AMS Acta
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Repository of the University of Bologna.

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Mostrando recursos 221 - 240 de 365

221.
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,...

222.
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...

224.
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.

226.
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,...

227.
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.

228.
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.

229.
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...

231.
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.

232.
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.

233.
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.

235.
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.

236.
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.

237.
OPTIMAL HOMEOMORPHISMS BETWEEN CLOSED CURVES - 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.

238.
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.

239.
A note on Harnack inequalities and propagation set for a class of hypoelliptic operators - Cinti, Chiara; Nystrom, Kaj; Polidoro, Sergio
In this paper we are concerned with Harnack inequalities for non-negative solutions to a class of second order hypoelliptic ultraparabolic partial differential equations in the form
$$L u:= X_1^2 u + ... + X_m^2 u + X_0 u - \partial_t u = 0$$
where the vector fields $X_1, \dots, X_m$ and $X_0 - \partial_t$ are invariant with respect to a suitable homogeneous Lie group on $R^{N+1}$.
Our main goal is the following result: consider any domain $Omega$ of $R^{N+1}$ and fix any $(x_0,t_0)$ in $Omega$. We give a geometric sufficient condition on the compact subsets $K$ of $Omega$ for which the Harnack inequality
$$\sup_{K}...

240.
On the stratication of secant varieties of Veronese varieties via symmetric rank. - Bernardi, Alessandra; Gimigliano, Alessandro; Idà, Monica
When considering $\sigma_r(X)$, the variety of r-secant $\PP {r-1}$ to a projective variety $X$, one question which arises is what are the possible values of the $X$-rank of points on $\sigma_r(X)$, apart from the generic value $r$? This geometric problem is of particular relevance (also for Applied Math) when $X$ is a variety parameterizing some kind of tensors. We study here the case when $X$ is a Veronese variety (i.e. the case of symmetric tensors). We find the complete description of the rank strata in some cases, and we give algorithms which compute the symmetric rank.