We prove existence results for systems of boundary value problems involving elliptic second-order differential operators. The assumptions involve lower and upper solutions, which may be either well-ordered, or not at all. The results are stated in an abstract framework, and can be translated also for systems of parabolic type.We prove existence results for systems of boundary value problems involving elliptic second-order differential operators. The assumptions involve lower and upper solutions, which may be either well-ordered, or not at all. The results are stated in an abstract framework, and can be translated also for systems of parabolic type.

SN - 0219-1997 UR - https://doi.org/10.1142/S0219199721500802 JO - Commun. Contemp. Math. ER - TY - JOUR T1 - Periodic Solutions of Second-Order Differential Equations in Hilbert Spaces Y1 - 2021 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci AB -We prove the existence of periodic solutions of some infinite-dimensional systems by the use of the lower/upper solutions method. Both the well-ordered and non-well-ordered cases are treated, thus generalizing to systems some well-established results for scalar equations.

VL - 18 SN - 1660-5454 UR - https://doi.org/10.1007/s00009-021-01857-8 IS - 5 JO - Mediterranean Journal of Mathematics ER - TY - JOUR T1 - Propagating geometry information to finite element computations JF - Transactions on Mathematical Software Y1 - 2021 A1 - Luca Heltai A1 - Wolfgang Bangerth A1 - Martin Kronbichler A1 - Andrea Mola VL - 47 IS - 4 ER - TY - JOUR T1 - A Reduced Order Cut Finite Element method for geometrically parametrized steady and unsteady Navier–Stokes problems JF - Computer & Mathematics With Applications Y1 - 2021 A1 - Efthymios N Karatzas A1 - Monica Nonino A1 - F. Ballarin A1 - Gianluigi Rozza KW - Cut Finite Element Method KW - Navier–Stokes equations KW - Parameter–dependent shape geometry KW - Reduced Order Models KW - Unfitted mesh AB -We focus on steady and unsteady Navier–Stokes flow systems in a reduced-order modeling framework based on Proper Orthogonal Decomposition within a levelset geometry description and discretized by an unfitted mesh Finite Element Method. This work extends the approaches of [1], [2], [3] to nonlinear CutFEM discretization. We construct and investigate a unified and geometry independent reduced basis which overcomes many barriers and complications of the past, that may occur whenever geometrical morphings are taking place. By employing a geometry independent reduced basis, we are able to avoid remeshing and transformation to reference configurations, and we are able to handle complex geometries. This combination of a fixed background mesh in a fixed extended background geometry with reduced order techniques appears beneficial and advantageous in many industrial and engineering applications, which could not be resolved efficiently in the past.

SN - 0898-1221 UR - https://www.sciencedirect.com/science/article/pii/S0898122121002790 JO - Computers & Mathematics with Applications ER - TY - JOUR T1 - Well-Ordered and Non-Well-Ordered Lower and Upper Solutions for Periodic Planar Systems JF - Advanced Nonlinear Studies Y1 - 2021 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci VL - 21 UR - https://doi.org/10.1515/ans-2021-2117 IS - 2 ER - TY - JOUR T1 - The deal.II finite element library: Design, features, and insights JF - Computers and Mathematics with Applications Y1 - 2020 A1 - Daniel Arndt A1 - Wolfgang Bangerth A1 - Denis Davydov A1 - Timo Heister A1 - Luca Heltai A1 - Martin Kronbichler A1 - Matthias Maier A1 - Jean-Paul Pelteret A1 - Bruno Turcksin A1 - David Wells UR - https://doi.org/10.1016/j.camwa.2020.02.022 ER - TY - JOUR T1 - The deal.II library, Version 9.2 JF - Journal of Numerical Mathematics Y1 - 2020 A1 - Daniel Arndt A1 - Wolfgang Bangerth A1 - Bruno Blais A1 - Thomas C. Clevenger A1 - Marc Fehling A1 - Alexander V. Grayver A1 - Timo Heister A1 - Luca Heltai A1 - Martin Kronbichler A1 - Matthias Maier A1 - Peter Munch A1 - Jean-Paul Pelteret A1 - Reza Rastak A1 - Ignacio Tomas A1 - Bruno Turcksin A1 - Zhuoran Wang A1 - David Wells VL - 28 ER - TY - JOUR T1 - On functions having coincident p-norms JF - Annali di Matematica Pura ed Applicata (1923 -) Y1 - 2020 A1 - Giuliano Klun AB -In a measure space $(X,{\mathcal {A}},\mu )$, we consider two measurable functions $f,g:E\rightarrow {\mathbb {R}}$, for some $E\in {\mathcal {A}}$. We prove that the property of having equal p-norms when p varies in some infinite set $P\subseteq [1,+\infty )$ is equivalent to the following condition: $\begin{aligned} \mu (\{x\in E:|f(x)|>\alpha \})=\mu (\{x\in E:|g(x)|>\alpha \})\quad \text { for all } \alpha \ge 0. \end{aligned}$

VL - 199 UR - https://doi.org/10.1007/s10231-019-00907-z ER - TY - JOUR T1 - Periodic solutions of nearly integrable Hamiltonian systems bifurcating from infinite-dimensional tori JF - NONLINEAR ANALYSIS Y1 - 2020 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci AB -We prove the existence of periodic solutions of some infinite-dimensional nearly integrable Hamiltonian systems, bifurcating from infinite-dimensional tori, by the use of a generalization of the Poincaré–Birkhoff Theorem.

UR - https://doi.org/10.1016/j.na.2019.111720 ER - TY - JOUR T1 - Projection-based reduced order models for a cut finite element method in parametrized domains JF - Computers and Mathematics with Applications Y1 - 2020 A1 - Efthymios N Karatzas A1 - F. Ballarin A1 - Gianluigi Rozza AB -This work presents a reduced order modeling technique built on a high fidelity embedded mesh finite element method. Such methods, and in particular the CutFEM method, are attractive in the generation of projection-based reduced order models thanks to their capabilities to seamlessly handle large deformations of parametrized domains and in general to handle topological changes. The combination of embedded methods and reduced order models allows us to obtain fast evaluation of parametrized problems, avoiding remeshing as well as the reference domain formulation, often used in the reduced order modeling for boundary fitted finite element formulations. The resulting novel methodology is presented on linear elliptic and Stokes problems, together with several test cases to assess its capability. The role of a proper extension and transport of embedded solutions to a common background is analyzed in detail.

