• Geodesic Webs and PDE Systems of Euler Equations 

      Goldberg, Vladislav V.; Lychagin, Valentin V. (Journal article; Tidsskriftartikkel; Peer reviewed, 2008-10-30)
      We find necessary and sufficient conditions for the foliation defined by level sets of a function f(x1, ..., xn) to be totally geodesic in a torsion-free connection and apply them to find the conditions for d-webs of hypersurfaces to be geodesic, and in the case of flat connections, for d-webs (d ≥ n + 1) of hypersurfaces to be hyperplanar webs. These conditions are systems of generalized Euler ...
    • On Rank Problems for Planar Webs and Projective Structures 

      Lychagin, Valentin V.; Goldberg, Vladislav V. (Chapter; Bokkapittel, 2008-12-03)
      We present some old and recent results on rank problems and linearizability of geodesic planar webs
    • Feedback Differential Invariants 

      Lychagin, Valentin V. (Journal article; Tidsskriftartikkel, 2008-12-07)
      The problem of feedback equivalence for control systems is considered. An algebra of differential invariants and criteria for the feedback equivalence for regular control systems are found.
    • Feedback Equivalence of 1-dimensional Control Systems of the 1-st Order 

      Lychagin, Valentin V. (Working paper; Arbeidsnotat, 2008-12-09)
      The problem of local feedback equivalence for 1-dimensional control systems of the 1-st order is considered. The algebra of differential invariants and criteria for the feedback equivalence for regular control systems are found.
    • Geodesic Webs of Hypersurfaces 

      Goldberg, Vladislav V.; Lychagin, Valentin V. (Working paper; Arbeidsnotat, 2008-12-11)
      In the present paper we study geometric structures associated with webs of hypersurfaces. We prove that with any geodesic (n + 2)-web on an n-dimensional manifold there is naturally associated a unique projective structure and, provide that one of web foliations is pointed, there is also associated a unique affine structure. The projective structure can be chosen by the claim that the leaves of all ...
    • On a class of linearizable planar geodesic webs 

      Goldberg, Vladislav V.; Lychagin, Valentin V. (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.
    • Finite Larmor radius influence on MHD solitary waves 

      Mjølhus, Einar (Journal article; Tidsskriftartikkel; Peer reviewed, 2009)
    • Involutivity of field equations 

      Kruglikov, Boris (Working paper; Arbeidsnotat, 2009-02-10)
      We prove involutivity of Einstein and Einstein-Maxwell equations by calculating the Spencer cohomology of these systems. Relation with Cartan method is traced in details. Basic implications through Cartan-Kähler theory are derived.
    • DeltaProt : a software toolbox for comparative genomics 

      Thorvaldsen, Steinar; Flå, Tor; Willassen, Nils P (Journal article; Tidsskriftartikkel; Peer reviewed, 2010)
    • Automatic Segmentation of Dermoscopic Images by Iterative Classification 

      Zortea, Maciel; Skrøvseth, Stein Olav; Schopf, Thomas Roger Griesbeck; Kirchesch, Herbert M.; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2011)
      Accurate detection of the borders of skin lesions is a vital first step for computer aided diagnostic systems. This paper presents a novel automatic approach to segmentation of skin lesions that is particularly suitable for analysis of dermoscopic images. Assumptions about the image acquisition, in particular, the approximate location and color, are used to derive an automatic rule to select small ...
    • Thousand years of winter surface air temperature variations in Svalbard and northern Norway reconstructed from ice-core data 

      Divine, Dmitry V; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2011)
      Two isotopic ice core records from western Svalbard are calibrated to reconstruct more than 1000 years of past winter surface air temperature variations in Longyearbyen, Svalbard, and Vardø, northern Norway. Analysis of the derived reconstructions suggests that the climate evolution of the last millennium in these study areas comprises three major sub-periods. The cooling stage in Svalbard (ca. ...
    • Scale Space Methods for Analysis of Type 2 Diabetes Patients' Blood Glucose Values 

