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AMS Acta (10,950 recursos)

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Mostrando recursos 221 - 240 de 367

  1. On the X-rank with respect to linearly normal curves.

    Ballico, Edoardo; Bernardi, Alessandra
    In this paper we study the X-rank of points with respect to smooth linearly normal curves X contained in P^n of genus g and degree n+g. We prove that, for such a curve X, under certain circumstances, the X-rank of a general point of X-border rank equal to s is less or equal than n+1-s. In the particular case of g=2 we give a complete description of the X-rank if n=3,4; while if n is greter or equal than 5 we study the X-rank of points belonging to the tangential variety of X.

  2. Longevity pattern in Emilia Romagna (Italy) in a dynamic perspective

    Roli, Giulia; Miglio, Rossella; Rettaroli, Rosella; Samoggia, Alessandra
    In this paper, we investigate the pattern of longevity during the last 15 years in Emilia Romagna, a North-Eastern region of Italy, at a municipality level. We consider a specific index of extreme longevity based on people aged 95 and over in two different periods (1995-1999 and 2005-2009). Spatio-temporal modeling is used to tackle at both periods the random variations in the occurrence of people 95+, due to the increasingly rareness of such events, especially in small areas. This method exploits the spatial proximity and the consequent interaction of the geographical areas to smooth the observations, as well as to...

  3. Estimation of multiple correlated effects on a disease outcome when the data are nested: a hierarchical Bayesian approach

    Roli, Giulia
    The paper deals with the analysis of multiple exposures on the occurrence of a disease. We consider observational case-control data in a multilevel setting, with subjects nested in clusters. A hierarchical Bayesian model is proposed to tackle the within-cluster dependence and the correlation among the exposures, simultaneously. To do that, we assign prior distributions on the crucial parameters by exploiting additional information at different levels, as well as reasonable assumptions and previous knowledge. The model is applied to a multi-centric study aiming to investigate the association of dietary exposures with the colon-rectum cancer. Compared with results obtained with conventional regressions,...

  4. Metodi statistici a variabili latenti per lo studio di fenomeni finanziari

    De Angelis, Luca
    Negli ultimi decenni il concetto di variabile latente ha riscosso un enorme successo nelle discipline statistiche come attestano i numerosi lavori scientifici presenti in letteratura. In particolare, nelle scienze sociali e in psicometria, l’uso del concetto di variabile latente è stato largamente adottato per far fronte al problema di misurare quantità che, in natura, non possono essere direttamente osservate. La vasta letteratura riguardante questa metodologia si espande, in maniera più limitata, anche al campo della ricerca economica ed econometrica. Nonostante esistano studi di modelli a struttura latente applicati a variabili di tipo economico, molto pochi sono i lavori che considerano...

  5. Valuing patents for accounting purposes

    Morricone, S

  6. Patent Commercialization Strategy and IPO Underpricing: Evidence from the US Semiconductor Industry

    Morricone, S; Munari, F; Oriani, R
    R&D and innovation are critical factors to succeed. Nevertheless, they generate information asymmetries between corporate insiders and external investors. Within the context of an IPO, information asymmetries increase the risk of underpricing. In this paper, we claim that the patent commercialization strategy of the firm going public affects information asymmetries and IPO underpricing. In particular, underpricing will be higher when a firm’s patent commercialization strategy is more based on licenses. A firm’s stock of patent attenuates this effect. Our results on a sample of 130 IPOs in the US semiconductor industry confirm these predictions.

  7. Studiare, documentare, incontrarsi: indagine sull'utenza delle Biblioteche Universitarie nei Poli della Romagna - Questionario

    Citti, Alessandra; Politi, Angela Maria; Sabattini, Fulvia; Semenzato, Chiara; Sangiorgi, Sergio

  8. On the X-rank with respect to linear projections of projective varieties

    Ballico, Edoardo; Bernardi, Alessandra
    In this paper we improve the known bound for the $X$-rank $R_{X}(P)$ of an element $P\in {\mathbb{P}}^N$ in the case in which $X\subset {\mathbb P}^n$ is a projective variety obtained as a linear projection from a general $v$-dimensional subspace $V\subset {\mathbb P}^{n+v}$. Then, if $X\subset {\mathbb P}^n$ is a curve obtained from a projection of a rational normal curve $C\subset {\mathbb P}^{n+1}$ from a point $O\subset {\mathbb P}^{n+1}$, we are able to describe the precise value of the $X$-rank for those points $P\in {\mathbb P}^n$ such that $R_{X}(P)\leq R_{C}(O)-1$ and to improve the general result. Moreover we give a stratification,...

  9. Viaggiatori e viaggiatrici. Letture storico-geografiche

    Marchi, Marzia
    Il viaggio- una pratica sociale profondamente mutata in Europa dai pellegrinaggi medievali fino ai turismi contemporanei- è qui indagato attraverso letture sul ruolo di alcuni fra i molti protagonisti, che hanno innovato le modalità di viaggiare e proposto diverse visioni sul mondo esterno. In particolare si dà conto del contributo di alcune figure femminili alla difficile conquista della libertà di viaggiare da parte delle donne europee, ed inoltre della più recente valorizzazione di altri viaggiatori non europei, che si sono cimentati, nei secoli scorsi, in lunghi itinerari attorno al mondo.

