Recent additions

  • Greedy weights for matroids 

    Johnsen, Trygve; Verdure, Hugues (Journal article; Tidsskriftartikkel; Peer reviewed, 2020-12-19)
    We introduce greedy weights of matroids, inspired by those for linear codes. We show that a Wei duality holds for two of these types of greedy weights for matroids. Moreover we show that in the cases where the matroids involved are associated to linear codes, our definitions coincide with those for codes. Thus our Wei duality is a generalization of that for linear codes given by Schaathun. In ...
  • Overview of the MOSAiC expedition: Physical oceanography 

    Rabe, Benjamin; Baumann, Till Martin; Divine, Dmitry; Fer, Ilker; Gradinger, Rolf Rudolf; Granskog, Mats; Koenig, Zoe Charlotte; Muilwijk, Morven; Sundfjord, Arild; Heuze´, Céline; Regnery, Julia; Aksenov, Yevgeny; Allerholt, Jacob; Athanase, Marylou; Bai, Youcheng; Basque, Chris; Bauch, Dorothea; Chen, Dake; Cole, Sylvia T.; Craw, Lisa; Davies, Andrew; Damm, Ellen; Dethloff, Klaus; Doglioni, Francesca; Ebert, Falk; Fang, Ying-Chih; Fong, Allison A.; Graupner, Rainer; Haas, Christian; He, Hailun; He, Yan; Hoppmann, Mario; Janout, Markus; Kadko, David; Kanzow, Torsten; Karam, Salar; Kawaguchi, Yusuke; Kong, Bin; Krishfield, Richard A.; Krumpen, Thomas; Kuhlmey, David; Kuznetsov, Ivan; Lan, Musheng; Laukert, Georgi; Lei, Ruibo; Li, Lao; Torres-Valdés, Sinhué; Lin, Lina; Lin, Long; Liu, Hailong; Liu, Na; Loose, Brice; Ma, Xiaobing; McKay, Rosalie; Mallet, Maria; Mallet, Robbie D .C.; Maslowski, Wieslaw; Mertens, Christian; Mohrholz, Volker; Nicolaus, Marcel; O'Brien, Jeffrey K.; Perovich, Donald; Ren, Jian; Rex, Markus; Ribeiro, Natalia; Rinke, Annette; Sokolov, Vladimir; Sommerfeld, Anja; Spreen, Gunnar; Stanton, Timothy; Stephens, Mark; Su, Jie; Sukhikh, Natalia; Thomisch, Karolin; Tippenhauer, Sandra; Toole, John M.; Vredenborg, Myriel; Walter, Maren; Wang, Hangzhou; Wang, Lei; Wang, Yuntao; Wendisch, Manfred; Zhao, Jinping; Zhou, Meng; Zhu, Jialiang (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. ...
  • 25 years of self-organized criticality: concepts and controversies 

    Watkins, Nicholas W.; Pruessner, Gunnar; Chapman, Sandra; Crosby, Norma B; Jensen, Henrik J (Journal article; Tidsskriftartikkel; Peer reviewed, 2015-05-28)
    Introduced by the late Per Bak and his colleagues, self-organized criticality (SOC) has been one of the most stimulating concepts to come out of statistical mechanics and condensed matter theory in the last few decades, and has played a significant role in the development of complexity science. SOC, and more generally fractals and power laws, have attracted much comment, ranging from the very ...
  • Classification of simply-transitive Levi non-degenerate hypersurfaces in C^3 

    Doubrov, Boris; Merker, Joël; The, Dennis (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-06-24)
    Holomorphically homogeneous Cauchy–Riemann (CR) real hypersurfaces <i>M<i/><sup>3</sup>⊂C<sup>2</sup> were classified by Élie Cartan in 1932. In the next dimension, we complete the classification of simply-transitive Levi non-degenerate hypersurfaces <i>M<i/><sup>5</sup>⊂C<sup>3</sup> using a novel Lie algebraic approach independent of any earlier classifications of abstract Lie algebras. Central ...
  • Constraining two climate field reconstruction methodologies over the North Atlantic realm using pseudo-proxy experiments 

    Nilsen, Tine; Talento, Stefanie; Werner, Johannes P (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-07-06)
    This study presents pseudo-proxy experiments to quantify the reconstruction skill of two climate field reconstruction methodologies for a marine proxy network subject to age uncertainties. The BARCAST methodology (Bayesian Algorithm for Reconstructing Climate Anomalies in Space and Time) is tested for sea surface temperature (SST) reconstruction for the first time over the northern North Atlantic ...
  • Symmetry classification of viscid flows on space curves 

    Duyunova, Anna; Lychagin, Valentin; Tychkov, Sergey (Journal article; Tidsskriftartikkel; Peer reviewed, 2020-11-01)
    Symmetries and differential invariants of viscid flows with viscosity depending on temperature on a space curve are given. Their dependence on thermodynamic states of media is studied, and a classification of thermodynamic states is given.
  • Continuum mechanics of media with inner structures 

    Duyunova, Anna; Lychagin, Valentin; Tychkov, Sergey (Journal article; Tidsskriftartikkel; Peer reviewed, 2020-11-05)
    We propose a geometrical approach to the mechanics of continuous media equipped with inner structures and give the basic (Navier–Stokes, mass conservation and energy conservation) equations of their motion.
  • Exact semidefinite programming bounds for packing problems 

