Viser treff 162-181 av 314

    • Laplace transformation of Lie class ω = 1 overdetermined systems 

      Kruglikov, Boris (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-01-20)
      In this paper, we investigate overdetermined systems of scalar PDEs on the plane with one common characteristic, whose general solution depends on one function of one variable. We describe linearization of such systems and their integration via Laplace transformation, relating this to Lie's integration theorem and formal theory of PDEs.
    • Late Quaternary temperature variability described as abrupt transitions on a 1/f noise background 

      Rypdal, Kristoffer; Rypdal, Martin Wibe (Journal article; Tidsskriftartikkel; Peer reviewed, 2016-03-31)
      In order to have a scaling description of the climate system that is not inherently non-stationary, the rapid shifts between stadials and interstadials during the last glaciation (the Dansgaard-Oeschger events) cannot be included in the scaling law. The same is true for the shifts between the glacial and interglacial states in the Quaternary climate. When these events are omitted from a scaling ...
    • Late quaternary temperature variability described as abrupt transitions on a 1/f noise background 

      Rypdal, Martin Wibe; Rypdal, Kristoffer (Journal article; Tidsskriftartikkel, 2015-11-09)
      We show that in order to have a scaling description of the climate system that is not inherently non-stationary, the rapid shifts between stadial and interstadial conditions during the last glaciation cannot be included in the scaling law. The same is true for the 5 shifts between the glacial and interglacial states in the quaternary climate. When these events are omitted from a scaling analysis ...
    • Lattice conditional independence models and Hibi ideals 

      Caines, Peter; Mohammadi, Fatemeh; Sáenz-de-Cabezón, Eduardo; Wynn, Henry (Journal article; Tidsskriftartikkel; Peer reviewed, 2022-06-10)
      Lattice conditional independence models [Andersson and Perlman, Lattice models for conditional independence in a multivariate normal distribution, Ann. Statist. 21 (1993), 1318–1358] are a class of models developed first for the Gaussian case in which a distributive lattice classifies all the conditional independence statements. The main result is that these models can equivalently be described via ...
    • Leakage from medically-certified to self-certified workplace absence among norwegian employees 

      Karlsen, Geir Runar; Ytterstad, Elinor (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-12-05)
      There are two different kinds of sickness absence in Norway: self-certified absence (SCA), and medically-certified absence (MCA). In this study of 6437 Norwegian employees, we applied logistic regression models for a dichotomous SCA variable on initially 38 independent vari- ables including age and gender. Our findings showed that employees reporting long-term health issues but no record of MCA ...
    • Learning similarities between irregularly sampled short multivariate time series from EHRs 

      Mikalsen, Karl Øyvind; Bianchi, Filippo Maria; Soguero-Ruiz, Cristina; Skrøvseth, Stein Olav; Lindsetmo, Rolv-Ole; Revhaug, Arthur; Jenssen, Robert (Conference object; Konferansebidrag, 2016-12-04)
      A large fraction of the Electronic Health Records consists of clinical multivariate time series. Building models for extracting information from these is important for improving the understanding of diseases, patient care and treatment. Such time series are oftentimes particularly challenging since they are characterized by multiple, possibly dependent variables, length variability and irregular ...
    • The Life and Death of the Recent Global Warming Hiatus Parsimoniously Explained 

      Rypdal, Kristoffer (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-07-21)
      The main features of the instrumental global mean surface temperature (GMST) are reasonably well described by a simple linear response model driven by anthropogenic, volcanic and solar forcing. This model acts as a linear long-memory filter of the forcing signal. The physical interpretation of this filtering is the delayed response due to the thermal inertia of the ocean. This description is ...
    • Lightboard – a new teaching tool at the Faculty of Science and Technology at UiT 

      Malyutina, Elena; Gustavsson, Björn Johan; Vierinen, Juha; Tveito, Torbjørn (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-05-05)
      We would like to present a new tool that was built by three lecturers at UiT last semester – Lightboard. This tool was used before in other countries and other universities, but never at UiT. The COVID-19 pandemic situation motivated the lecturers to find a way to do online lectures differently. Blackboard and chalk work well for natural sciences as long as the lecture is physical and the teacher ...
    • Limits to the quantification of local climate change 

      Chapman, Sandra; Stainforth, David A.; Watkins, Nicholas W. (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-09-16)
      Abstract Wedemonstrate how the fundamental timescales of anthropogenic climate change limit the identification of societally relevant aspects of changes in precipitation.Weshow that it is nevertheless possible to extract, solely from observations, some confident quantified assessments of change at certain thresholds and locations. Maps of such changes, for a variety of hydrologically-relevant, ...
    • Linear codes associated to symmetric determinantal varieties: Even rank case 

      Beelen, Peter; Johnsen, Trygve; Singh, Prasant (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-06-20)
      We consider linear codes over a finite field Fq, for odd q, derived from determinantal varieties, obtained from symmetric matrices of bounded ranks. A formula for the weight of a codeword is derived. Using this formula, we have computed the minimum distance for the codes corresponding to matrices upperbounded by any fixed, even rank. A conjecture is proposed for the cases where the upper bound is ...
    • Linear scaling Coulomb interaction in the multiwavelet basis, a parallel implementation 