      Skrøvseth, Stein Olav; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2011)
      We describe how scale space methods can be used for quantitative analysis of blood glucose concentrations from type 2 diabetes patients. Blood glucose values were recorded voluntarily by the patients over one full year as part of a self-management process, where the timeand frequency of the recordings are decided by the patients. This makes a unique datasetin its extent, though with a large variation ...
    • Causality in Scale Space as an Approach to Change Detection 

      Skrøvseth, Stein Olav; Godtliebsen, Fred; Bellika, Johan Gustav (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      Kernel density estimation and kernel regression are useful ways to visualize and assess the structure of data. Using these techniques we define a temporal scale space as the vector space spanned by bandwidth and a temporal variable. In this space significance regions that reflect a significant derivative in the kernel smooth similar to those of SiZer (Significant Zero-crossings of derivatives) are ...
    • SDiff(2) and uniqueness of the Plebanski equation 

      Kruglikov, Boris; Morozov, Oleg (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      The group of area preserving diffeomorphisms showed importance in the problems of self-dual gravity and integrability theory. We discuss how representations of this infinite-dimensional Lie group can arise in mathematical physics from pure local considerations. Then using Lie algebra extensions and cohomology we derive the second Plebański equation and its geometry. We do not use Kähler or other ...
    • Wei-type duality theorems for matroids 

      Britz, Thomas; Johnsen, Trygve; Mayhew, Dillon; Shiromoto, Keisuke (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      We present several fundamental duality theorems for matroids and more general combinatorial structures. As a special case, these results show that the maximal cardinalities of fixed-ranked sets of a matroid determine the corresponding maximal cardinalities of the dual matroid. Our main results are applied to perfect matroid designs, graphs, transversals, and linear codes over division rings, in each ...
    • A toolbox for fitting complex spatial point process models using integrated nested Laplace approximation (INLA) 

      Illian, Janine; Sørbye, Sigrunn Holbek; Rue, Håvard (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      This paper develops methodology that provides a toolbox for routinely fitting complex models to realistic spatial point pattern data. We consider models that are based on log-Gaussian Cox processes and include local interaction in these by considering constructed covariates. This enables us to use integrated nested Laplace approximation and to considerably speed up the inferential task. In addition, ...
    • A Two-Component Generalization of the Integrable rdDym Equation 

      Morozov, Oleg (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      We find a two-component generalization of the integrable case of rdDym equation. The reductions of this system include the general rdDym equation, the Boyer-Finley equation, and the deformed Boyer-Finley equation. Also we find a Bäcklund transformation between our generalization and Bodganov's two-component generalization of the universal hierarchy equatio
    • Automatic nematode detection in cod fillets (Gadus morhua L.) by hyperspectral imaging 

      Sivertsen, Agnar Holten; Heia, Karsten; Hindberg, Kristian; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      Detection of objects embedded in tissue, using visible light, is difficult due to light scattering. The optical properties of the surrounding tissue will influence the spectral characteristics of the light interacting with the object, and the spectral signature observed from the object will be directly affected. A method for calibrating the spectral signature of small objects, embedded in translucent ...
    • Bayesian multiscale analysis of images modeled as Gaussian Markov random fields 

      Thon, Kevin Otto; Rue, Håvard; Skrøvseth, Stein Olav; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      A Bayesian multiscale technique for the detection of statistically significant features in noisy images is proposed. The prior is defined as a stationary intrinsic Gaussian Markov random field on a toroidal graph, which enables efficient computation of the relevant posterior marginals. Hence the method is applicable to large images produced by modern digital cameras. The technique is demonstrated ...
    • Pairwise scale space comparison of time series with application to climate research 

      Godtliebsen, Fred; Holmström, L.; Miettinen, A.; Erästö, P.; Divine, Dmitry V; Koc, Nalan (Journal article; Tidsskriftartikkel; Peer reviewed, 2012)
      In this paper, we study how sea surface temperature variations in the North Atlantic and the Norwegian Sea are correlated with the climate in the Northern Hemisphere in late Holocene. The analysis is performed by testing statistical hypotheses through novel scale space methodologies. In late Holocene, the proposed techniques reveal that the climate development in the subpolar North Atlantic has been ...