VL - 79 UR - https://www.scopus.com/inward/record.uri?eid=2-s2.0-85070900852&doi=10.1016%2fj.camwa.2019.08.003&partnerID=40&md5=2d222ab9c7832955d155655d3c93e1b1 ER - TY - CONF T1 - A Reduced Order Approach for the Embedded Shifted Boundary FEM and a Heat Exchange System on Parametrized Geometries T2 - IUTAM Symposium on Model Order Reduction of Coupled Systems, Stuttgart, Germany, May 22–25, 2018 Y1 - 2020 A1 - Efthymios N Karatzas A1 - Giovanni Stabile A1 - Nabib Atallah A1 - Guglielmo Scovazzi A1 - Gianluigi Rozza ED - Fehr, Jörg ED - Bernard Haasdonk AB -A model order reduction technique is combined with an embedded boundary finite element method with a POD-Galerkin strategy. The proposed methodology is applied to parametrized heat transfer problems and we rely on a sufficiently refined shape-regular background mesh to account for parametrized geometries. In particular, the employed embedded boundary element method is the Shifted Boundary Method (SBM) recently proposed. This approach is based on the idea of shifting the location of true boundary conditions to a surrogate boundary, with the goal of avoiding cut cells near the boundary of the computational domain. This combination of methodologies has multiple advantages. In the first place, since the Shifted Boundary Method always relies on the same background mesh, there is no need to update the discretized parametric domain. Secondly, we avoid the treatment of cut cell elements, which usually need particular attention. Thirdly, since the whole background mesh is considered in the reduced basis construction, the SBM allows for a smooth transition of the reduced modes across the immersed domain boundary. The performances of the method are verified in two dimensional heat transfer numerical examples.

JF - IUTAM Symposium on Model Order Reduction of Coupled Systems, Stuttgart, Germany, May 22–25, 2018 PB - Springer International Publishing UR - https://arxiv.org/abs/1807.07753 ER - TY - JOUR T1 - A reduced-order shifted boundary method for parametrized incompressible Navier–Stokes equations JF - Computer Methods in Applied Mechanics and Engineering Y1 - 2020 A1 - Efthymios N Karatzas A1 - Giovanni Stabile A1 - Leo Nouveau A1 - Guglielmo Scovazzi A1 - Gianluigi Rozza AB -We investigate a projection-based reduced order model of the steady incompressible Navier–Stokes equations for moderate Reynolds numbers. In particular, we construct an “embedded” reduced basis space, by applying proper orthogonal decomposition to the Shifted Boundary Method, a high-fidelity embedded method recently developed. We focus on the geometrical parametrization through level-set geometries, using a fixed Cartesian background geometry and the associated mesh. This approach avoids both remeshing and the development of a reference domain formulation, as typically done in fitted mesh finite element formulations. Two-dimensional computational examples for one and three parameter dimensions are presented to validate the convergence and the efficacy of the proposed approach.

VL - 370 UR - https://www.scopus.com/inward/record.uri?eid=2-s2.0-85087886522&doi=10.1016%2fj.cma.2020.113273&partnerID=40&md5=d864e4808190b682ecb1c8b27cda72d8 ER - TY - JOUR T1 - The deal.II Library, Version 9.1 JF - Journal of Numerical Mathematics Y1 - 2019 A1 - Daniel Arndt A1 - Wolfgang Bangerth A1 - Thomas C. Clevenger A1 - Denis Davydov A1 - Marc Fehling A1 - Garcia-Sanchez, Daniel A1 - Harper, Graham A1 - Timo Heister A1 - Luca Heltai A1 - Martin Kronbichler A1 - Maguire Kynch, Ross A1 - Matthias Maier A1 - Jean-Paul Pelteret A1 - Bruno Turcksin A1 - David Wells AB - This paper provides an overview of the new features of the finite element library deal.II, version 9.1. ER - TY - JOUR T1 - The deal.II Library, Version 9.1 JF - Journal of Numerical Mathematics Y1 - 2019 A1 - Daniel Arndt A1 - Wolfgang Bangerth A1 - Thomas C. Clevenger A1 - Denis Davydov A1 - Marc Fehling A1 - Garcia-Sanchez, Daniel A1 - Harper, Graham A1 - Timo Heister A1 - Luca Heltai A1 - Martin Kronbichler A1 - Maguire Kynch, Ross A1 - Matthias Maier A1 - Jean-Paul Pelteret A1 - Bruno Turcksin A1 - David Wells ER - TY - JOUR T1 - On the Number of Flats Tangent to Convex Hypersurfaces in Random Position JF - Discrete & Computational Geometry Y1 - 2019 A1 - Khazhgali Kozhasov A1 - Antonio Lerario UR - https://doi.org/10.1007/s00454-019-00067-0 ER - TY - JOUR T1 - The Real Polynomial Eigenvalue Problem is Well Conditioned on the Average JF - Foundations of Computational Mathematics Y1 - 2019 A1 - Carlos Beltrán A1 - Khazhgali Kozhasov AB -We study the average condition number for polynomial eigenvalues of collections of matrices drawn from some random matrix ensembles. In particular, we prove that polynomial eigenvalue problems defined by matrices with random Gaussian entries are very well conditioned on the average.

UR - https://doi.org/10.1007/s10208-019-09414-2 ER - TY - JOUR T1 - A reduced basis approach for PDEs on parametrized geometries based on the shifted boundary finite element method and application to a Stokes flow JF - Computer Methods in Applied Mechanics and Engineering Y1 - 2019 A1 - Efthymios N Karatzas A1 - Giovanni Stabile A1 - Leo Nouveau A1 - Guglielmo Scovazzi A1 - Gianluigi Rozza AB -We propose a model order reduction technique integrating the Shifted Boundary Method (SBM) with a POD-Galerkin strategy. This approach allows to deal with complex parametrized domains in an efficient and straightforward way. The impact of the proposed approach is threefold. First, problems involving parametrizations of complex geometrical shapes and/or large domain deformations can be efficiently solved at full-order by means of the SBM. This unfitted boundary method permits to avoid remeshing and the tedious handling of cut cells by introducing an approximate surrogate boundary. Second, the computational effort is reduced by the development of a Reduced Order Model (ROM) technique based on a POD-Galerkin approach. Third, the SBM provides a smooth mapping from the true to the surrogate domain, and for this reason, the stability and performance of the reduced order basis are enhanced. This feature is the net result of the combination of the proposed ROM approach and the SBM. Similarly, the combination of the SBM with a projection-based ROM gives the great advantage of an easy and fast to implement algorithm considering geometrical parametrization with large deformations. The transformation of each geometry to a reference geometry (morphing) is in fact not required. These combined advantages will allow the solution of PDE problems more efficiently. We illustrate the performance of this approach on a number of two-dimensional Stokes flow problems.

VL - 347 UR - https://www.scopus.com/inward/record.uri?eid=2-s2.0-85060107322&doi=10.1016%2fj.cma.2018.12.040&partnerID=40&md5=1a3234f0cb000c91494d946428f8ebef ER - TY - CHAP T1 - A Spectral Element Reduced Basis Method in Parametric CFD T2 - Numerical Mathematics and Advanced Applications - ENUMATH 2017 Y1 - 2019 A1 - Martin W. Hess A1 - Gianluigi Rozza ED - Radu, Florin Adrian ED - Kumar, Kundan ED - Berre, Inga ED - Nordbotten, Jan Martin ED - Pop, Iuliu Sorin AB -We consider the Navier-Stokes equations in a channel with varying Reynolds numbers. The model is discretized with high-order spectral element ansatz functions, resulting in 14 259 degrees of freedom. The steady-state snapshot solu- tions define a reduced order space, which allows to accurately evaluate the steady- state solutions for varying Reynolds number with a reduced order model within a fixed-point iteration. In particular, we compare different aspects of implementing the reduced order model with respect to the use of a spectral element discretization. It is shown, how a multilevel static condensation in the pressure and velocity boundary degrees of freedom can be combined with a reduced order modelling approach to enhance computational times in parametric many-query scenarios.

