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  • Resultat 371-380 av 894
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371.
  • Carling, Kenneth, et al. (författare)
  • Testing the gravity p-median model empirically
  • 2015
  • Ingår i: Operations Research Perspectives. - : Elsevier. - 2214-7160. ; 2:124
  • Tidskriftsartikel (refereegranskat)abstract
    • Regarding the location of a facility, the presumption in the widely used p-median model is that the customer opts for the shortest route to the nearest facility. However, this assumption is problematic on free markets since the customer is presumed to gravitate to a facility by the distance to and the attractiveness of it. The recently introduced gravity p-median model offers an extension to the p-median model that account for this. The model is therefore potentially interesting, although it has not yet been implemented and tested empirically. In this paper, we have implemented the model in an empirical problem of locating vehicle inspections, locksmiths, and retail stores of vehicle spare-parts for the purpose of investigating its superiority to the p-median model. We found, however, the gravity p-median model to be of limited use for the problem of locating facilities as it either gives solutions similar to the p-median model, or it gives unstable solutions due to a non-concave objective function.
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372.
  • Carlson, Johan, 1972, et al. (författare)
  • Minimizing Dimensional Variation and Robot Traveling Time in Welding Stations
  • 2014
  • Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 23:C, s. 77-82
  • Konferensbidrag (refereegranskat)abstract
    • Complex assembled products as an automotive car body consist of about 300 sheet metal parts joined by up to 4000 spot welds. In the body factory, there are several hundred robots organized into lines of welding stations. The distribution of welds between robots and the welding sequences have a significant influence on both dimensional quality and throughput. Therefore, this paper proposes a novel method for quality and throughput optimization based on a systematic search algorithm which exploits properties of the welding process. It uses approximated lower bounds to speed up the search and to estimate the quality of the solution. The method is successfully tested on reference assemblies, including detailed fixtures, welding robots and guns.
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373.
  • Casselgren, Carl Johan (författare)
  • Anote on one-sided interval edge colorings of bipartite graphs
  • 2022
  • Ingår i: Discrete Mathematics. - : Elsevier. - 0012-365X .- 1872-681X. ; 345:2
  • Tidskriftsartikel (refereegranskat)abstract
    • For a bipartite graph G with parts X and Y, an X-interval coloring is a proper edge coloring of G by integers such that the colors on the edges incident to any vertex in X form an interval. Denote by chi(int) (G, X) the minimum k such that G has an X-interval coloring with k colors. Casselgren and Toft (2016) [12] asked whether there is a polynomial P(Delta) such that if G has maximum degree at most A, then chi(int)(G, X) <= P(A). In this short note, we answer this question in the affirmative; in fact, we prove that a cubic polynomial suffices. We also deduce some improved upper bounds on chi(int)(G, X) for bipartite graphs with small maximum degree. (C) 2021 The Author(s). Published by Elsevier B.V.
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374.
  • Casselgren, Carl Johan, et al. (författare)
  • Avoiding and Extending Partial Edge Colorings of Hypercubes
  • 2022
  • Ingår i: Graphs and Combinatorics. - : Springer. - 0911-0119 .- 1435-5914. ; 38:3
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider the problem of extending and avoiding partial edge colorings of hypercubes; that is, given a partial edge coloring φ of the d-dimensional hypercube Qd, we are interested in whether there is a proper d-edge coloring of Qd that agrees with the coloring φ on every edge that is colored under φ; or, similarly, if there is a proper d-edge coloring that disagrees with φ on every edge that is colored under φ. In particular, we prove that for any d≥ 1 , if φ is a partial d-edge coloring of Qd, then φ is avoidable if every color appears on at most d/8 edges and the coloring satisfies a relatively mild structural condition, or φ is proper and every color appears on at most d- 2 edges. We also show that φ is avoidable if d is divisible by 3 and every color class of φ is an induced matching. Moreover, for all 1 ≤ k≤ d, we characterize for which configurations consisting of a partial coloring φ of d- k edges and a partial coloring ψ of k edges, there is an extension of φ that avoids ψ.
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375.
  • Casselgren, Carl Johan (författare)
  • Brooks' theorem with forbidden colors
  • 2023
  • Ingår i: Journal of Graph Theory. - : WILEY. - 0364-9024 .- 1097-0118.
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider extensions of Brooks' classic theorem on vertex coloring where some colors cannot be used on certain vertices. In particular we prove that if G is a connected graph with maximum degree Delta(G) >= 4 that is not a complete graph and P subset of V (G) is a set of vertices where either (i) at most Delta(G) - 2 colors are forbidden for every vertex in P, and any two vertices of P are at distance at least 4, or (ii) at most Delta(G) - 3 colors are forbidden for every vertex in P, and any two vertices of P are at distance at least 3, then there is a proper Delta(G)-coloring of G respecting these constraints. In fact, we shall prove that these results hold in the more general setting of list colorings. These results are sharp.
