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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QDD127 - 12/07/2012
Gazzola, F.; Pavani, R.
Wide oscillations finite time blow up for solutions to nonlinear fourth order differential equations | Abstract | | We give sufficient conditions for local solutions to some fourth order semilinear ordinary differential equations to blow up in finite time with wide oscillations. This phenomenon is not visible for lower order equations. This result is then applied to several classes of semilinear partial differential equations in order to characterize the blow up of solutions. In particular, its applications to a suspension bridge model are widely discussed. We also give numerical results which describe this oscillating blow up and allow to suggest several open problems and formulate some related conjectures. |
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QDD126 - 11/07/2012
Gazzola, F.
On the moments of solutions to linear parabolic equations involving the biharmonic operator | Abstract | | We consider the solutions to Cauchy problems for the parabolic equation $u_ tau + Delta^2u=0$ in $ mathbb{R}_+ times mathbb{R}^n$, with fast decay initial data. We study the behavior of their moments. This enables us to give a more precise description of the sign-changing behavior of solutions corresponding to positive initial data. |
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QDD125 - 01/07/2012
Pagani, C.D.; Pierotti, D.
A three dimensional Steklov eigenvalue problem with exponential nonlinearity on the boundary | Abstract | | We investigate the existence of pairs (λ, u), with λ > 0 and u harmonic function in a bounded domain Ω ⊂ R3, such that the nonlinear boundary condition ∂u = λ μ sinh u holds on ∂Ω, where μ is a non negative weight function. This type of exponential boundary condition arises in corrosion modeling (Butler Volmer condition). |
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QDD124 - 27/06/2012
Bramanti, M.; Niu, P.; Zhu, M.
Interior HW^{1,p} estimates for divergence degenerate elliptic systems in Carnot groups | Abstract | | Let X_{1},…,X_{q} be the basis of the space of horizontal vector fields on a homogeneous Carnot group in R^n (q |
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QDD122 - 14/05/2012
Bonforte, M.; Grillo, G.; Vazquez, J.L.
Quantitative Local Bounds for Subcritical Semilinear Elliptic Equations | Abstract | | The purpose of this paper is to prove local upper and lower bounds for weak solutions of suitable classes of semilinear elliptic equations, defined on bounded Euclidean domains, without reference to the boundary behaviour. We give an explicit expression for all the involved constants. As a consequence, we obtain local Harnack inequalities with explicit constants, as well as gradient bounds. |
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QDD123 - 14/05/2012
Grillo, G.; Muratori, M.; Porzio, M.M.
Porous media equations with two weights: existence, uniqueness, smoothing and decay properties of energy solutions via Poincaré inequalities | Abstract | | We study weighted porous media equations on Euclidean domains, either with Dirichlet or with Neumann homogeneous boundary conditions. Existence of weak solutions and uniqueness in a suitable class is studied in detail. Moreover, short time smoothing effects in Lebesgue spaces are discussed for short time, in connection with the validity of a Poincaré inequality in appropriate weighted Sobolev spaces, and the long-time asymptotic behaviour is also studied. In fact, we prove full equivalence between certain smoothing effects and suitable weighted Poincaré-type inequalities. Particular emphasis is given to the Neumann problem, which is much less studied in the literature, as well as to the case of the whole Euclidean case when the corresponding weight makes its measure finite, so that solutions converge to their weighted average instead than to zero. Examples are given in terms of wide classes of weights. |
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QDD121 - 02/04/2012
Baraldo, S.; Ieva, F.; Mainardi, L.; Paganoni, A. M.
Estimation approaches for the Apparent Diffusion Coefficient in Rice-distributed MR signals | Abstract | | The Apparent Diffusion Coefficient (ADC) is often considered in the differential diagnosis of tumors, since the analysis of a field of ADCs on a particular region of the body allows to identify regional necrosis. This quantity can be estimated from magnitude signals obtained in diffusion
Magnetic Resonance (MR), but in some situations, like total body MRs, it is possible to repeat only few measurements on the same patient, thus providing a limited amount of data for the estimation of ADCs. In this work we consider a Rician distributed magnitude signal with an exponential
dependence on the so-called b-value. Different pixelwise estimators for the ADC, both frequentist and Bayesian, are proposed and compared by a simulation study, focusing on issues caused by low signal-to-noise ratios and small sample sizes. |
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QDD120 - 10/02/2012
Barchielli, A.; Gregoratti, M.
Entanglement Protection and Generation Under Continuous Monitoring | Abstract | | Entanglement between two quantum systems is a resource in quantum information, but dissipation usually destroys it. In this article we consider two qubits without direct interaction and we show that, even in cases where the open system dynamics destroys any initial entanglement, the mere monitoring of the environment can preserve or create the entanglement, by filtering the state of the qubits. While the systems we study are very simple, we can show examples with entanglement protection or entanglement birth, death, rebirth due to monitoring. |
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