JF - Numerical Mathematics and Advanced Applications - ENUMATH 2017 PB - Springer International Publishing VL - 126 UR - https://arxiv.org/abs/1712.06432 ER - TY - JOUR T1 - On the topological degree of planar maps avoiding normal cones JF - TOPOLOGICAL METHODS IN NONLINEAR ANALYSIS Y1 - 2019 A1 - Alessandro Fonda A1 - Giuliano Klun AB -The classical Poincaré-Bohl theorem provides the existence of a zero for a function avoiding external rays. When the domain is convex, the same holds true when avoiding normal cones.

We consider here the possibility of dealing with nonconvex sets having inward corners or cusps, in which cases the normal cone vanishes. This allows us to deal with situations where the topological degree may be strictly greater than $1$.

We construct generic real symmetric tensors with only real eigenvectors or, equivalently, real homogeneous polynomials with the maximum possible finite number of critical points on the sphere.

PB - SIAM VL - 2 UR - https://epubs.siam.org/doi/pdf/10.1137/17M1145902 ER - TY - JOUR T1 - Minimizing movements for mean curvature flow of droplets with prescribed contact angle JF - Journal de Mathématiques Pures et Appliquées Y1 - 2018 A1 - Giovanni Bellettini A1 - Matteo Novaga A1 - Shokhrukh Kholmatov KW - Capillary functional KW - Mean curvature flow with prescribed contact angle KW - Minimizing movements KW - Sets of finite perimeter AB -We study the mean curvature motion of a droplet flowing by mean curvature on a horizontal hyperplane with a possibly nonconstant prescribed contact angle. Using the solutions constructed as a limit of an approximation algorithm of Almgren–Taylor–Wang and Luckhaus–Sturzenhecker, we show the existence of a weak evolution, and its compatibility with a distributional solution. We also prove various comparison results. Résumé Nous étudions le mouvement par courbure moyenne d'une goutte qui glisse par courbure moyenne sur un hyperplan horizontal avec un angle de contact prescrit éventuellement non constant. En utilisant les solutions construites comme limites d'un algorithme d'approximation dû à Almgren, Taylor et Wang et Luckhaus et Sturzenhecker, nous montrons l'existence d'une évolution faible, et sa compatibilité avec une solution au sens des distributions. Nous démontrons également plusieurs résultats de comparaison.

VL - 117 UR - http://www.sciencedirect.com/science/article/pii/S0021782418300825 ER - TY - JOUR T1 - Minimizing Movements for Mean Curvature Flow of Partitions JF - SIAM Journal on Mathematical Analysis Y1 - 2018 A1 - Giovanni Bellettini A1 - Shokhrukh Kholmatov AB -We prove the existence of a weak global in time mean curvature flow of a bounded partition of space using the method of minimizing movements. The result is extended to the case when suitable driving forces are present. We also prove some consistency results for a minimizing movement solution with smooth and viscosity solutions when the evolution starts from a partition made by a union of bounded sets at a positive distance. In addition, the motion starting from the union of convex sets at a positive distance agrees with the classical mean curvature flow and is stable with respect to the Hausdorff convergence of the initial partitions.

VL - 50 UR - https://doi.org/10.1137/17M1159294 ER - TY - JOUR T1 - Numerical study of the Kadomtsev-Petviashvili equation and dispersive shock waves JF - Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences Y1 - 2018 A1 - Tamara Grava A1 - Christian Klein A1 - Giuseppe Pitton AB -A detailed numerical study of the long time behaviour of dispersive shock waves in solutions to the Kadomtsev–Petviashvili (KP) I equation is presented. It is shown that modulated lump solutions emerge from the dispersive shock waves. For the description of dispersive shock waves, Whitham modulation equations for KP are obtained. It is shown that the modulation equations near the soliton line are hyperbolic for the KPII equation while they are elliptic for the KPI equation leading to a focusing effect and the formation of lumps. Such a behaviour is similar to the appearance of breathers for the focusing nonlinear Schrödinger equation in the semiclassical limit.

VL - 474 UR - https://royalsocietypublishing.org/doi/abs/10.1098/rspa.2017.0458 ER - TY - JOUR T1 - Symplectic invariants for parabolic orbits and cusp singularities of integrable systems JF - Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences Y1 - 2018 A1 - Alexey Bolsinov A1 - Lorenzo Guglielmi A1 - Elena Kudryavtseva AB -We discuss normal forms and symplectic invariants of parabolic orbits and cuspidal tori in integrable Hamiltonian systems with two degrees of freedom. Such singularities appear in many integrable systems in geometry and mathematical physics and can be considered as the simplest example of degenerate singularities. We also suggest some new techniques which apparently can be used for studying symplectic invariants of degenerate singularities of more general type. This article is part of the theme issue ‘Finite dimensional integrable systems: new trends and methods’.

VL - 376 UR - https://royalsocietypublishing.org/doi/abs/10.1098/rsta.2017.0424 ER - TY - CONF T1 - On Uniqueness of Weak Solutions to Transport Equation with Non-smooth Velocity Field T2 - Theory, Numerics and Applications of Hyperbolic Problems I Y1 - 2018 A1 - Paolo Bonicatto ED - Klingenberg, Christian ED - Westdickenberg, Michael JF - Theory, Numerics and Applications of Hyperbolic Problems I PB - Springer International Publishing CY - Cham SN - 978-3-319-91545-6 UR - https://link.springer.com/chapter/10.1007/978-3-319-91545-6_15 ER - TY - JOUR T1 - The deal.II Library, Version 8.5 JF - JOURNAL OF NUMERICAL MATHEMATICS Y1 - 2017 A1 - Daniel Arndt A1 - Wolfgang Bangerth A1 - Denis Davydov A1 - Timo Heister A1 - Luca Heltai A1 - Martin Kronbichler A1 - Matthias Maier A1 - Jean-Paul Pelteret A1 - Bruno Turcksin A1 - David Wells VL - 25 UR - https://www.dealii.org/deal85-preprint.pdf ER - TY - JOUR T1 - Minimizers of anisotropic perimeters with cylindrical norms JF - Communications on Pure & Applied Analysis Y1 - 2017 A1 - Giovanni Bellettini A1 - Matteo Novaga A1 - Shokhrukh Kholmatov KW - anisotropic Bernstein problem; KW - minimal cones KW - Non parametric minimal surfaces KW - Sets of finite perimeter AB -We study various regularity properties of minimizers of the Φ–perimeter, where Φ is a norm. Under suitable assumptions on Φ and on the dimension of the ambient space, we prove that the boundary of a cartesian minimizer is locally a Lipschitz graph out of a closed singular set of small Hausdorff dimension. Moreover, we show the following anisotropic Bernstein-type result: any entire cartesian minimizer is the subgraph of a monotone function depending only on one variable.

