Ara es mostren els items 1-19 de 19

  • An explicit construction for neighborly centrally symmetric polytopes 

    Pfeifle, Julián (2006-06-29)
    Article
    Accés obert
    We give an explicit construction, based on Hadamard matrices, for an infinite series of $\big\lfloor\frac12\sqrt{d}\big\rfloor$-neighborly centrally symmetric $d$-dimensional polytopes with $4d$~vertices. This ...
  • Basis expansions and roots of Ehrhart polynomials 

    Pfeifle, Julián; Beck, Matthias; Loera, Jesús de; Develin, Mike; Richard, Stanley (2004)
    Article
    Accés obert
  • Bounding the volume of facet-empty lattice tetrahedra 

    Pfeifle, Julián; Duong, Han; Haase, Christian; Reznick, Bruce (2005)
    Article
    Accés obert
  • Búsqueda de comunidades en grafos grandes mediante configuraciones implícitas de vectores 

    Muntés Mulero, Víctor; Padrol Sureda, Arnau; Perarnau Llobet, Guillem; Pfeifle, Julián (PUBL. UNIVERSIDAD DE ZARAGOZA, 2009)
    Text en actes de congrés
    Accés obert
    Presentamos el algoritmo OCA para buscar comunidades solapadas en grafos grandes, como por ejemplo la Wikipedia con 1,6×107 nodos y 1,8×108 aristas. OCA se basa en la búsqueda iterativa de subconjuntos localmente óptimos ...
  • Complejos de homomorfismos y disecciones de polígonos 

    Pfeifle, Julián (Secretariado de Publicaciones. Universidad de Valladolid., 2006)
    Comunicació de congrés
    Accés obert
    Encontramos realizaciones canónicas de los complejos politopales Hom(G,H)formados por homomorfismos de grafos y estudiados por Babson y Kozlov. Si G es un grafo completo, caracterizamos en qué casos cierta proyección ...
  • Continuous symmetry and shape measures 

    Perarnau Llobet, Guillem; Pfeifle, Julián; Saludes Closa, Jordi (Centre de Recerca Matemàtica, 2010-06)
    Capítol de llibre
    Accés obert
  • Dissections, Hom-complexes and the Cayley trick 

    Pfeifle, Julián (2006-06-27)
    Article
    Accés obert
    We show that certain canonical realizations of the complexes $\Hom(G,H)$ and $\Hom_+(G,H)$ of (partial) graph homomorphisms studied by Babson and Kozlov are in fact instances of the polyhedral Cayley trick. For $G$~a ...
  • Exploiting symmetry on the Universal Polytope 

    Pfeifle, Julián (2012)
    Text en actes de congrés
    Accés restringit per política de l'editorial
    The most successful method to date for finding lower bounds on the number of simplices needed to triangulate a given polytope P involves optimizing a linear functional over the associated Universal Polytope U(P). However, ...
  • Gale duality bounds on the location of roots of polynomials with nonnegative coefficients 

    Pfeifle, Julián (2010-04)
    Article
    Accés obert
    We bound the location of roots of polynomials that have nonnegative coefficients with respect to a fixed but arbitrary basis of the vector space of polynomials of degree at most d. For this, we interpret the basis polynomials ...
  • Graph operations and Laplacian eigenpolytopes 

    Padrol Sureda, Arnau; Pfeifle, Julián (2010-07)
    Text en actes de congrés
    Accés obert
    We introduce the Laplacian eigenpolytopes ("L-polytopes") associated to a simple undirected graph G, investigate how they change under basic operations such as taking the union, join, complement, line graph and cartesian ...
  • Kalai's squeezed 3-spheres are polytopal 

    Pfeifle, Julián (2002)
    Article
    Accés obert
    In 1988, Kalai [5] extended a construction of Billera and Lee to produce many triangulated(d−1)-spheres. In fact, in view of upper bounds on the number of simplicial d-polytopes by Goodman and Pollack [2, 3], he derived ...
  • Kalai's squeezed three-spheres are polytopal 

    Pfeifle, Julián (2001)
    Article
    Accés obert
    In 1988, Kalai [5] extended a construction of Billera and Lee to produce many triangulated(d−1)-spheres. In fact, in view of upper bounds on the number of simplicial d-polytopes by Goodman and Pollack [2,3], he derived ...
  • On polytopality of Cartesian products of graphs 

    Pfeifle, Julián; Pilaud, Vincent; Santos Pérez, Francisco Javier (2010-07)
    Text en actes de congrés
    Accés obert
    We study the polytopality of Cartesian products of non-polytopal graphs. On the one hand, we prove that a product of graphs is the graph of a simple polytope if and only if its factors are. On the other hand, we provide ...
  • On the monotone upper bound problem 

    Pfeifle, Julián; Ziegler, Günter M. (2004)
    Article
    Accés obert
    The Monotone Upper Bound Problem asks for the maximal number M(d,n) of vertices on a strictly-increasing edge-path on a simple d-polytope with n facets. More specifically, it asks whether the upper bound M(d,n) ≤ ...
  • Overlapping community search for social networks 

    Padrol Sureda, Arnau; Perarnau Llobet, Guillem; Pfeifle, Julián; Muntés Mulero, Víctor (IEEE Press. Institute of Electrical and Electronics Engineers, 2010)
    Text en actes de congrés
    Accés obert
    Finding decompositions of a graph into a family of clusters is crucial to understanding its underlying structure. While most existing approaches focus on partitioning the nodes, real-world datasets suggest the presence ...
  • Polygons as sections of higher-dimensional polytopes 

    Padrol Sureda, Arnau; Pfeifle, Julián (2015-02-09)
    Article
    Accés obert
    We show that every heptagon is a section of a 3-polytope with 6 vertices. This implies that every n-gon with n >= 7 can be obtained as a section of a (2 + [n/7])-dimensional polytope with at most [6n/7] vertices; and ...
  • Prodsimplicial-neighborly polytopes 

    Matschke, Benjamin; Pfeifle, Julián; Pilaud, Vincent (2010-11)
    Article
    Accés obert
    We introduce PSN polytopes whose k-skeleton is combinatorially equivalent to that of a product of r simplices. They simultaneously generalize both neighborly and neighborly cubical polytopes. We construct PSN polytopes ...
  • Root polytopes and growth series of root lattices 

    Ardila, Federico; Beck, Matthias; Hosten, Serkan; Pfeifle, Julián; Seashore, Kim (2011)
    Article
    Accés obert
    The convex hull of the roots of a classical root lattice is called a root polytope. We determine explicit unimodular triangulations of the boundaries of the root polytopes associated to the root lattices $A_n$, $C_n$, and ...
  • The rotation graph of k-ary trees is Hamiltonian 

    Huemer, Clemens; Hurtado Díaz, Fernando Alfredo; Pfeifle, Julián (Crete University Press, 2006)
    Article
    Accés restringit per política de l'editorial
    In this paper we show that the graph of k-ary trees, connected by rotations, contains a Hamilton cycle. Our proof is constructive and thus provides a cyclic Gray code for k-ary trees. Furthermore, we identify a basic ...