Blar i tittel Artikler, rapporter og annet (matematikk og statistikk)
Viser treff 196-215 av 314
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Objective Momentum Barriers in Wall Turbulence
(Journal article; Tidsskriftartikkel; Peer reviewed, 2022-04-28)We use the recent frame-indifferent theory of diffusive momentum transport to identify internal barriers in wall-bounded turbulence. Formed by the invariant manifolds of the Laplacian of the velocity field, the barriers block the viscous part of the instantaneous momentum flux in the flow. We employ the level sets of single-trajectory Lagrangian diagnostic tools, the trajectory rotation average ... -
ODEs whose Symmetry Groups are not Fiber-Preserving
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023)We observe that, up to conjugation, a majority of higher order ODEs (ordinary differential equations) and ODE systems have only fiber-preserving point symmetries. By exploiting Lie's classification of Lie algebras of vector fields, we describe all exceptions to this in the case of scalar ODEs and systems of ODEs on a pair of functions. The scalar ODEs whose symmetry algebras are not fiber preserving ... -
On a class of integrable systems of Monge-Ampère type
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-08)We investigate a class of multi-dimensional two-component systems of Monge-Ampère type that can be viewed as generalisations of heavenly type equations appearing in a self-dual Ricci-flat geometry. Based on the Jordan-Kronecker theory of the skew-symmetric matrix pencils, a classification of normal forms of such systems is obtained. All two-component systems of Monge-Ampère type turn out to be ... -
On a class of linearizable planar geodesic webs
(Working paper; Arbeidsnotat, 2008-12-13)We present a complete description of a class of linearizable planar geodesic webs which contain a parallelizable 3-subweb. -
On C-class equations
(Journal article; Tidsskriftartikkel; Peer reviewed, 2023-09-29)The concept of a C-class of differential equations goes back to E. Cartan with the upshot that generic equations in a C-class can be solved without integration. While Cartan’s definition was in terms of differential invariants being first integrals, all results exhibiting C-classes that we are aware of are based on the fact that a canonical Cartan geometry associated to the equations in the class ... -
On Data-Independent Properties for Density-Based Dissimilarity Measures in Hybrid Clustering
(Journal article; Tidsskriftartikkel; Peer reviewed, 2016-09-12)Hybrid clustering combines partitional and hierarchical clustering for computational effectiveness and versatility in cluster shape. In such clustering, a dissimilarity measure plays a crucial role in the hierarchical merging. The dissimilarity measure has great impact on the final clustering, and data-independent properties are needed to choose the right dissimilarity measure for the problem ... -
On hybrid classification using model assisted posterior estimates
(Journal article; Tidsskriftartikkel; Peer reviewed, 2012)Traditional parametric and nonparametric classifiers used for statistical pattern recognition have their own strengths and limitations. While parametric methods assume some specific parametric models for density functions or posterior probabilities of competing classes, nonparametric methods are free from such assumptions. So, when these model assumptions are correct, parametric methods outperform ... -
On integrability of certain rank 2 sub-Riemannian structures
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-10-01)We discuss rank 2 sub-Riemannian structures on low-dimensional manifolds and prove that some of these structures in dimensions 6, 7 and 8 have a maximal amount of symmetry but no integrals polynomial in momenta of low degrees, except for those coming from the Killing vector fields and the Hamiltonian, thus indicating nonintegrability of the corresponding geodesic flows. -
On Jordan classes for Vinberg's theta-groups
(Journal article; Tidsskriftartikkel; Peer reviewed, 2021-10-23)V. L. Popov has recently introduced an analogue of Jordan classes (packets or decomposition classes) for the action of a θ-group (G0, V), showing that they are finitely-many, locally-closed, irreducible unions of G0-orbits of constant dimension partitioning V. We carry out a local study of their closures showing that Jordan classes are smooth and that their closure is a union of Jordan classes. We ... -
On Multi-Rule and Probability-Dependant Adaptations of Conway’s Game of Life and Their Character