VL - 16 UR - http://aimsciences.org//article/id/47054f15-00c7-40b7-9da1-4c0b1d0a103d ER - TY - JOUR T1 - A natural framework for isogeometric fluid-structure interaction based on BEM-shell coupling JF - COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING Y1 - 2017 A1 - Luca Heltai A1 - Kiendl, J. A1 - Antonio DeSimone A1 - Alessandro Reali VL - 316 UR - http://cdsads.u-strasbg.fr/abs/2017CMAME.316..522H ER - TY - RPRT T1 - Random spectrahedra Y1 - 2017 A1 - Paul Breiding A1 - Khazhgali Kozhasov A1 - Antonio Lerario ER - TY - JOUR T1 - Symplectic geometry of the moduli space of projective structures in homological coordinates JF - Inventiones Mathematicae Y1 - 2017 A1 - Marco Bertola A1 - Dmitry Korotkin A1 - Chaya Norton UR - https://arxiv.org/abs/1506.07918 ER - TY - JOUR T1 - Adaptivity and blow-up detection for nonlinear evolution problems JF - SIAM J. Sci. Comput. Y1 - 2016 A1 - Andrea Cangiani A1 - E.H. Georgoulis A1 - Kyza, Irene A1 - Metcalfe, Stephen VL - 38 UR - https://doi.org/10.1137/16M106073X ER - TY - JOUR T1 - The deal.II Library, Version 8.3 JF - ARCHIVE OF NUMERICAL SOFTWARE Y1 - 2016 A1 - Wolfgang Bangerth A1 - Timo Heister A1 - Luca Heltai A1 - G. Kanschat A1 - Martin Kronbichler A1 - Matthias Maier A1 - Bruno Turcksin VL - 4 UR - http://nbn-resolving.de/urn:nbn:de:bsz:16-ans-231226 ER - TY - JOUR T1 - The deal.II library, Version 8.4 JF - JOURNAL OF NUMERICAL MATHEMATICS Y1 - 2016 A1 - Wolfgang Bangerth A1 - Denis Davydov A1 - Timo Heister A1 - Luca Heltai A1 - G. Kanschat A1 - Martin Kronbichler A1 - Matthias Maier A1 - Bruno Turcksin A1 - David Wells VL - 24 UR - https://www.math.clemson.edu/ heister/preprints/deal84-preprint.pdf ER - TY - JOUR T1 - Error Estimates of B-spline based finite-element method for the wind-driven ocean circulation JF - JOURNAL OF SCIENTIFIC COMPUTING Y1 - 2016 A1 - Rotundo, N. A1 - Kim, T. -Y. A1 - Jiang, W. A1 - Luca Heltai A1 - Fried, E. VL - 69 ER - TY - RPRT T1 - Large KAM tori for perturbations of the dNLS equation Y1 - 2016 A1 - Massimiliano Berti A1 - Thomas Kappeler A1 - Riccardo Montalto AB - We prove that small, semi-linear Hamiltonian perturbations of the defocusing nonlinear Schr\"odinger (dNLS) equation on the circle have an abundance of invariant tori of any size and (finite) dimension which support quasi-periodic solutions. When compared with previous results the novelty consists in considering perturbations which do not satisfy any symmetry condition (they may depend on x in an arbitrary way) and need not be analytic. The main difficulty is posed by pairs of almost resonant dNLS frequencies. The proof is based on the integrability of the dNLS equation, in particular the fact that the nonlinear part of the Birkhoff coordinates is one smoothing. We implement a Newton-Nash-Moser iteration scheme to construct the invariant tori. The key point is the reduction of linearized operators, coming up in the iteration scheme, to 2×2 block diagonal ones with constant coefficients together with sharp asymptotic estimates of their eigenvalues. UR - http://preprints.sissa.it/handle/1963/35284 U1 - 35589 U2 - Mathematics ER - TY - JOUR T1 - Pimsner algebras and Gysin sequences from principal circle actions JF - Journal of Noncommutative Geometry Y1 - 2016 A1 - Francesca Arici A1 - Jens Kaad A1 - Giovanni Landi VL - 10 UR - http://hdl.handle.net/2066/162951 ER - TY - JOUR T1 - Refined node polynomials via long edge graphs JF - Communications in Number Theory and Physics Y1 - 2016 A1 - Lothar Göttsche A1 - Benjamin Kipkirui Kikwai AB -The generating functions of the Severi degrees for sufficiently ample line bundles on algebraic surfaces are multiplicative in the topological invariants of the surface and the line bundle. Recently new proofs of this fact were given for toric surfaces by Block, Colley, Kennedy and Liu, Osserman, using tropical geometry and in particular the combinatorial tool of long-edged graphs. In the first part of this paper these results are for $\mathbb{P}^2$ and rational ruled surfaces generalised to refined Severi degrees. In the second part of the paper we give a number of mostly conjectural generalisations of this result to singular surfaces, and curves with prescribed multiple points. The formulas involve modular forms and theta functions.

