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AMS Acta (8,910 recursos)
Repository of the University of Bologna.

["viewname_eprint_types" not defined] = Preprint

Mostrando recursos 221 - 240 de 354

221. 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.

222. 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.

223. Conditionally identically distributed species sampling sequences - Bassetti, Federico; Crimaldi, Irene; Leisen, Fabrizio

224. 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.

225. 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.

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.

227. 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.

228. 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}...

229. 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.

230. Central limit theorems for multicolor urns with dominated colors - Berti, Patrizia; Crimaldi, Irene; Pratelli, Luca; Rigo, Pietro

231. Denoising and Segmentation of MR Images by Coupled Diffusive Filters - Zama, Fabiana
The image denoising and segmentation is a fundamental task in many medical applications based on magnetic resonance image processing. This problem can be solved by means of nonlinear diffusive filters requiring the solution of evolutive partial differential equations. In this work a coupled system of linear and nonlinear diffusion-reaction equations is proposed and tested for denoising and segmentation of magnetic resonance images. The discretization of the coupled system by means of the Finite Element method is reported. The effectiveness of the model has been tested on MR images affected by gaussian, impulsive noise and also in the case of dynamic magnetic resonance images where the data are affected by noise in...

232. Computation of Regularization Parameters using the Fourier Coefficients - Zama, Fabiana
In the solution of ill-posed problems by means of regularization methods, a crucial issue is the computation of the regularization parameter. In this work we focus on the Truncated Singular Value Decomposition (TSVD) and Tikhonov method and we define a method for computing the regularization parameter based on the behavior of Fourier coefficients. We compute a safe index for truncating the TSVD and consequently a value for the regularization parameter of the Tikhonov method. An extensive numerical experimentation is carried out on the Hansen's Regtool test problems and the results confirm the effectiveness and robustness of the method proposed.

233. Harnack inequality and no-arbitrage bounds for self-financing portfolios - Carciola, Alessandro; Pascucci, Andrea; Polidoro, Sergio
We give a direct proof of the Harnack inequality for a class of Kolmogorov operators associated with a linear SDE and we find the explicit expression of the optimal Harnack constant. We discuss some possible implication of the Harnack inequality in finance: specifically we infer no-arbitrage bounds for the value of self-financing portfolios in terms of the initial wealth.

234. La legislazione della bonifica e i Consorzi di Bonifica in Italia - Galvani, Adriana
Per bonifica si intende quella attività di progettazione, esecuzione, manutenzione ed esercizio della rete idrografica, dei manufatti, degli impianti idrovori e di sollevamento, avente la finalità di mettere in sicurezza i territori urbanizzati e produttivi - che altrimenti sarebbero soggetti ad inondazioni o a dissesti idrogeologici - e di rendere coltivabili i terreni mediante irrigazione. L’attività di bonifica riveste quindi due funzioni che si integrano in un delicato equilibrio, da una parte la bonifica si pone a salvaguardia del territorio, dall’altra consente il razionale sviluppo dello stesso sia a fini strettamente agricoli sia a fini produttivi (Lenzi, 2001, p.4).

235. A central limit theorem and its applications to multicolor randomly reinforced urns - Berti, Patrizia; Crimaldi, Irene; Pratelli, Luca; Rigo, Pietro

236. On the variety parametrizing completely decomposable polynomials. - Arrondo, Enrique; Bernardi, Alessandra
The purpose of this paper is to relate the variety parameterizing completely decomposable homogeneous polynomials of degree $d$ in $n+1$ variables on an algebraically closed field, called $\Split_{d}(\PP n)$, with the Grassmannian of $n-1$ dimensional projective subspaces of $\PP {n+d-1}$. We compute the dimension of some secant varieties to $\Split_{d}(\PP n)$ and find a counterexample to a conjecture that wanted its dimension related to the one of the secant variety to $\GG (n-1, n+d-1)$. Moreover by using an invariant embedding of the Veronse variety into the Pl\"ucker space, then we are able to compute the intersection of $\GG (n-1, n+d-1)$ with $\Split_{d}(\PP n)$, some of its secant variety, the tangential variety and the second...

237. Introduzione al valore economico dell'intelligenza connettiva nell'architettura orientata ai servizi - Vagnozzi, Ercole

238. Dal cloud computing al SOA: l'importanza dell'interazione con il processo e l'ecosistema - Vagnozzi, Ercole

239. Rate of convergence of predictive distributions for dependent data - Berti, Patrizia; Crimaldi, Irene; Pratelli, Luca; Rigo, Pietro

240. Studio per la conoscenza e la conservazione del manoscritto Phillipps 9589 - Lorusso, Salvatore; Natali, Andrea; Matteucci, Chiara; Savigni, Raffaele
Il presente lavoro riguarda lo studio del Codice dantesco miniato su pergamena conservato presso la Biblioteca del Centro Dantesco dei Frati Minori Conventuali di Ravenna. Tale codice è noto alla comunità scientifica internazionale con la sigla di Phillipps 9589 e deve la sua notorietà al fatto non solo di essere l’unico palinsesto dantesco, ma anche di conservare, nella scriptio superior, uno tra i testimoni più antichi di quel ramo della tradizione manoscritta della Commedia di Dante chiamata “tradizione α”, la cosiddetta “antica vulgata”. La ricerca, svolta in più fasi, si è proposta di approfondire gli aspetti tecnico-diagnostici, in particolare le finalità...


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