One of the fundamental algorithmic problems in computer science involves selecting the kth smallest element in a collection A of n elements. We propose an algorithm design methodology to solve the selection problem on...
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One of the fundamental algorithmic problems in computer science involves selecting the kth smallest element in a collection A of n elements. We propose an algorithm design methodology to solve the selection problem on meshes with multiple broadcasting. Our methodology leads to a selection algorithm that runs in O(n1/8(log n)3/4) time on a mesh with multiple broadcasting of size n3/8(log n)1/4 X n5/8/(log n)1/4 . This result is optimal over a large class of selection algorithms. Our result shows that just as for semigroup computations, selection can be done faster on suitably chosen rectangular meshes than on square meshes.
Multivariable trial functions that depend on random parameters are maximized by crude global search. Analytical and numerical investigations of error distributions confirm recent conclusions that in practice random se...
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Multivariable trial functions that depend on random parameters are maximized by crude global search. Analytical and numerical investigations of error distributions confirm recent conclusions that in practice random searching points perform better than rectangular lattices, and that quasi-random searching points are even more efficient.
We present four polylog-time parallel algorithms for matching parentheses on an exclusive-read and exclusive-write (EREW) parallel random-access machine (PRAM) model. These algorithms provide new insights into the par...
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We present four polylog-time parallel algorithms for matching parentheses on an exclusive-read and exclusive-write (EREW) parallel random-access machine (PRAM) model. These algorithms provide new insights into the parentheses-matching problem. The first algorithm has a time complexity of O(log2 n) employing O(n/log n) processors for an input string containing n parentheses. Although this algorithm is not cost-optimal, it is extremely simple to implement. The remaining three algorithms, which are based on a different approach, achieve O(log n) time complexity in each case, and represent successive improvements. The second algorithm requires O(n) processors and working space, and it is comparable to the first algorithm in its ease of implementation. The third algorithm uses O(n/log n) processors and O(n log n) space. Thus, it is cost-optimal, but uses extra space compared to the standard stack-based sequential algorithm. The last algorithm reduces the space complexity to O(n) while maintaining the same processor and time complexities. Compared to other existing time-optimal algorithms for the parentheses-matching problem that either employ extensive pipelining or use linked lists and comparable data structures, and employ sorting or a linked list ranking algorithm as subroutines, our last two algorithms have two distinct advantages. First, these algorithms employ arrays as their basic data structures, and second, they do not use any pipelining, sorting, or linked list ranking algorithms.
This paper summarizes theoretical and practical investigations into the effect of parallelization by grid-partitioning on the performance of multigrid methods for the solution of partial differential equations on gene...
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This paper summarizes theoretical and practical investigations into the effect of parallelization by grid-partitioning on the performance of multigrid methods for the solution of partial differential equations on general two-dimensional domains. Particular emphasis will be placed on the algorithmic scalability for MIMD distributed memory systems. Experimental results for two Navier-Stokes test problems, presented in the last section of the paper, show that the theoretically predicted dependency of the combined numerical and parallel efficiencies of multigrid methods on the number of processors employed is in fact very weak. This leads to the conclusion that multigrid is an appropriate candidate for solving partial differential equations on massively parallel machines.
We prove that prefix sums of n integers of at most b bits can be found on a COMMON CRCW PRAM in time with a linear time-processor product. The algorithm is optimally fast, for any polynomial number of processors. In p...
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We prove that prefix sums of n integers of at most b bits can be found on a COMMON CRCW PRAM in time with a linear time-processor product. The algorithm is optimally fast, for any polynomial number of processors. In particular, if the time taken is . This is a generalisation of previous result. The previous time algorithm was valid only for O(log n)-bit numbers. Application of this algorithm to r-way parallel merge sort algorithm is also considered. We also consider a more realistic PRAM variant, in which the word size, m, may be smaller than b (m≥log n). On this model, prefix sums can be found in optimal time.
A family of intervals on the real line provides a natural model for a vast number of scheduling and VLSI problems. Recently, a number of parallel algorithms to solve a variety of practical problems on such a family of...
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A family of intervals on the real line provides a natural model for a vast number of scheduling and VLSI problems. Recently, a number of parallel algorithms to solve a variety of practical problems on such a family of intervals have been proposed in the literature. We develop computational tools and show how they can be used for the purpose of devising cost-optimal parallel algorithms for a number of interval-related problems including finding a largest subset of pairwise nonoverlapping intervals, a minimum dominating subset of intervals, along with algorithms to compute the shortest path between a pair of intervals and, based on the shortest path, a parallel algorithm to find the center of the family of intervals. More precisely, with an arbitrary family of n intervals as input, all our algorithms run in O(log n) time using O(n) processors in the EREW-PRAM model of computation.
In this paper we propose time-optimal convex hull algorithms for two classes of enhanced meshes. Our first algorithm computes the convex hull of an arbitrary set of n points in the plane in O(log n) time on a mesh wit...
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In this paper we propose time-optimal convex hull algorithms for two classes of enhanced meshes. Our first algorithm computes the convex hull of an arbitrary set of n points in the plane in O(log n) time on a mesh with multiple broadcasting of size n x n. The second algorithm shows that the same problem can be solved in O(1) time on a reconfigurable mesh of size n x n. Both algorithms achieve time lower bounds for their respective model of computation.
Consider a set of S points in the plane. A point p in S is said to be k-maximal if exactly k elements in S dominate p. We propose a very simple, cost-optimal, EREW algorithm to solve the 1-maximal elements problem in ...
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Consider a set of S points in the plane. A point p in S is said to be k-maximal if exactly k elements in S dominate p. We propose a very simple, cost-optimal, EREW algorithm to solve the 1-maximal elements problem in the plane.
A function is unimodal if it strictly increases to a unique maximum and then strictly decreases. The problem of determining the smallest possible interval containing the maximum of a unimodal function, by probing only...
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A function is unimodal if it strictly increases to a unique maximum and then strictly decreases. The problem of determining the smallest possible interval containing the maximum of a unimodal function, by probing only at integer values is studied. In the finite case, the search takes place over the range 0 to N, while in the infinite case the search takes place over the nonnegative integers. The analyses are based on an unusual Fibonacci version of Kraft's inequality.
A technique to accelerate convergence of stochastic approximation algorithms is studied. It is based on Kesten's idea of equalization of the gain coefficient for the Robbins-Monro algorithm. Convergence with proba...
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A technique to accelerate convergence of stochastic approximation algorithms is studied. It is based on Kesten's idea of equalization of the gain coefficient for the Robbins-Monro algorithm. Convergence with probability is proved for the multidimensional analog of the Kesten accelerated stochastic approximation algorithm. Asymptotic normality of the delivered estimates is also shown. Results of numerical simulations are presented that demonstrate the efficiency of the acceleration procedure.
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