PB - International Press of Boston VL - 10 UR - http://dx.doi.org/10.4310/CNTP.2016.v10.n2.a2 ER - TY - JOUR T1 - On Sobolev instability of the interior problem of tomography JF - Journal of Mathematical Analysis and Applications Y1 - 2016 A1 - Marco Bertola A1 - Alexander Katsevich A1 - Alexander Tovbis ER - TY - JOUR T1 - The deal.II Library, Version 8.2 JF - Archive of Numerical Software, vol. 3, n. 100, (2015), pages : 1-8 Y1 - 2015 A1 - Wolfgang Bangerth A1 - Timo Heister A1 - Luca Heltai A1 - G. Kanschat A1 - Martin Kronbichler A1 - Matthias Maier A1 - Bruno Turcksin A1 - T. D. Young AB - This paper provides an overview of the new features of the finite element library deal.II version 8.2 UR - http://urania.sissa.it/xmlui/handle/1963/34464 U1 - 34637 U2 - Mathematics U4 - 1 U5 - MAT/08 ER - TY - JOUR T1 - Gli abachi: antichi strumenti precursori delle moderne macchine da calcolo Y1 - 2015 A1 - Giuliano Klun UR - http://hdl.handle.net/10077/10884 ER - TY - JOUR T1 - A topological join construction and the Toda system on compact surfaces of arbitrary genus JF - Analysis & PDE Y1 - 2015 A1 - Aleks Jevnikar A1 - Kallel, Sadok A1 - Andrea Malchiodi PB - Mathematical Sciences Publishers VL - 8 ER - TY - RPRT T1 - Translation and adaptation of Birman's paper "On the theory of self-adjoint extensions of positive definite operators" (1956) Y1 - 2015 A1 - Mikhail Khotyakov A1 - Alessandro Michelangeli AB - This is an accurate translation from Russian and adaptation to the modern mathematical jargon of a classical paper by M. Sh. Birman published in 1956, which is still today central in the theory of self-adjoint extensions of semi-bounded operators, and for which yet no English version was available so far. PB - SISSA UR - http://urania.sissa.it/xmlui/handle/1963/34443 U1 - 34570 ER - TY - JOUR T1 - Adler-Gelfand-Dickey approach to classical W-algebras within the theory of Poisson vertex algebras Y1 - 2014 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - We put the Adler-Gelfand-Dickey approach to classical W-algebras in the framework of Poisson vertex algebras. We show how to recover the bi-Poisson structure of the KP hierarchy, together with its generalizations and reduction to the N-th KdV hierarchy, using the formal distribution calculus and the lambda-bracket formalism. We apply the Lenard-Magri scheme to prove integrability of the corresponding hierarchies. We also give a simple proof of a theorem of Kupershmidt and Wilson in this framework. Based on this approach, we generalize all these results to the matrix case. In particular, we find (non-local) bi-Poisson structures of the matrix KP and the matrix N-th KdV hierarchies, and we prove integrability of the N-th matrix KdV hierarchy. PB - SISSA UR - http://hdl.handle.net/1963/7242 N1 - 45 pages ER - TY - JOUR T1 - Classical W-algebras and generalized Drinfeld-Sokolov hierarchies for minimal and short nilpotents JF - Communications in Mathematical Physics 331, nr. 2 (2014) 623-676 Y1 - 2014 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - We derive explicit formulas for lambda-brackets of the affine classical W-algebras attached to the minimal and short nilpotent elements of any simple Lie algebra g. This is used to compute explicitly the first non-trivial PDE of the corresponding intgerable generalized Drinfeld-Sokolov hierarchies. It turns out that a reduction of the equation corresponding to a short nilpotent is Svinolupov's equation attached to a simple Jordan algebra, while a reduction of the equation corresponding to a minimal nilpotent is an integrable Hamiltonian equation on 2h-3 functions, where h is the dual Coxeter number of g. In the case when g is sl_2 both these equations coincide with the KdV equation. In the case when g is not of type C_n, we associate to the minimal nilpotent element of g yet another generalized Drinfeld-Sokolov hierarchy. PB - SISSA UR - http://hdl.handle.net/1963/6979 N1 - 46 pages U1 - 6967 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - Dirac reduction for Poisson vertex algebras JF - Communications in Mathematical Physics 331, nr. 3 (2014) 1155-1190 Y1 - 2014 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - We construct an analogue of Dirac's reduction for an arbitrary local or non-local Poisson bracket in the general setup of non-local Poisson vertex algebras. This leads to Dirac's reduction of an arbitrary non-local Poisson structure. We apply this construction to an example of a generalized Drinfeld-Sokolov hierarchy. PB - SISSA UR - http://hdl.handle.net/1963/6980 N1 - 31 pages U1 - 6968 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - Existence of immersed spheres minimizing curvature functionals in compact 3-manifolds JF - Mathematische Annalen Y1 - 2014 A1 - Kuwert, Ernst A1 - Andrea Mondino A1 - Johannes Schygulla AB -We study curvature functionals for immersed 2-spheres in a compact, three-dimensional Riemannian manifold $M$. Under the assumption that the sectional curvature $K^M$ is strictly positive, we prove the existence of a smooth immersion $f:{\mathbb{S}}^2 \rightarrow M$ minimizing the $L^2$ integral of the second fundamental form. Assuming instead that $K^M \leq 2 $ and that there is some point $\bar{x}\in M$ with scalar curvature $R^M(\bar{x})>6$, we obtain a smooth minimizer $f:{\mathbb{S}}^2 \rightarrow M$ for the functional $\int \frac{1}{4}|H|^2+1$, where $H$ is the mean curvature.

VL - 359 UR - https://doi.org/10.1007/s00208-013-1005-3 ER - TY - RPRT T1 - Integrability of Dirac reduced bi-Hamiltonian equations Y1 - 2014 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - First, we give a brief review of the theory of the Lenard-Magri scheme for a non-local bi-Poisson structure and of the theory of Dirac reduction. These theories are used in the remainder of the paper to prove integrability of three hierarchies of bi-Hamiltonian PDE's, obtained by Dirac reduction from some generalized Drinfeld-Sokolov hierarchies. PB - SISSA UR - http://hdl.handle.net/1963/7247 N1 - 15 pages U1 - 7286 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - On an isomonodromy deformation equation without the Painlevé property Y1 - 2014 A1 - Boris Dubrovin A1 - Andrey Kapaev AB - We show that the fourth order nonlinear ODE which controls the pole dynamics in the general solution of equation $P_I^2$ compatible with the KdV equation exhibits two remarkable properties: 1) it governs the isomonodromy deformations of a $2\times2$ matrix linear ODE with polynomial coefficients, and 2) it does not possesses the Painlev\'e property. We also study the properties of the Riemann--Hilbert problem associated to this ODE and find its large $t$ asymptotic solution for the physically interesting initial data. PB - Maik Nauka-Interperiodica Publishing UR - http://hdl.handle.net/1963/6466 N1 - 34 pages, 8 figures, references added U1 - 6410 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - Local behavior of fractional p-minimizers Y1 - 2014 A1 - Agnese Di Castro A1 - Tuomo Kuusi A1 - Giampiero Palatucci KW - fractional Sobolev spaces AB -We extend the De Giorgi-Nash Moser theory to nonlocal, possibly degerate integro-differential operators