(Journal article; Tidsskriftartikkel; Peer reviewed, 2022)We introduce some novel adaptations of Life-Like Automata. We focus mainly on some modifications of the original rules where the evolution of the system is depending from the step and the zone of the grid. We study also the properties of some novel non-deterministic adaptations. -
On Rank Problems for Planar Webs and Projective Structures
(Chapter; Bokkapittel, 2008-12-03)We present some old and recent results on rank problems and linearizability of geodesic planar webs -
On structure of linear differential operators, acting on line bundles
(Journal article; Tidsskriftartikkel; Peer reviewed, 2019-11-14)We study differential invariants of linear differential operators and use them to find conditions for equivalence of differential operators acting on line bundles over smooth manifolds with respect to groups of automorphisms. -
On the Blaschke Conjecture for 3-Webs
(Journal article; Tidsskriftartikkel; Peer reviewed, 2004-11-21)We find relative differential invariants of orders eight and nine for a planar nonparallelizable 3-web such that their vanishing is necessary and sufficient for a 3-web to be linearizable. This solves the Blaschke conjecture for 3-webs. As a side result, we show that the number of linearizations in the Gronwall conjecture does not exceed fifteen and give criteria for rigidity of 3-webs. -
On the equivariant Betti numbers of symmetric definable sets: vanishing, bounds and algorithms
(Journal article; Tidsskriftartikkel; Peer reviewed, 2018-03-02)Let R be a real closed field. We prove that for any fixed <i>d</i>, the equivariant rational cohomology groups of closed symmetric semi-algebraic subsets of R<sup><i>k</i></sup> defined by polynomials of degrees bounded by <i>d</i> vanishes in dimensions <i>d</i> and larger. This vanishing result is tight. Using a new geometric approach we also prove an upper bound of [mathematical formula] on the ... -
On the Ewald Oseen scattering formulation for light scattering: stability, singularity and parallelization.
(Journal article; Tidsskriftartikkel; Peer reviewed, 2019-11-15)In this paper we discuss some of the mathematical and numerical issues that have to be addressed when calculating wave scattering using the Ewald Oseen scattering (EOS) formulation which is newly developed for solving linear and nonlinear scattering problems. The discussion is framed in context of light scattering by objects whose optical response can be of a nonlinear and/or inhomogeneous nature. ... -
On the Isotypic Decomposition of Cohomology Modules of Symmetric Semi-algebraic Sets: Polynomial Bounds on Multiplicities
(Journal article; Tidsskriftartikkel; Peer reviewed, 2018-04-30)We consider symmetric (under the action of products of finite symmetric groups) real algebraic varieties and semi-algebraic sets, as well as symmetric complex varieties in affine and projective spaces, defined by polynomials of degrees bounded by a fixed constant <i>d</i>. We prove that if a Specht module, S<sup>λ</sup>, appears with positive multiplicity in the isotypic decomposition of the ... -
On the symmetry algebras of 5-dimensional CR-manifolds
(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-11-13)We show that for a real-analytic connected holomorphically nondegenerate 5-dimensional CR-hypersurface <i>M</i> and its symmetry algebra <i>s</i> one has either: (i) dim <i>s</i> = 15 and <i>M</i> is spherical (with Levi form of signature either (2,0), or (1,1), everywhere), or (ii) dim <i>s</i> ≤ 11 where dim <i>s</i> = 11 can only occur if on a dense open subset <i>M</i> is spherical with Levi ... -
On uniqueness of submaximally symmetric parabolic geometries
(Journal article; Tidsskriftartikkel, 2021)Among (regular, normal) parabolic geometries of type (G,P), there is a locally unique maximally symmetric structure and it has symmetry dimension dim(G). The symmetry gap problem concerns the determination of the next realizable (submaximal) symmetry dimension. When G is a complex or split-real simple Lie group of rank at least three or when (G,P)=(G2,P2), we establish a local classification result ... -
On uniqueness of submaximally symmetric parabolic geometries
(Journal article; Tidsskriftartikkel; Peer reviewed, 2024-01-24)Among (regular, normal) parabolic geometries of type (<i>G,P</i>), there is a locally unique maximally symmetric structure and it has symmetry dimension dim(<i>G</i>). The symmetry gap problem concerns the determination of the next realizable (submaximal) symmetry dimension. When <i>G</i> is a complex or split-real simple Lie group of rank at least three or when (<i>G,P</i>) = (<i>G<sub>2</sub></i>, ... -
Overview of the MOSAiC expedition: Physical oceanography
(Journal article; Tidsskriftartikkel; Peer reviewed, 2022-02-07)Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. ...