hal@UTDALLAS.EDU (Hal Sudborough) (09/25/90)
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ANNOUNCEMENT !! **************************** ( 2nd Meeting of ) Workshop on Algorithmic Research in the Midsouthwest ( WARM ) Friday, October 12 Final Schedule ******************************************************************************** ** LOCATION: Performance Hall ( Jonsson 2.604 ) The University of Texas at Dallas Richardson, Texas 75083-0688 CONTACT: Hal Sudborough, Head Computer Science Program (214)-690-2184 Please call or send e-mail if you'd like information on a special rate at a local hotel, routing information from the airport, etc. ******************************************************************************** ** [11:00 - 11:45 a.m.] SYMMETRY IN INTERCONNECTION NETWORKS S.Lakshmivarahan Parallel Processing Institute School of Electrical Engineering and Computer Science University of Oklahoma, Norman, Oklahoma Abstract: This talk will review various notions of symmetry in graphs including vertex symmetry, edge symmetry, distance transitivity, distance regularity, etc. Analysis of symmetry of interconnection networks based on Star graphs and other graphs relating to the symmetric permutation group, even graphs and orthogonal graphs will be presented. ******************************************************************************** ** [11:45-12:30 noon] LOWER BOUNDS FOR PARALLEL COMPUTATION ON LINKED STRUCTURES Vijaya Ramachandran Department of Computer Sciences The University of Texas at Austin (joint work with Faith Fich.) Abstract: We present the following results on parallel computation on linked structures: We give a tight relationship between the time required on a decision tree and the time required on a CREW PRAM to compute any function of a collection of circular doubly linked lists. We show that this relationship does not hold for singly linked lists. We derive a tight lower bound of OMEGA(log^*n) time on a decision tree for the problem of coloring an n-node doubly linked circular linked list with a constant number of colors. By applying the first result, we then derive a tight lower bound of OMEGA(log log^*n) time on a CREW PRAM for the problem of coloring a collection of circular doubly linked lists on n nodes. ******************************************************************************** ** [12:30 noon - 2:00 p.m] --------- lunch --------------- ******************************************************************************** ** [2:00 - 2:45 p.m.] THE COMPUTATIONAL COMPLEXITY OF OPTIMAL SORTING NETWORK VERIFICATION by Ian Parberry Center for Research in Parallel and Distributed Computing Department of Computer Sciences University of North Texas Denton, Texas A sorting network is a combinational circuit for sorting, constructed from comparison-swap units. The depth of such a circuit is a measure of its running time. It is reasonable to hypothesize that only the fastest (that is, the shallowest) sorting networks are likely to be fabricated. It is shown that the verification of shallow sorting networks is computationally intractable. More precisely, it is shown that the problem of verifying sorting networks with depth very close to the optimum is Co-NP complete, and further that any deterministic or randomized algorithm that has access only to the inputs and outputs of the sorting network must take exponential time to verify it. Despite the computational intractability of the shallow sorting network verification problem, we describe a construction and verification algorithm which was recently executed on a supercomputer to demonstrate that there is no 9-input sorting network of depth 6, a question which had remained open for over 15 years. ******************************************************************************** ** [2:45 -3:30 p.m.] A COMBINATORIAL VIEW OF VISIBILITY GRAPHS by James M. Abello Distributed Laboratory for Algorithms Design Department of Computer Science Texas A&M University College Station, Texas ( joint work with Omer Egecioglu and Krishna Kumar ) We address the polynomial time recognition problem for vertex graphs of staircase-like polygons. These graphs are given a coordinate free combinatorial representation in terms of linear orders induced on Ferrer's diagrams of shape (n-1, n-2, .. , 1). This opens up a surprising connection between these graphs and maximal chains in the Permutohedron. It turns out that various properties of the underlying graphs can be studied in terms of restricted types of Coxeter transformations. As a byproduct a generalization of chordal graphs is obtained settling several open questions posed by O'Rourke. ******************************************************************************** ** [3:30 -4:00 p.m.] ----------Break------------- ******************************************************************************** ** [4:00 - 4:45 p.m.] PARALLEL ALGORITHMS FOR COMBINATORIAL SEARCH PROBLEMS by Yanjun Zhang Department of Computer Science Southern Methodist University Dallas, Texas Abstract: In this talk we present parallel algorithms for backtrack search, branch-and-bound computation and game-tree search. Our model of parallel computation is a network of processors communicating via messages. Our primary interest in a parallel algorithm is its speed-up over the sequential ones. Our goal is to design parallel algorithms that achieve a speed-up proportional to the number of processors used. For backtrack search, we propose a simple randomized method for parallelizing sequential backtrack search algorithms for solving enumeration problems. We show that, uniformly on all instances, this method is likely to achieve a nearly best possible speed-up. For branch-and-bound computation, we present a randomized method called Local Best-First Search for parallelizing sequential branch-and-bound algorithms. We show that, uniformly on all instances, the execution time of Local Best-First Search is unlikely to exceed a certain inherent lower bound by more than a constant factor. For game-tree search, we present a class of parallel algorithms that parallelize the `left-to-right' algorithm for evaluating AND/OR trees and the alpha-beta pruning algorithm for evaluating MIN/MAX trees, and prove that the algorithm achieves a linear speed-up over the left-to-right algorithm on uniform AND/OR trees when the number of processors used is close to the height of the input tree. We conjecture that the same conclusion holds for the speed-up of the algorithm over the alpha-beta pruning algorithm. ******************************************************************************** ** There will be a dinner trip afterward to a restaurant in North Dallas for those interested.