PB - SISSA U1 - 7301 U2 - Mathematics U4 - 1 U5 - MAT/05 ANALISI MATEMATICA ER - TY - JOUR T1 - Singular Value Decomposition of a Finite Hilbert Transform Defined on Several Intervals and the Interior Problem of Tomography: The Riemann-Hilbert Problem Approach JF - Comm. Pure Appl. Math. Y1 - 2014 A1 - Marco Bertola A1 - Alexander Katsevich A1 - Alexander Tovbis ER - TY - RPRT T1 - Structure of classical (finite and affine) W-algebras Y1 - 2014 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - First, we derive an explicit formula for the Poisson bracket of the classical finite W-algebra W^{fin}(g,f), the algebra of polynomial functions on the Slodowy slice associated to a simple Lie algebra g and its nilpotent element f. On the other hand, we produce an explicit set of generators and we derive an explicit formula for the Poisson vertex algebra structure of the classical affine W-algebra W(g,f). As an immediate consequence, we obtain a Poisson algebra isomorphism between W^{fin}(g,f) and the Zhu algebra of W(g,f). We also study the generalized Miura map for classical W-algebras. PB - SISSA UR - http://hdl.handle.net/1963/7314 N1 - 40 pages U1 - 7359 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - Weighted quantile correlation test for the logistic family Y1 - 2014 A1 - Ferenc Balogh A1 - Éva Krauczi AB - We summarize the results of investigating the asymptotic behavior of the weighted quantile correlation tests for the location-scale family associated to the logistic distribution. Explicit representations of the limiting distribution are given in terms of integrals of weighted Brownian bridges or alternatively as infinite series of independent Gaussian random variables. The power of this test and the test for the location logistic family against some alternatives are demonstrated by numerical simulations. PB - University of Szeged UR - http://urania.sissa.it/xmlui/handle/1963/35025 U1 - 35261 U2 - Mathematics U4 - 1 ER - TY - JOUR T1 - Classical W-algebras and generalized Drinfeld-Sokolov bi-Hamiltonian systems within the theory of Poisson vertex algebras JF - Communications in Mathematical Physics 323, nr. 2 (2013) 663-711 Y1 - 2013 A1 - Alberto De Sole A1 - Victor G. Kac A1 - Daniele Valeri AB - We provide a description of the Drinfeld-Sokolov Hamiltonian reduction for the construction of classical W-algebras within the framework of Poisson vertex algebras. In this context, the gauge group action on the phase space is translated in terms of (the exponential of) a Lie conformal algebra action on the space of functions. Following the ideas of Drinfeld and Sokolov, we then establish under certain sufficient conditions the applicability of the Lenard-Magri scheme of integrability and the existence of the corresponding integrable hierarchy of bi-Hamiltonian equations. PB - Springer UR - http://hdl.handle.net/1963/6978 N1 - 43 pages. Second version with minor editing and corrections U1 - 6966 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - RPRT T1 - On critical behaviour in systems of Hamiltonian partial differential equations Y1 - 2013 A1 - Boris Dubrovin A1 - Tamara Grava A1 - Christian Klein A1 - Antonio Moro AB -We study the critical behaviour of solutions to weakly dispersive Hamiltonian systems considered as perturbations of elliptic and hyperbolic systems of hydrodynamic type with two components. We argue that near the critical point of gradient catastrophe of the dispersionless system, the solutions to a suitable initial value problem for the perturbed equations are approximately described by particular solutions to the Painlev\'e-I (P$_I$) equation or its fourth order analogue P$_I^2$. As concrete examples we discuss nonlinear Schr\"odinger equations in the semiclassical limit. A numerical study of these cases provides strong evidence in support of the conjecture.

PB - SISSA U1 - 7280 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - RPRT T1 - The deal.II Library, Version 8.1 Y1 - 2013 A1 - Wolfgang Bangerth A1 - Timo Heister A1 - Luca Heltai A1 - G. Kanschat A1 - Martin Kronbichler A1 - Matthias Maier A1 - Bruno Turcksin A1 - T. D. Young AB - This paper provides an overview of the new features of the finite element library deal.II version 8.0. PB - SISSA UR - http://hdl.handle.net/1963/7236 N1 - 5 pages U1 - 7272 U2 - Mathematics U4 - 1 U5 - MAT/08 ANALISI NUMERICA ER - TY - JOUR T1 - Free Form Deformation Techniques Applied to 3D Shape Optimization Problems JF - Communications in Applied and Industrial Mathematics Y1 - 2013 A1 - Anwar Koshakji A1 - Alfio Quarteroni A1 - Gianluigi Rozza AB - The purpose of this work is to analyse and study an efficient parametrization technique for a 3D shape optimization problem. After a brief review of the techniques and approaches already available in literature, we recall the Free Form Deformation parametrization, a technique which proved to be efficient and at the same time versatile, allowing to manage complex shapes even with few parameters. We tested and studied the FFD technique by establishing a path, from the geometry definition, to the method implementation, and finally to the simulation and to the optimization of the shape. In particular, we have studied a bulb and a rudder of a race sailing boat as model applications, where we have tested a complete procedure from Computer-Aided-Design to build the geometrical model to discretization and mesh generation. ER - TY - JOUR T1 - Inversion formulae for the $\romancosh$-weighted Hilbert transform JF - Proc. Amer. Math. Soc. Y1 - 2013 A1 - Marco Bertola A1 - Katsevich, A. A1 - Alexander Tovbis VL - 141 UR - http://dx.doi.org/10.1090/S0002-9939-2013-11642-4 ER - TY - RPRT T1 - On the tritronquée solutions of P$_I^2$ Y1 - 2013 A1 - Tamara Grava A1 - Andrey Kapaev A1 - Christian Klein AB -For equation P$_I^2$, the second member in the P$_I$ hierarchy, we prove existence of various degenerate solutions depending on the complex parameter $t$ and evaluate the asymptotics in the complex $x$ plane for $|x|\to\infty$ and $t=o(x^{2/3})$. Using this result, we identify the most degenerate solutions $u^{(m)}(x,t)$, $\hat u^{(m)}(x,t)$, $m=0,\dots,6$, called {\em tritronqu\'ee}, describe the quasi-linear Stokes phenomenon and find the large $n$ asymptotics of the coefficients in a formal expansion of these solutions. We supplement our findings by a numerical study of the tritronqu\'ee solutions.

