Now showing items 181-200 of 316

    • The gap phenomenon in parabolic geometries 

      Kruglikov, Boris; The, Dennis (Journal article; Tidsskriftartikkel; Peer reviewed, 2014-09-14)
      The infinitesimal symmetry algebra of any Cartan geometry has maximum dimension realized by the flat model, but often this dimension drops significantly when considering non-flat geometries, so a gap phenomenon arises. For general (regular, normal) parabolic geometries of type (G,P), we use Tanaka theory to derive a universal upper bound on the submaximal symmetry dimension. We use Kostant’s version ...
    • Empirical Growth Models for the Renewable Energy Sector 

      Rypdal, Kristoffer (Journal article; Tidsskriftartikkel; Peer reviewed, 2018-07-25)
      Three simple, empirical models for growth of power consumption in the renewable energy sector are compared. These are the exponential, logistic, and power-law models. The exponential model describes growth at a fixed relative growth rate, the logistic model saturates at a fixed limit, while the power-law model describes slowing, but unlimited, growth. The model parameters are determined by regression ...
    • Finite dimensional dynamics for nonlinear filtration equation 

      Akhmetzyanov, Atlas V.; Kushner, Alexei G.; Lychagin, Valentin (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-09-01)
      We construct new finite dimensional submanifolds in the solution space of nonlinear differential filtration equations and describe the corresponding evolutionary dynamics. This method is implemented in a computer program of symbolic computations Maple.
    • You Just Keep on Pushing My Love over the Borderline: A Rejoinder 

      Simpson, Daniel; Rue, Håvard; Riebler, Andrea Ingeborg; Martins, Thiago Guerrera; Sørbye, Sigrunn Holbek (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-06)
      <i>INTRODUCTION</i>: The point of departure for our paper is that most modern statistical models are built to be flexible enough to model diverse data generating mechanisms. Good statistical practice requires us to limit this flexibility, which is typically controlled by a small number of parameters, to the amount “needed” to model the data at hand. The Bayesian framework provides a natural method ...
    • On a class of integrable systems of Monge-Ampère type 

      Doubrov, Boris; Ferapontov, Eugene V.; Kruglikov, Boris; Novikov, Vladimir S. (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 ...
    • Penalising Model Component Complexity: A Principled, Practical Approach to Constructing Priors 

      Simpson, Daniel; Rue, Håvard; Riebler, Andrea Ingeborg; Martins, Thiago Guerrera; Sørbye, Sigrunn Holbek (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-06)
      In this paper, we introduce a new concept for constructing prior distributions. We exploit the natural nested structure inherent to many model components, which defines the model component to be a flexible extension of a base model. Proper priors are defined to penalise the complexity induced by deviating from the simpler base model and are formulated after the input of a user-defined scaling parameter ...
    • Extreme-value statistics from Lagrangian convex hull analysis for homogeneous turbulent Boussinesq convection and MHD convection 

      Pratt, J; Busse, A; Muller, WC; Watkins, NW; Chapman, Sandra (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-20)
      We investigate the utility of the convex hull of many Lagrangian tracers to analyze transport properties of turbulent flows with different anisotropy. In direct numerical simulations of statistically homogeneous and stationary Navier–Stokes turbulence, neutral fluid Boussinesq convection, and MHD Boussinesq convection a comparison with Lagrangian pair dispersion shows that convex hull statistics ...
    • 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 ...
    • Submaximally symmetric almost quaternionic structures 

      Kruglikov, Boris; Winther, Henrik; Zalabová, Lenka (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-11-10)
      The symmetry dimension of a geometric structure is the dimension of its symmetry algebra. We investigate symmetries of almost quaternionic structures of quaternionic dimension <i>n</i>. The maximal possible symmetry is realized by the quaternionic projective space H<i>P<sup> n</sup></i>, which is flat and has the symmetry algebra sl(<i>n</i> + 1, H) of dimension 4<i>n</i><sup> 2</sup> + ...
    • Penalised Complexity Priors for Stationary Autoregressive Processes 

      Sørbye, Sigrunn Holbek; Rue, Håvard (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-05-23)
      The autoregressive (AR) process of order p(AR(p)) is a central model in time series analysis. A Bayesian approach requires the user to define a prior distribution for the coefficients of the AR(p) model. Although it is easy to write down some prior, it is not at all obvious how to understand and interpret the prior distribution, to ensure that it behaves according to the users' prior knowledge. In ...
    • Jet-determination of symmetries of parabolic geometries 

      Kruglikov, Boris; The, Dennis (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-04-24)
      We establish 2-jet determinacy for the symmetry algebra of the underlying structure of any (complex or real) parabolic geometry. At non-flat points, we prove that the symmetry algebra is in fact 1-jet determined. Moreover, we prove 1-jet determinacy at any point for a variety of non-flat parabolic geometries—in particular torsion-free, parabolic contact, and several other classes.
    • Fractional Gaussian noise: Prior specification and model comparison 

      Sørbye, Sigrunn Holbek; Rue, Håvard (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-07-07)
      Fractional Gaussian noise (fGn) is a stationary stochastic process used to model anti-persistent or persistent dependency structures in observed time series. Properties of the autocovariance function of fGn are characterised by the Hurst exponent (<i>H)</i>, which in Bayesian contexts typically has been assigned a uniform prior on the unit interval. This paper argues why a uniform prior is ...
    • Non-degenerate para-complex structures in 6D with large symmetry groups 

