Quaderni di Dipartimento
Collezione dei preprint del Dipartimento di Matematica.
La presenza del full-text è lacunosa per i prodotti antecedenti maggio 2006.
Trovati 866 prodotti
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QDD30 - 10/04/2008
Biroli M. ; Marchi S.
Wiener criterion for relaxed problems related to p-homogeneous Riemannian Dirichlet forms | Abstract | | We state a Wiener criterion for the regularity of points with respect to a relaxed Dirichlet problem for a p-homogeneous Riemannian Dirichlet form. |
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QDD29 - 07/04/2008
Barchielli, A. ; Gregoratti, M. ; Licciardo, M.
Feedback control of the fluorescence light squeezing | Abstract | | We consider a two-level atom stimulated by a coherent monochromatic laser and we study how to
enhance the squeezing of the fluorescence light and of the atom itself in the presence of a Wiseman-Milburn feedback mechanism, based on the homodyne detection of a fraction of the emitted light. Besides analyzing the effect of the control parameters on the squeezing properties of the light and of the atom, we also discuss the relations among these. The problem is tackled inside the framework of quantum trajectory theory. |
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QDD28 - 26/03/2008
Bassetti, F. ; Ladelli, L. ; Regazzini, E.
Probabilistic study of the speed of approach to equilibrium for an inelastic Kac model | Abstract | | This paper deals with a one--dimensional model for granular materials, which boils down to an inelastic version of the Kac kinetic equation, with inelasticity parameter p>0. In particular, the paper provides bounds for certain distances -- such as specific weighted chi--distances and the Kolmogorov distance -- between the solution of that equation and the limit. It is assumed that the even part of the initial datum (which determines the asymptotic properties of the solution) belongs to the domain of normal attraction of a symmetric stable distribution with characteristic exponent alpha=2/(1+p). With such initial data, it turns out that the limit exists and is just the aforementioned stable distribution. A necessary condition for the relaxation to equilibrium is also proved. Some bounds are obtained without introducing any extra--condition. Sharper bounds, of an exponential type, are exhibited in the presence of additional assumptions concerning either the behaviour, near to the origin, of the initial characteristic function, or the behaviour, at infinity, of the initial probability distribution function.
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QDD27 - 06/03/2008
Fragala , I. ; Gazzola, F.
Partially overdetermined elliptic problems | Abstract | | We consider semilinear elliptic Dirichlet problems in bounded domains, overdetermined with a Neumann condition on a proper part of the boundary. Under different kinds of assumptions, we show that these problems admit a solution only if the domain is a ball. When these assumptions are not fulfilled, we discuss possible counterexamples to symmetry.
We also consider Neumann problems overdetermined with a Dirichlet condition on a proper part of the boundary, and the case of partially overdetermined problems on exterior domains.
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QDD26 - 04/03/2008
Barchielli, A.; Gregoratti, M.
Quantum continual measurements: the spectrum of the output | Abstract | | When a quantum system is monitored in continuous time, the result of the measurement is a stochastic process. When the output process is stationary, at least in the long run, the spectrum of the process can be introduced and its properties studied. A typical continual measurement for quantum optical systems is the so called homodyne detection. In this paper we show how the Heisenberg uncertainty relations give rise to characteristic bounds on the
possible homodyne spectra and we discuss how this is related to the typical quantum phenomenon
of squeezing. |
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QDD25 - 05/02/2008
Barchielli, A.; Gregoratti, M.; Licciardo, M.
Quantum trajectories, feedback and squeezing | Abstract | | Quantum trajectory theory is the best mathematical set up to model continual observations of a quantum system and feedback based on the observed output. Inside this framework, we study how to enhance the squeezing of the fluorescence light emitted by a two-level atom, stimulated by a coherent monochromatic laser. In the presence of a Wiseman-Milburn feedback scheme, based on the homodyne detection of a fraction of the emitted light, we analyze the squeezing dependence on the various control parameters. |
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QDD22 - 14/09/2007
Higazy, M. Sh. ; El-Shanawany, R. ; Scapellato, R.
Orthogonal double covers of complete bipartite graphs by the union of a cycle and a star | Abstract | | Let H be a graph on n vertices and C a collection of subgraphs of H, one for each vertex. Then C is an orthogonal double cover (ODC) of H if every edge of H occurs in exactly two members of C and any two members of C share exactly an edge whenever the corresponding vertices are adjacent in H. If all subgraphs in C are isomorphic to a given spanning subgraph G, then C is said to be an ODC of H by G. We construct ODCs of complete bipartite graphs by the union of a cycle and a star, that can be either disjoint or have in common the center of the star. |
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QDD20 - 19/06/2007
Garavaglia, E.; Guagenti E.; Pavani R.; Petrini, L.
La predicibilita' di un terremoto caratteristico nell'ipotesi dei processi di rinnovo del tipo mistura | Abstract | | Nel presente lavoro ci proponiamo di studiare il problema della previsione dei terremoti, considerando il caso di un terremoto che diremo "caratteristico disturbato" nel senso che, accanto a tempi di intercorrenza piuttosto lunghi e regolari, sono presenti nelle aree sismogenetiche italiane sotto indagine, tempi di intercorrenza brevi e irregolarmente distribuiti nel tempo.
In questo lavoro proponiamo l'ipotesi di un terremoto caratteristico disturbato da una specie di rumore, rappresentabile a sua volta come processo poissoniano non prevedibile, cosicche' il processo finale (limitato ai terremoti forti) viene modellato secondo un renewal process con distribuzione mistura dei failure times: esponenziale per i tempi brevi, Weibull con hazard rate crescente per tempi lunghi.
Vengono discusse la metodologia adottata, le procedure di stima a confronto, la credibilita' delle procedure, la robustezza del modello e vengono fornite applicazioni alle zone sismogenetiche italiane. |
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