PB - SISSA U1 - 7282 U2 - Mathematics U4 - 1 U5 - MAT/07 FISICA MATEMATICA ER - TY - JOUR T1 - Detection of transcriptional triggers in the dynamics of microbial growth: application to the respiratory-versatile bacterium Shewanella oneidensis JF - Nucleic Acids Research, Volume 40, Issue 15, August 2012, Pages 7132-7149 Y1 - 2012 A1 - Q Beg A1 - Mattia Zampieri A1 - N Klitgord A1 - S Collins A1 - M Serres A1 - Daniel Segrè A1 - Claudio Altafini AB - The capacity of microorganisms to respond to variable external conditions requires a coordination of environment-sensing mechanisms and decisionmaking regulatory circuits. Here, we seek to understand the interplay between these two processes by combining high-throughput measurement of time-dependent mRNA profiles with a novel computational approach that searches for key genetic triggers of transcriptional changes. Our approach helped us understand the regulatory strategies of a respiratorily versatile bacterium with promising bioenergy and bioremediation applications, Shewanella oneidensis, in minimal and rich media. By comparing expression profiles across these two conditions, we unveiled components of the transcriptional program that depend mainly on the growth phase. Conversely, by integrating our time-dependent data with a previously available large compendium of static perturbation responses, we identified transcriptional changes that cannot be explained solely by internal network dynamics, but are rather triggered by specific genes acting as key mediators of an environment-dependent response. These transcriptional triggers include known and novel regulators that respond to carbon, nitrogen and oxygen limitation. Our analysis suggests a sequence of physiological responses, including a coupling between nitrogen depletion and glycogen storage, partially recapitulated through dynamic flux balance analysis, and experimentally confirmed by metabolite measurements. Our approach is broadly applicable to other systems PB - SISSA UR - http://hdl.handle.net/1963/6506 U1 - 6452 U2 - Mathematics U4 - 1 ER - TY - JOUR T1 - Numerical study of the small dispersion limit of the Korteweg-de Vries equation and asymptotic solutions JF - Physica D 241, nr. 23-24 (2012): 2246-2264 Y1 - 2012 A1 - Tamara Grava A1 - Christian Klein KW - Korteweg-de Vries equation AB - We study numerically the small dispersion limit for the Korteweg-de Vries (KdV) equation $u_t+6uu_x+\epsilon^{2}u_{xxx}=0$ for $\epsilon\ll1$ and give a quantitative comparison of the numerical solution with various asymptotic formulae for small $\epsilon$ in the whole $(x,t)$-plane. The matching of the asymptotic solutions is studied numerically. PB - Elsevier U1 - 7069 U2 - Physics U4 - -1 ER - TY - JOUR T1 - Numerical Study of breakup in generalized Korteweg-de Vries and Kawahara equations JF - SIAM J. Appl. Math. 71 (2011) 983-1008 Y1 - 2011 A1 - Boris Dubrovin A1 - Tamara Grava A1 - Christian Klein AB - This article is concerned with a conjecture in [B. Dubrovin, Comm. Math. Phys., 267 (2006), pp. 117–139] on the formation of dispersive shocks in a class of Hamiltonian dispersive regularizations of the quasi-linear transport equation. The regularizations are characterized by two arbitrary functions of one variable, where the condition of integrability implies that one of these functions must not vanish. It is shown numerically for a large class of equations that the local behavior of their solution near the point of gradient catastrophe for the transport equation is described by a special solution of a Painlevé-type equation. This local description holds also for solutions to equations where blowup can occur in finite time. Furthermore, it is shown that a solution of the dispersive equations away from the point of gradient catastrophe is approximated by a solution of the transport equation with the same initial data, modulo terms of order $\\\\epsilon^2$, where $\\\\epsilon^2$ is the small dispersion parameter. Corrections up to order $\\\\epsilon^4$ are obtained and tested numerically. PB - SIAM UR - http://hdl.handle.net/1963/4951 U1 - 4732 U2 - Mathematics U3 - Mathematical Physics U4 - -1 ER - TY - JOUR T1 - Gene expression analysis of the emergence of epileptiform activity after focal injection of kainic acid into mouse hippocampus. JF - The European journal of neuroscience. 2010 Oct; 32(8):1364-79 Y1 - 2010 A1 - Dario Motti A1 - Caroline Le Duigou A1 - Nicole Chemaly A1 - Lucia Wittner A1 - Dejan Lazarevic A1 - Helena Krmac A1 - Troels Torben Marstrand A1 - Eivind Valen A1 - Remo Sanges A1 - Elia Stupka A1 - Albin Sandelin A1 - Enrico Cherubini A1 - Stefano Gustincich A1 - Richard Miles AB -We report gene profiling data on genomic processes underlying the progression towards recurrent seizures after injection of kainic acid (KA) into the mouse hippocampus. Focal injection enabled us to separate the effects of proepileptic stimuli initiated by KA injection. Both the injected and contralateral hippocampus participated in the status epilepticus. However, neuronal death induced by KA treatment was restricted to the injected hippocampus, although there was some contralateral axonal degeneration. We profiled gene expression changes in dorsal and ventral regions of both the injected and contralateral hippocampus. Changes were detected in the expression of 1526 transcripts in samples from three time-points: (i) during the KA-induced status epilepticus, (ii) at 2 weeks, before recurrent seizures emerged, and (iii) at 6 months after seizures emerged. Grouping genes with similar spatio-temporal changes revealed an early transcriptional response, strong immune, cell death and growth responses at 2 weeks and an activation of immune and extracellular matrix genes persisting at 6 months. Immunostaining for proteins coded by genes identified from array studies provided evidence for gliogenesis and suggested that the proteoglycan biglycan is synthesized by astrocytes and contributes to a glial scar. Gene changes at 6 months after KA injection were largely restricted to tissue from the injection site. This suggests that either recurrent seizures might depend on maintained processes including immune responses and changes in extracellular matrix proteins near the injection site or alternatively might result from processes, such as growth, distant from the injection site and terminated while seizures are maintained.