      Kruglikov, Boris; Winther, Henrik (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-05-20)
      For an almost product structure J on a manifold M of dimension 6 with non-degenerate Nijenhuis tensor N J, we show that the automorphism group G=Aut(M,J) has dimension at most 14. In the case of equality G is the exceptional Lie group G∗2. The next possible symmetry dimension is proved to be equal to 10, and G has Lie algebra sp(4,R). Both maximal and submaximal symmetric structures are globally ...
    • Conformal differential invariants 

      Kruglikov, Boris (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-27)
      We compute the Hilbert polynomial and the Poincar´e function counting the number of fixed jet-order differential invariants of conformal metric structures modulo local diffeomorphisms, and we describe the field of rational differential invariants separating generic orbits of the diffeomorphism pseudogroup action. This resolves the local recognition problem for conformal structures.
    • Erratum to: Almost complex structures in 6D with nondegenerate Nijenhuis tensors and large symmetry groups 

      Kruglikov, Boris; Winther, Henrik (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-02-23)
      We correct an error in the second part of Theorem 3 of our original paper.
    • Statistical significance of rising and oscillatory trends in global ocean and land temperature in the past 160 years 

      Østvand, Lene; Rypdal, Kristoffer; Rypdal, Martin Wibe (Journal article; Tidsskriftartikkel, 2014-03-20)
      Various interpretations of the notion of a trend in the context of global warming are discussed, contrasting the difference between viewing a trend as the deterministic response to an external forcing and viewing it as a slow variation which can be separated from the background spectral continuum of long-range persistent climate noise. The emphasis in this paper is on the latter notion, and a general ...
    • Antarctic climate variability on regional and continental scales over the last 2000 years 

      Stenni, Barbara; Curran, Mark A.J.; Abram, Nerilie J.; Orsi, Anais; Goursaud, Sentia; Masson-Delmotte, Valerie; Neukom, Raphael; Goosse, Hugues; Divine, Dmitry V; Van Ommen, Tas; Steig, Eric J.; Dixon, Daniel A.; Thomas, Elizabeth R.; Bertler, Nancy A.N.; Isaksson, Elisabeth; Ekaykin, Alexey; Werner, Martin; Frezzotti, Massimo (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-11-17)
      Climate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. ...
    • Generalized Hamming Weights for Almost Affine Codes 

      Johnsen, Trygve; Verdure, Hugues (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-01-17)
      We define generalized Hamming weights for almost affine codes. We show that this definition is natural since we can extend some well known properties of t he generalized Hamming weights for linear codes, to almost affine codes. In addition we discus s duality of almost affine codes, and of the smaller class of multilinear codes.
    • Relative generalized hamming weights and extended weight polynomials of almost affine codes 

      Johnsen, Trygve; Verdure, Hugues (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-08-23)
      This paper is devoted to giving a generalization from linear codes to the larger class of almost affine codes of two different results. One such result is how one can express the relative generalized Hamming weights of a pair of codes in terms of intersection properties between the smallest of these codes and subcodes of the largest code. The other result tells how one can find the extended ...
    • A global multiproxy database for temperature reconstructions of the Common Era 

      Emile-Geay, Julien; McKay, Nicholas P; Kaufman, Darrell S; Von Gunten, Lucien; Wang, Jianghao; Anchukaitis, Kevin J; Abram, Nerilie J; Addison, Jason A; Curran, Mark AJ; Evans, Michael N; Henley, Benjamin J; Hao, Zhixin; Martrat, Belen; McGregor, Helen V; Neukom, Raphael; Pederson, Gregory T; Stenni, Barbara; Thirumalai, Kaustubh; Werner, Johannes P; Xu, Chenxi; Divine, Dmitry V; Dixon, Bronwyn C; Gergis, Joelle; Mundo, Ignacio A; Nakatsuka, Takeshi; Phipps, Steven J; Routson, Cody C; Steig, Eric J; Tierney, Jessica E; Tyler, Jonathan J; Allen, Kathryn J; Bertler, Nancy AN; Bjorklund, Jesper; Chase, Brian M; Chen, Min-Te; Cook, Ed; De Jong, Rixt; DeLong, Kristine L; Dixon, Daniel A; Ekaykin, Alexey A; Ersek, Vasile; Filipsson, Helena L; Francus, Pierre; Freund, Mandy B; Frezzotti, Massimo; Gaire, Narayan P; Gajewski, Konrad; Ge, Quansheng; Goosse, Hugues; Gornostaeva, Anastasia; Grosjean, Martin; Horiuchi, Kazuho; Hormes, Anne; Husum, Katrine; Isaksson, Elisabeth; Kandasamy, Selvaraj; Kawamura, Kenji; Kilbourne, K Halimeda; Koc, Nalan; Leduc, Guillaume; Linderholm, Hans W; Lorrey, Andrew M; Mikhalenko, Vladimir; Mortyn, P Graham; Motoyama, Hideaki; Moy, Andrew D; Mulvaney, Robert; Munz, Philipp M; Nash, David J; Oerter, Hans; Opel, Thomas; Orsi, Anais J; Ovchinnikov, Dmitriy V; Porter, Trevor J; Roop, Heidi A; Saenger, Casey; Sano, Masaki; Sauchyn, David; Saunders, Krystyna M; Seidenkrantz, Marit-Solveig; Severi, Mirko; Shao, Xuemei; Sicre, Marie-Alexandrine; Sigl, Michael; Sinclair, Kate; St George, Scott; St Jacques, Jeannine-Marie; Thamban, Meloth; Thapa, Udya Kuwar; Thomas, Elizabeth R; Turney, Chris; Uemura, Ryu; Viau, Andre E; Vladimirova, Diana O; Wahl, Eugene R; White, James WC; Yu, Zicheng; Zinke, Jens (Journal article; Tidsskriftartikkel; Peer reviewed, 2017-07-11)
      Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records ...