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376.
  • Casselgren, Carl Johan, et al. (författare)
  • Coloring Complete and Complete Bipartite Graphs from Random Lists
  • 2016
  • Ingår i: Graphs and Combinatorics. - : Springer Science and Business Media LLC. - 0911-0119 .- 1435-5914. ; 32:2, s. 533-542
  • Tidskriftsartikel (refereegranskat)abstract
    • Assign to each vertex v of the complete graph on n vertices a list L(v) of colors by choosing each list independently and uniformly at random from all f(n)-subsets of a color set , where f(n) is some integer-valued function of n. Such a list assignment L is called a random (f(n), [n])-list assignment. In this paper, we determine the asymptotic probability (as ) of the existence of a proper coloring of , such that for every vertex v of . We show that this property exhibits a sharp threshold at . Additionally, we consider the corresponding problem for the line graph of a complete bipartite graph with parts of size m and n, respectively. We show that if , , and L is a random (f(n), [n])-list assignment for the line graph of , then with probability tending to 1, as , there is a proper coloring of the line graph of with colors from the lists.
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377.
  • Casselgren, Carl Johan, et al. (författare)
  • Completing partial Latin squares with one filled row, column and symbol
  • 2013
  • Ingår i: Discrete Mathematics. - : Elsevier. - 0012-365X .- 1872-681X. ; 313:9, s. 1011-1017
  • Tidskriftsartikel (refereegranskat)abstract
    • Let P be an n×n partial Latin square every non-empty cell of which lies in a fixed row r, a fixed column c or contains a fixed symbol s. Assume further that s is the symbol of cell (r,c) in P. We prove that P is completable to a Latin square if n≥8 and n is divisible by 4, or n≤7 and n∉{3,4,5}. Moreover, we present a polynomial algorithm for the completion of such a partial Latin square.
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378.
  • Casselgren, Carl Johan, 1982-, et al. (författare)
  • Completing partial Latin squares with two filled rows and three filled columns
  • 2023
  • Ingår i: Journal of Combinatorics. - : INT PRESS BOSTON, INC. - 2156-3527 .- 2150-959X. ; 14:1, s. 139-153
  • Tidskriftsartikel (refereegranskat)abstract
    • Consider a partial Latin square P where the first two rows and first three columns are completely filled, and every other cell of P is empty. It has been conjectured that all such partial Latin squares of order at least 8 are completable. Based on a technique by Kuhl and McGinn we describe a framework for completing partial Latin squares in this class. Moreover, we use our method for proving that all partial Latin squares from this family, where the intersection of the nonempty rows and columns form a Latin rectangle with three distinct symbols, are completable.
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379.
  • Casselgren, Carl Johan, et al. (författare)
  • Edge precoloring extension of trees
  • 2021
  • Ingår i: AUSTRALASIAN JOURNAL OF COMBINATORICS. - : CENTRE DISCRETE MATHEMATICS & COMPUTING. - 2202-3518. ; 81, s. 233-244
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider the problem of extending partial edge colorings of trees. We obtain analogues of classical results on extending partial Latin squares by characterizing exactly which partial edge colorings with at most Delta(T) precolored edges of a tree T with maximum degree Delta(T) are extendable to proper Delta(T)-edge colorings of T. Furthermore, we prove sharp conditions on when it is possible to extend a partial edge coloring where the precolored edges form a matching or a collection of connected subgraphs. Finally, we consider the problem of avoiding a given (not necessarily proper) partial edge coloring.
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380.
  • Casselgren, Carl Johan, et al. (författare)
  • Edge Precoloring Extension of Trees II
  • 2022
  • Ingår i: Discussiones Mathematicae. Graph Theory. - : Uniwersytet Zielonogorski * Wydzial Matematyki, Informatyli i Ekonometrii,Technical University Zielona Gora, Institute of Mathematics. - 1234-3099 .- 2083-5892.
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider the problem of extending and avoiding partial edge colorings of trees; that is, given a partial edge coloring phi of a tree T we are interested in whether there is a proper Delta(T )-edge coloring of T that agrees with the coloring phi on every edge that is colored under phi; or, similarly, if there is a proper Delta(T )-edge coloring that disagrees with phi on every edge that is colored under phi. We characterize which partial edge colorings with at most Delta(T ) + 1 precolored edges in a tree T are extendable, thereby proving an analogue of a result by Andersen for Latin squares. Furthermore we obtain some "mixed" results on extending a partial edge coloring subject to the condition that the extension should avoid a given partial edge coloring; in particular, for all 0 <= k <= Delta(T ), we characterize for which configurations consisting of a partial coloring phi of Delta(T ) - k edges and a partial coloring psi of k + 1 edges of a tree T, there is an extension of phi that avoids psi.
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