper presents explicit formulas for its algebraic operations which allow to conclude about their behaviour in LSC(L) and USC(L). As applications, idempotent functions are characterized and the results of [10] about strict insertion of functions are signi cantly improved: general pointfree formulations that correspond exactly to the classical strict insertion results of Dowker and Michael regarding, respectively, normal countably paracompact spaces and perfectly normal spaces are derived. The paper ends with a...

  9. Relative asymptotics for orthogonal matrix polynomials

    Branquinho, A.; Marcellán, F.; Mendes, A.
    In this paper we study sequences of matrix polynomials that satisfy a non-symmetric recurrence relation. To study this kind of sequences we use a vector interpretation of the matrix orthogonality. In the context of these sequences of matrix polynomials we introduce the concept of the generalized matrix Nevai class and we give the ratio asymptotics between two consecutive polynomials belonging to this class. We study the generalized matrix Chebyshev polynomials and we deduce its explicit expression as well as we show some illustrative examples. The concept of a Dirac delta functional is introduced. We show how the vector model that includes a Dirac delta functional is a representation...

  10. A new characterization of Goursat categories

    Gran, Marino; Rodelo, Diana
    We present a new characterisation of Goursat categories in terms of special kind of pushouts, that we call Goursat pushouts. This allows one to prove that, for a regular category, the Goursat property is actually equivalent to the validity of the denormalised 3-by-3 Lemma. Goursat pushouts are also useful to clarify, from a categorical perspective, the existence of the quaternary operations characterising 3-permutable varieties

  11. Local smoothing with given marginals

    Jacob, Pierre; Oliveira, Paulo Eduardo
    In models using categorical data one may use adjacency relations to justify smoothing to improve upon simple histogram approximations of the probabilities. This is particularly convenient for sparsely observed or rather peaked distributions. Moreover, in a few models, prior knowledge of a marginal distribution is available. We adapt local polynomial estimators to include this partial information about the underlying distribution and give explicit representations for the proposed estimators. An application to a set of anthropological data is included.

  12. Drug delivery: from a contact lens to the anterior chamber

    Ferreira, J. A.; Oliveira, P. de; Silva, P. M. da; Murta, J. N.
    Mathematical models to describe drug concentration profiles of topically administered drug in the anterior chamber aqueous humor have been proposed by several authors. The aim of this paper is to present a mathematical model to predict the drug concentration in the anterior chamber when a therapeutical contact lens with the drug is entrapped in nanoparticles is used.

  13. Flux Tracking in delivery polymeric systems

    Ferreira, J. A.; Oliveira, P. de; Silva, P. M. da; Simon, L.
    The dynamics of diffusive and stress-induced transport in polymeric delivery systems was investigated. Partial and ordinary differential equations were first written to describe drug release behaviors in Maxwell and Maxwell-Voigt materials. The time constants governing the flux and concentration responses of a permeating species were determined from a Laplace transform solution of the original model. A ”tracking strategy”, based on the estimated characteristic times, was proposed to estimate the delivery rate and the concentration near the exit side of the membrane. The methodology was more efficient at times greater than the time constant and the prediction error decreased further as the process approached steady state. Numerical illustrations and comparisons...

  14. Optimal convergence rates for the Strong Law of Large Numbers under Association

    Oliveira, Paulo Eduardo

  15. The flashing ratchet and unidirectional transport of matter

    Vorotnikov, Dmitry
    We study the ashing ratchet model of a Brownian motor, which consists in cyclical switching between the Fokker-Planck equation with an asymmetric ratchet-like potential and the pure di usion equation. We show that the motor really performs unidirectional transport of mass, for proper parameters of the model, by analyzing the attractor of the problem and the stationary vector of a related Markov chain

  16. Bilevel derivative-free optimization and its application to robust optimization

    Conn, Andrew R.; Vicente, L. N.
    We address bilevel programming problems when the derivatives of both the upper and the lower level objective functions are unavailable. The core algorithms used for both levels are trust-region interpolation-based methods, using minimum Frobenius norm quadratic models when the number of points is smaller than the number of basis components. We take advantage of the problem structure to derive conditions (related to the global convergence theory of the underlying trust-region methods, as far as possible) under which the lower level can be solved inexactly and sample points can be reused for model building. In addition, we indicate numerically how effective these expedients can be. A number of other issues...

  17. Worst case complexity of direct search

    Vicente, L. N.
    In this paper we prove that direct search of directional type shares the worst case complexity bound of steepest descent when sufficient decrease is imposed using a quadratic function of the step size parameter. This result is proved under smoothness of the objective function and using a framework of the type of GSS (generating set search). We also discuss the worst case complexity of direct search when only simple decrease is imposed and when the objective function is non-smooth.

  18. Direct multisearch for multiobjective optimization

    Custódio, A. L.; Madeira, J. F. A.; Vaz, A. I. F.; Vicente, L. N.
    In practical applications of optimization it is common to have several conflicting objective functions to optimize. Frequently, these functions are subject to noise or can be of black-box type, preventing the use of derivative-based techniques. We propose a novel multiobjective derivative-free methodology, calling it direct multisearch (DMS), which does not aggregate any of the objective functions. Our framework is inspired by the search/poll paradigm of direct-search methods of directional type and uses the concept of Pareto dominance to maintain a list of nondominated points (from which the new iterates or poll centers are chosen). The aim of our method is to generate as many points in the Pareto...

  19. Local Hölder continuity for doubly nonlinear parabolic equations

    Kuusi, Tuomo; Siljander, Juhana; Urbano, José Miguel
    We give a proof of the Hölder continuity of weak solutions of certain degenerate doubly nonlinear parabolic equations in measure spaces. We only assume the measure to be a doubling non-trivial Borel measure which supports a Poincaré inequality. The proof discriminates between large scales, for which a Harnack inequality is used, and small scales, that require intrinsic scaling methods.

  20. Image-driven parameter estimation in absorption-diffusion models of chromoscopy

    Figueiredo, Isabel N.; Figueiredo, Pedro; Almeida, Nuno
    The administration of dyes and subsequent examination, with a colorimetry visual criterium, is a gastroenterology procedure for distinguishing, in endoscopic images, normal and aberrant colonic crypts. These are thought to be possible precursors of colon cancer. In this paper a combined image segmentation and parameter estimation model is proposed for in vivo colonic crypts’ images, obtained with chromoscopic colonoscopy. The parameter estimation is an inverse problem. It is formulated as a partial differential equation constrained optimization problem, and involves an absorption-diffusion equation. A Lagrange multiplier formulation is employed and analyzed for resolving this inverse problem. Using only the segmentation of the medical endoscopic image, which separates normal and aberrant crypts,...

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