    Dostert, Maria; de Laat, David; Moustrou, Philippe (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-05-25)
    In this paper we give an algorithm to round the floating point output of a semidefinite programming solver to a solution over the rationals or a quadratic extension of the rationals. This algorithm does not require the solution to be strictly feasible and works for large problems. We apply this to get sharp bounds for packing problems, and we use these sharp bounds to prove that certain optimal ...
  • 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 ...
  • Symmetries and Differential Invariants for Inviscid Flows on a Curve 

    Duyunova, Anna; Lychagin, Valentin; Tychkov, Sergey (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-02-04)
    Symmetries and the corresponding fields of differential invariants of the inviscid flows on a curve are given. Their dependence on thermodynamic states of media is studied, and a classification of thermodynamic states is given.
  • Families of Gröbner Degenerations, Grassmannians and Universal Cluster Algebras 

    Bossinger, Lara; Mohammadi, Fatemeh; Nájera Chávez, Alfredo (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-06-10)
    Let V be the weighted projective variety defined by a weighted homogeneous ideal J and C a maximal cone in the Gröbner fan of J with m rays. We construct a flat family over A<sup>m</sup> that assembles the Gröbner degenerations of V associated with all faces of C. This is a multi-parameter generalization of the classical one-parameter Gröbner degeneration associated to a weight. We explain how our ...
  • Cosheaves 

    Prasolov, Andrei (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-10-21)
    The categories pCS(X,Pro(k)) of precosheaves and CS(X,Pro(k)) of cosheaves on a small Grothendieck site X, with values in the category Pro(k) of prok-modules, are constructed. It is proved that pCS(X,Pro(k)) satisfies the AB4 and AB5* axioms, while CS(X,Pro(k)) satisfies AB3 and AB5*. Homology theories for cosheaves and precosheaves, based on quasi-projective resolutions, are constructed and investigated.
  • Explicit values for Ramanujan's theta function φ(q) 

    Berndt, Bruce C.; Rebák, Örs (Journal article; Tidsskriftartikkel; Peer reviewed, 2022-01-09)
    This paper provides a survey of particular values of Ramanujan's theta function φ(q) = ∑ q^(n^2), n = −∞ ... ∞, when q = e^(−π√n), where n is a positive rational number. First, descriptions of the tools used to evaluate theta functions are given. Second, classical values are briefly discussed. Third, certain values due to Ramanujan and later authors are given. Fourth, the methods that are used to ...
  • Coloring the Voronoi tessellation of lattices 

    Dutour Sikirić, Mathieu; Madore, David A.; Moustrou, Philippe; Vallentin, Frank (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-05-03)
    In this paper we define the chromatic number of a lattice: It is the least number of colors one needs to color the interiors of the cells of the Voronoi tessellation of a lattice so that no two cells sharing a facet are of the same color. We compute the chromatic number of the root lattices, their duals, and of the Leech lattice, we consider the chromatic number of lattices of Voronoi’s first ...
  • Finite-sample properties of estimators for first and second order autoregressive processes 

    Sørbye, Sigrunn Holbek; Nicolau, Pedro Guilherme; Rue, Håvard (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-12-05)
    The class of autoregressive (AR) processes is extensively used to model temporal dependence in observed time series. Such models are easily available and routinely fitted using freely available statistical software like R. A potential problem is that commonly applied estimators for the coefficients of AR processes are severely biased when the time series are short. This paper studies the ...
  • On uniqueness of submaximally symmetric parabolic geometries 

    The, Dennis (Journal article; Tidsskriftartikkel; Preprint; Manuskript, 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 ...
  • Symmetry gaps for higher order ordinary differential equations 

    The, Dennis; Kessy, Johnson Allen (Journal article; Tidsskriftartikkel; Preprint; Manuskript, 2021)
    The maximal contact symmetry dimensions for scalar ODEs of order ≥4 and vector ODEs of order ≥3 are well known. Using a Cartan-geometric approach, we determine for these ODE the next largest realizable (submaximal) symmetry dimension. Moreover, finer curvature-constrained submaximal symmetry dimensions are also classified.
  • Second-Order PDEs in 3D with Einstein–Weyl Conformal Structure 

    Berjawi, S.; Ferapontov, E.V.; Kruglikov, Boris; Novikov, V.S. (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-12-07)
    Einstein–Weyl geometry is a triple (D,g,ω) where D is a symmetric connection, [g] is a conformal structure and ω is a covector such that ∙ connection D preserves the conformal class [g], that is, Dg=ωg; ∙ trace-free part of the symmetrised Ricci tensor of D vanishes. Three-dimensional Einstein–Weyl structures naturally arise on solutions of second-order dispersionless integrable PDEs in 3D. In this ...
  • Differential invariants of Kundt spacetimes 

    Kruglikov, Boris; Schneider, Eivind (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-09-07)
    We find generators for the algebra of rational differential invariants for general and degenerate Kundt spacetimes and relate this to other approaches to the equivalence problem for Lorentzian metrics. Special attention is given to dimensions three and four.
  • Spatiotemporal Analysis of COVID-19 Incidence Data 

    Spassiani, Ilaria; Sebastiani, Giovanni; Palu, Giorgio (Journal article; Tidsskriftartikkel; Peer reviewed, 2021-03-11)
    High-density lipoproteins (HDL) are a heterogenous group of plasma molecules with a large variety in composition. There is a wide specter in lipid content and the number of different proteins that has been associated with HDL is approaching 100. Given this heterogeneity and the fact that the total amount of HDL is inversely related to the risk of coronary heart disease (CHD), there has been ...

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