PB - Wiley UR - http://hdl.handle.net/1963/4480 U1 - 4244 U2 - Neuroscience U3 - Neurobiology U4 - -1 ER - TY - RPRT T1 - Numerical Solution of the Small Dispersion Limit of the Camassa-Holm and Whitham Equations and Multiscale Expansions Y1 - 2010 A1 - Simonetta Abenda A1 - Tamara Grava A1 - Christian Klein AB - The small dispersion limit of solutions to the Camassa-Holm (CH) equation is characterized by the appearance of a zone of rapid modulated oscillations. An asymptotic description of these oscillations is given, for short times, by the one-phase solution to the CH equation, where the branch points of the corresponding elliptic curve depend on the physical coordinates via the Whitham equations. We present a conjecture for the phase of the asymptotic solution. A numerical study of this limit for smooth hump-like initial data provides strong evidence for the validity of this conjecture.... UR - http://hdl.handle.net/1963/3840 U1 - 487 U2 - Mathematics U3 - Mathematical Physics ER - TY - JOUR T1 - On universality of critical behaviour in the focusing nonlinear Schrödinger equation, elliptic umbilic catastrophe and the \\\\it tritronquée solution to the Painlevé-I equation JF - J. Nonlinear Sci. 19 (2009) 57-94 Y1 - 2009 A1 - Boris Dubrovin A1 - Tamara Grava A1 - Christian Klein AB - We argue that the critical behaviour near the point of ``gradient catastrophe\\\" of the solution to the Cauchy problem for the focusing nonlinear Schr\\\\\\\"odinger equation $ i\\\\epsilon \\\\psi_t +\\\\frac{\\\\epsilon^2}2\\\\psi_{xx}+ |\\\\psi|^2 \\\\psi =0$ with analytic initial data of the form $\\\\psi(x,0;\\\\epsilon) =A(x) e^{\\\\frac{i}{\\\\epsilon} S(x)}$ is approximately described by a particular solution to the Painlev\\\\\\\'e-I equation. UR - http://hdl.handle.net/1963/2525 U1 - 1593 U2 - Mathematics U3 - Mathematical Physics ER - TY - JOUR T1 - Numerical study of a multiscale expansion of the Korteweg-de Vries equation and Painlevé-II equation JF - Proc. R. Soc. A 464 (2008) 733-757 Y1 - 2008 A1 - Tamara Grava A1 - Christian Klein AB - The Cauchy problem for the Korteweg de Vries (KdV) equation with small dispersion of order $\\\\e^2$, $\\\\e\\\\ll 1$, is characterized by the appearance of a zone of rapid modulated oscillations. These oscillations are approximately described by the elliptic solution of KdV where the amplitude, wave-number and frequency are not constant but evolve according to the Whitham equations. Whereas the difference between the KdV and the asymptotic solution decreases as $\\\\epsilon$ in the interior of the Whitham oscillatory zone, it is known to be only of order $\\\\epsilon^{1/3}$ near the leading edge of this zone. To obtain a more accurate description near the leading edge of the oscillatory zone we present a multiscale expansion of the solution of KdV in terms of the Hastings-McLeod solution of the Painlev\\\\\\\'e-II equation. We show numerically that the resulting multiscale solution approximates the KdV solution, in the small dispersion limit, to the order $\\\\epsilon^{2/3}$. UR - http://hdl.handle.net/1963/2592 U1 - 1530 U2 - Mathematics U3 - Mathematical Physics ER - TY - JOUR T1 - On finite-dimensional projections of distributions for solutions of randomly forced PDE\\\'s JF - Ann. Inst. Henri Poincare-Prob. Stat. 43 (2007) 399-415 Y1 - 2007 A1 - Andrei A. Agrachev A1 - Sergei Kuksin A1 - Andrey Sarychev A1 - Armen Shirikyan AB - The paper is devoted to studying the image of probability measures on a Hilbert space under finite-dimensional analytic maps. We establish sufficient conditions under which the image of a measure has a density with respect to the Lebesgue measure and continuously depends on the map. The results obtained are applied to the 2D Navier-Stokes equations perturbed by various random forces of low dimension. UR - http://hdl.handle.net/1963/2012 U1 - 2184 U2 - Mathematics U3 - Functional Analysis and Applications ER - TY - RPRT T1 - Numerical solution of the small dispersion limit of Korteweg de Vries and Whitham equations Y1 - 2007 A1 - Tamara Grava A1 - Christian Klein AB - The Cauchy problem for the Korteweg de Vries (KdV) equation with small dispersion of order $\\\\epsilon^2$, is characterized by the appearance of a zone of rapid modulated oscillations of wave-length of order $\\\\epsilon$. These oscillations are approximately described by the elliptic solution of KdV where the amplitude, wave-number and frequency are not constant but evolve according to the Whitham equations. In this manuscript we give a quantitative analysis of the discrepancy between the numerical solution of the KdV equation in the small dispersion limit and the corresponding approximate solution for values of $\\\\epsilon$ between $10^{-1}$ and $10^{-3}$. The numerical results are compatible with a difference of order $\\\\epsilon$ within the `interior\\\' of the Whitham oscillatory zone, of order $\\\\epsilon^{1/3}$ at the left boundary outside the Whitham zone and of order $\\\\epsilon^{1/2}$ at the right boundary outside the Whitham zone. JF - Comm. Pure Appl. Math. 60 (2007) 1623-1664 UR - http://hdl.handle.net/1963/1788 U1 - 2756 U2 - Mathematics U3 - Mathematical Physics ER - TY - RPRT T1 - Numerical study of a multiscale expansion of KdV and Camassa-Holm equation Y1 - 2007 A1 - Tamara Grava A1 - Christian Klein AB - We study numerically solutions to the Korteweg-de Vries and Camassa-Holm equation close to the breakup of the corresponding solution to the dispersionless equation. The solutions are compared with the properly rescaled numerical solution to a fourth order ordinary differential equation, the second member of the Painlev\\\\\\\'e I hierarchy. It is shown that this solution gives a valid asymptotic description of the solutions close to breakup. We present a detailed analysis of the situation and compare the Korteweg-de Vries solution quantitatively with asymptotic solutions obtained via the solution of the Hopf and the Whitham equations. We give a qualitative analysis for the Camassa-Holm equation UR - http://hdl.handle.net/1963/2527 U1 - 1591 U2 - Mathematics U3 - Mathematical Physics ER - TY - JOUR T1 - 2-d stability of the Néel wall JF - Calc. Var. Partial Differential Equations 27 (2006) 233-253 Y1 - 2006 A1 - Antonio DeSimone A1 - Hans Knuepfer A1 - Felix Otto AB - We are interested in thin-film samples in micromagnetism, where the magnetization m is a 2-d unit-length vector field. More precisely we are interested in transition layers which connect two opposite magnetizations, so called Néel walls. UR - http://hdl.handle.net/1963/2194 U1 - 2050 U2 - Mathematics U3 - Functional Analysis and Applications ER - TY - CHAP T1 - Recent analytical developments in micromagnetics T2 - The science of hysteresis / eds. Giorgio Bertotti, Isaak D. Mayergoyz. - Amsterdam: Elsevier, 2006. Vol.2, 269-381. Y1 - 2006 A1 - Antonio DeSimone A1 - Robert V. Kohn A1 - Stefan Müller A1 - Felix Otto JF - The science of hysteresis / eds. Giorgio Bertotti, Isaak D. Mayergoyz. - Amsterdam: Elsevier, 2006. Vol.2, 269-381. SN - 978-0-12-480874-4 UR - http://hdl.handle.net/1963/2230 U1 - 2014 U2 - Mathematics U3 - Functional Analysis and Applications ER - TY - JOUR T1 - The duality of spectral curves that arises in two-matrix models JF - Teoret. Mat. Fiz. Y1 - 2003 A1 - Marco Bertola A1 - B. Eynard A1 - Kharnad, Dzh. VL - 134 ER - TY - JOUR T1 - Non-linear sigma-models in noncommutative geometry: fields with values in finite spaces JF - Mod. Phys. Lett. A 18 (2003) 2371-2379 Y1 - 2003 A1 - Ludwik Dabrowski A1 - Thomas Krajewski A1 - Giovanni Landi AB - We study sigma-models on noncommutative spaces, notably on noncommutative tori. We construct instanton solutions carrying a nontrivial topological charge q and satisfying a Belavin-Polyakov bound. The moduli space of these instantons is conjectured to consists of an ordinary torus endowed with a complex structure times a projective space $CP^{q-1}$. PB - World Scientific UR - http://hdl.handle.net/1963/3215 U1 - 1086 U2 - Mathematics U3 - Mathematical Physics ER - TY - JOUR T1 - Some Properties of Non-linear sigma-Models in Noncommutative Geometry JF - Int. J. Mod. Phys. B 14 (2000) 2367-2382 Y1 - 2000 A1 - Ludwik Dabrowski A1 - Thomas Krajewski A1 - Giovanni Landi AB - We introduce non-linear $\\\\sigma$-models in the framework of noncommutative geometry with special emphasis on models defined on the noncommutative torus. We choose as target spaces the two point space and the circle and illustrate some characteristic features of the corresponding $\\\\sigma$-models. In particular we construct a $\\\\sigma$-model instanton with topological charge equal to 1. We also define and investigate some properties of a noncommutative analogue of the Wess-Zumino-Witten model. PB - SISSA Library UR - http://hdl.handle.net/1963/1373 U1 - 3082 U2 - Mathematics U3 - Mathematical Physics ER -