  10. Computing symmetric rank for symmetric tensors.

    Bernardi, Alessandra; Gimigliano, Alessandro; Idà, Monica
    We consider the problem of determining the symmetric tensor rank for symmetric tensors with an algebraic geometry approach. We give algorithms for computing the symmetric rank for $2\times \cdots \times 2$ tensors and for tensors of small border rank. From a geometric point of view, we describe the symmetric rank strata for some secant varieties of Veronese varieties.

  11. Dealing with multiple criteria in test assembly

    Veldkamp, Bernard P.; Matteucci, Mariagiulia
    It is quite common that tests or exams are being used for more then one purpose. First of all, they are used to measure the ability of the students in a reliable manner. Besides, they can be used for pass/fail decisions or to predict future behavior of the candidate, like future job behavior or academic performance. The question remains how to assemble a test that can be used for all these different purposes, that is, how to assemble a multi-objective test. Besides, multiple objectives can result from different purposes, but also from the way test specifications have been implemented. For...

  12. Misspecification Resistant Model Selection Using Information Complexity With Applications

    Bozdogan, Hamparsum; Howe, Andrew; Katragadda, Suman; Liberati, Caterina

  13. Computer adaptive testing with empirical prior information: a Gibbs sampler approach for ability estimation

    Matteucci, Mariagiulia; Veldkamp, Bernard P.
    In this paper, empirical prior information is introduced in computer adaptive testing. Despite its increasing use, the method suffers from a weak measurement precision, especially under particular conditions. Therefore, it is shown how the inclusion of background variables both in the initialization and the ability estimation is able to improve the accuracy of ability estimates. In particular, a Gibbs sampler scheme is proposed in the phases of interim and final ability estimation. By using simulated data, it is demonstrated that the method produces more accurate ability estimates, especially for short tests and when reproducing boundary abilities.

  14. Complexity, Heegaard diagrams and generalized Dunwoody manifolds

    Cattabriga, Alessia; Mulazzani, Michele; Vesnin, Andrei
    We deal with Matveev complexity of compact orientable 3- manifolds represented via Heegaard diagrams. This lead us to the definition of modified Heegaard complexity of Heegaard diagrams and of manifolds. We define a class of manifolds which are generalizations of Dunwoody manifolds, including cyclic branched coverings of two-bridge knots and links, torus knots, some pretzel knots, and some theta-graphs. Using modified Heegaard complexity, we obtain upper bounds for their Matveev complexity, which linearly depend on the order of the covering. Moreover, using homology arguments due to Matveev and Pervova we obtain lower bounds.

  15. Hilden braid groups

    Bellingeri, Paolo; Cattabriga, Alessia
    Let H_g be a genus g handlebody and MCG_{2n}(T_g) be the group of the isotopy classes of orientation preserving homeomorphisms of \partial T_g = H_g, fixing a given set of 2n points. In this paper we study two particular subgroups of MCG_{2n}(T_g) which generalize Hilden groups defined by Hilden in [16]. As well as Hilden groups are related to plat closures of braids, these generalizations are related to Heegaard splittings of manifolds and to bridge decompositions of links. Connections between these subgroups and motion groups of links in closed 3-manifolds are also provided.

  16. Conditionally identically distributed species sampling sequences

    Bassetti, Federico; Crimaldi, Irene; Leisen, Fabrizio

  17. Regularity near the Initial State in the Obstacle Problem for a class of Hypoelliptic Ultraparabolic Operators

    Nystrom, Kaj; Pascucci, Andrea; Polidoro, Sergio
    This paper is devoted to a proof of regularity, near the initial state, for solutions to the Cauchy-Dirichlet and obstacle problem for a class of second order differential operators of Kolmogorov type. The approach used here is general enough to allow us to consider smooth obstacles as well as non-smooth obstacles.

  18. An Iterative Method for the Solution of Nonlinear Regularization Problems with Regularization Parameter Estimation

    Loli Piccolomini, Elena; Zama, Fabiana
    Ill posed problems constitute the mathematical model of a large variety of applications. Aim of this paper is to define an iterative algorithm finding the solution of a regularization problem. The method minimizes a function constituted by a least squares term and a generally nonlinear regularization term, weighted by a regularization parameter. The proposed method computes a sequence of iterates approximating the regularization parameter and a sequence of iterates approximating the solution. The numerical experiments performed on 1D test problems show that the algorithm gives good results with different regularization functions both in terms of precision and computational efficiency. Moreover, it could be easily applied to large size regularization problems.


    Cerri, Andrea; Di Fabio , Barbara
    The concept of natural pseudo-distance has proven to be a powerful tool for measuring the dissimilarity between topological spaces endowed with continuous real-valued functions. Roughly speaking, the natural pseudo-distance is defined as the infimum of the change of the functions' values, when moving from one space to the other through homeomorphisms, if possible. In this paper, we prove the first available result about the existence of optimal homeomorphisms between closed curves, i.e. inducing a change of the function that equals the natural pseudo-distance.

  20. An Iterative Tikhonov Method for Large Scale Computations

    Loli Piccolomini, Elena; Zama, Fabiana
    In this paper we present an iterative method for the minimization of the Tikhonov regularization functional in the absence of information about noise. Each algorithm iteration updates both the estimate of the regularization parameter and the Tikhonov solution. In order to reduce the number of iterations, an inexact version of the algorithm is also proposed. In this case the inner Conjugate Gradient (CG) iterations are truncated before convergence. In the numerical experiments the methods are tested on inverse ill posed problems arising both in signal and image processing.

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