94,042 research outputs found

    Distributed local approximation algorithms for maximum matching in graphs and hypergraphs

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    We describe approximation algorithms in Linial's classic LOCAL model of distributed computing to find maximum-weight matchings in a hypergraph of rank rr. Our main result is a deterministic algorithm to generate a matching which is an O(r)O(r)-approximation to the maximum weight matching, running in O~(rlogΔ+log2Δ+logn)\tilde O(r \log \Delta + \log^2 \Delta + \log^* n) rounds. (Here, the O~()\tilde O() notations hides polyloglog Δ\text{polyloglog } \Delta and polylog r\text{polylog } r factors). This is based on a number of new derandomization techniques extending methods of Ghaffari, Harris & Kuhn (2017). As a main application, we obtain nearly-optimal algorithms for the long-studied problem of maximum-weight graph matching. Specifically, we get a (1+ϵ)(1+\epsilon) approximation algorithm using O~(logΔ/ϵ3+polylog(1/ϵ,loglogn))\tilde O(\log \Delta / \epsilon^3 + \text{polylog}(1/\epsilon, \log \log n)) randomized time and O~(log2Δ/ϵ4+logn/ϵ)\tilde O(\log^2 \Delta / \epsilon^4 + \log^*n / \epsilon) deterministic time. The second application is a faster algorithm for hypergraph maximal matching, a versatile subroutine introduced in Ghaffari et al. (2017) for a variety of local graph algorithms. This gives an algorithm for (2Δ1)(2 \Delta - 1)-edge-list coloring in O~(log2Δlogn)\tilde O(\log^2 \Delta \log n) rounds deterministically or O~((loglogn)3)\tilde O( (\log \log n)^3 ) rounds randomly. Another consequence (with additional optimizations) is an algorithm which generates an edge-orientation with out-degree at most (1+ϵ)λ\lceil (1+\epsilon) \lambda \rceil for a graph of arboricity λ\lambda; for fixed ϵ\epsilon this runs in O~(log6n)\tilde O(\log^6 n) rounds deterministically or O~(log3n)\tilde O(\log^3 n ) rounds randomly

    Universality of the Future Chronological Boundary

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    The purpose of this note is to establish, in a categorical manner, the universality of the Geroch-Kronheimer-Penrose causal boundary when considering the types of causal structures that may profitably be put on any sort of boundary for a spacetime. Actually, this can only be done for the future causal boundary (or the past causal boundary) separately; furthermore, only the chronology relation, not the causality relation, is considered, and the GKP topology is eschewed. The final result is that there is a unique map, with the proper causal properties, from the future causal boundary of a spacetime onto any ``reasonable" boundary which supports some sort of chronological structure and which purports to consist of a future completion of the spacetime. Furthermore, the future causal boundary construction is categorically unique in this regard.Comment: 25 pages, AMS-TeX; 2 figures, PostScript (separate); captions (separate); submitted to Class. Quantum Grav, slight revision: bottom lines legible, figures added, expanded discussion and example

    Some results on chromatic number as a function of triangle count

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    A variety of powerful extremal results have been shown for the chromatic number of triangle-free graphs. Three noteworthy bounds are in terms of the number of vertices, edges, and maximum degree given by Poljak \& Tuza (1994), and Johansson. There have been comparatively fewer works extending these types of bounds to graphs with a small number of triangles. One noteworthy exception is a result of Alon et. al (1999) bounding the chromatic number for graphs with low degree and few triangles per vertex; this bound is nearly the same as for triangle-free graphs. This type of parametrization is much less rigid, and has appeared in dozens of combinatorial constructions. In this paper, we show a similar type of result for χ(G)\chi(G) as a function of the number of vertices nn, the number of edges mm, as well as the triangle count (both local and global measures). Our results smoothly interpolate between the generic bounds true for all graphs and bounds for triangle-free graphs. Our results are tight for most of these cases; we show how an open problem regarding fractional chromatic number and degeneracy in triangle-free graphs can resolve the small remaining gap in our bounds

    Deterministic parallel algorithms for bilinear objective functions

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    Many randomized algorithms can be derandomized efficiently using either the method of conditional expectations or probability spaces with low independence. A series of papers, beginning with work by Luby (1988), showed that in many cases these techniques can be combined to give deterministic parallel (NC) algorithms for a variety of combinatorial optimization problems, with low time- and processor-complexity. We extend and generalize a technique of Luby for efficiently handling bilinear objective functions. One noteworthy application is an NC algorithm for maximal independent set. On a graph GG with mm edges and nn vertices, this takes O~(log2n)\tilde O(\log^2 n) time and (m+n)no(1)(m + n) n^{o(1)} processors, nearly matching the best randomized parallel algorithms. Other applications include reduced processor counts for algorithms of Berger (1997) for maximum acyclic subgraph and Gale-Berlekamp switching games. This bilinear factorization also gives better algorithms for problems involving discrepancy. An important application of this is to automata-fooling probability spaces, which are the basis of a notable derandomization technique of Sivakumar (2002). Our method leads to large reduction in processor complexity for a number of derandomization algorithms based on automata-fooling, including set discrepancy and the Johnson-Lindenstrauss Lemma

    A Two Term Truncation of the Multiple Ising Model Coupled to 2d Gravity

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    We consider a model of p independent Ising spins on a dynamical planar phi-cubed graph. Truncating the free energy to two terms yields an exactly solvable model that has a third order phase transition from a pure gravity region (gamma=-1/2) to a tree-like region (gamma=1/2), with gamma=1/3 on the critical line. We are able to make an order of magnitude estimate of the value of p above which there exists a branched polymer (ie tree-like) phase in the full model, that is, p is approximately 13-23, which corresponds to a central charge c of about 6-12.Comment: 18 pages, LaTeX, 12 figure
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