      Jensen, Stig Rune; Juselius, Jonas; Durdek, Antoine Pacifique Romain; Flå, Tor; Wind, Peter; Frediani, Luca (Journal article; Tidsskriftartikkel; Peer reviewed, 2014-08-27)
      We present a parallel and linear scaling implementation of the calculation of the electrostatic potential arising from an arbitrary charge distribution. Our approach is making use of the multi-resolution basis of multiwavelets. The potential is obtained as the direct solution of the Poisson equation in its Green’s function integral form. In the multiwavelet basis, the formally non local ...
    • Linear slices of Hyperbolic polynomials and positivity of symmetric polynomial functions 

      Riener, Cordian Benedikt; Schabert, Robin Leonid (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-10-17)
      A real univariate polynomial of degree n is called hyperbolic if all of its n roots are on the real line. Such polynomials appear quite naturally in different applications, for example, in combinatorics and optimization. The focus of this article is on families of hyperbolic polynomials which are determined through k linear conditions on the coefficients. The coefficients corresponding to such a ...
    • Linearizability of d-webs, d ≥ 4, on two-dimensional manifolds 

      Goldberg, Vladislav V.; Lychagin, Valentin V.; Akivis, Maks A. (Journal article; Tidsskriftartikkel; Peer reviewed, 2004-03-31)
      We find d − 2 relative differential invariants for a d-web, d ≥ 4, on a two-dimensional manifold and prove that their vanishing is necessary and sufficient for a d-web to be linearizable. If one writes the above invariants in terms of web functions f(x, y) and g4(x, y), ..., gd(x, y), then necessary and sufficient conditions for the linearizabilty of a d-web are two PDEs of the fourth order ...
    • Long-memory effects in linear response models of Earth's temperature and implications for future global warming 

      Rypdal, Martin Wibe; Rypdal, Kristoffer (Journal article; Tidsskriftartikkel; Peer reviewed, 2014-07-15)
      A linearized energy-balance model for global temperature is formulated, featuring a scale-invariant longrange memory (LRM) response and stochastic forcing representing the influence on the ocean heat reservoir from atmospheric weather systems. The model is parameterized by an effective response strength, the stochastic forcing strength, and the memory exponent. The instrumental global surface ...
    • Long-range memory in Earth's surface temperature on time scales from months to centuries 

      Rypdal, Kristoffer; Østvand, Lene; Rypdal, Martin wibe (Journal article; Tidsskriftartikkel; Peer reviewed, 2013)
      The paper explores the hypothesis that the temporal global temperature response can be modeled as a long-range memory (LRM) stochastic process characterized by a Hurst exponent 0.5 < H≲1.0 on time scales from months to decades. The LRM is a mathematical representation of the multitude of response times associated with the various subsystems. By analysis of instrumental and reconstructed temperature ...
    • Long-Range Memory in Millennium-Long ESM and AOGCM Experiments 

      Nilsen, Tine (Others; Andre, 2014)
      Consider the Earth’s global mean surface temperature time series (GMST) as a realization of a stochastic process. Based on a number of studies, a long-range memory (LRM) stochastic process seems to describe the GMST better than a shortrange memory model, such as the AR(1)-process. We want to study the persistence in climate model simulations, to find out if simulated temperature data exhibit the ...
    • Long-range persistence in global surface temperatures explained by linear multibox energy balance models 

      Fredriksen, Hege-Beate; Rypdal, Martin Wibe (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-09-15)
      The temporal fluctuations in global mean surface temperature are an example of a geophysical quantity that can be described using the notions of long-range persistence and scale invariance/scaling, but this description has suffered from lack of a generally accepted physical explanation. Processes with these statistical signatures can arise from nonlinear effects, for instance, through cascade-like ...
    • Machine Learning in Chronic Pain Research: A Scoping Review 

      Jenssen, Marit Dagny Kristine; Bakkevoll, Per Atle; Ngo, Phuong; Budrionis, Andrius; Fagerlund, Asbjørn Johansen; Tayefi, Maryam; Bellika, Johan Gustav; Godtliebsen, Fred (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-04-02)
      Given the high prevalence and associated cost of chronic pain, it has a significant impact on individuals and society. Improvements in the treatment and management of chronic pain may increase patients’ quality of life and reduce societal costs. In this paper, we evaluate state-of-the-art machine learning approaches in chronic pain research. A literature search was conducted using the PubMed, IEEE ...
    • Mapping climate change in European temperature distributions 

      Stainforth, David A.; Chapman, Sandra; Watkins, Nicholas W. (Journal article; Tidsskriftartikkel; Peer reviewed, 2013)
      Climate change poses challenges for decision makers across society, not just in preparing for the climate of the future but even when planning for the climate of the present day. When making climate sensitive decisions, policy makers and adaptation planners would benefit from information on local scales and for user-specific quantiles (e.g. the hottest/coldest 5% of days) and thresholds (e.g. ...
    • Mapping the shape and dimension of three-dimensional Lagrangian coherent structures and invariant manifolds 

      Aksamit, Nikolas Olson (Journal article; Tidsskriftartikkel; Peer reviewed, 2023-03-10)
      We introduce maps of Cauchy–Green strain tensor eigenvalues to barycentric coordinates to quantify and visualize the full geometry of three-dimensional deformation in stationary and non-stationary fluid flows. As a natural extension of Lagrangian coherent structure diagnostics, which provide separate scalar fields and a one-dimensional quantification of fluid deformation, our barycentric mapping ...