4,968 research outputs found

    A survey on performance analysis of warehouse carousel systems

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    This paper gives an overview of recent research on the performance evaluation and design of carousel systems. We discuss picking strategies for problems involving one carousel, consider the throughput of the system for problems involving two carousels, give an overview of related problems in this area, and present an extensive literature review. Emphasis has been given on future research directions in this area

    Monte Carlo methods of PageRank computation

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    We describe and analyze an on-line Monte Carlo method of PageRank computation. The PageRank is being estimated basing on results of a large number of short independent simulation runs initiated from each page that contains outgoing hyperlinks. The method does not require any storage of the hyperlink matrix and is highly parallelizable. We study confidence intervals, and discover drawbacks of the absolute error criterion and the relative error criterion. Further, we suggest a so-called weighted relative error criterion, which ensures a good accuracy in a relatively small number of simulation runs. Moreover, with the weighted relative error measure, the complexity of the algorithm does not depend on the web structure

    Optimal picking of large orders in carousel systems

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    A carousel is an automated storage and retrieval system which consists of a circular disk with a large number of shelves and drawers along its circumference. The disk can rotate either direction past a picker who has a list of items that have to be collected from nn different drawers. In this paper, we assume that locations of the nn items are independent and have a continous non-uniform distribution over the carousel circumference. For this model, we determine a limiting behavior of the shortest rotation time needed to collect one large order. In particular, our limiting result indicates that if an order is large, then it is optimal to allocate {\it less} frequently asked items {\it close} to the picker's starting position. This is in contrast with picking of small orders where the optimal allocation rule is clearly the opposite. We also discuss travel times and allocation issues for optimal picking of sequential orders

    Decomposition of the Google pagerank and optimal linking strategy

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    We provide the analysis of the Google PageRank from the perspective of the Markov Chain Theory. First we study the Google PageRank for a Web that can be decomposed into several connected components which do not have any links to each other. We show that in order to determine the Google PageRank for a completely decomposable Web, it is sufficient to compute a subPageRank for each of the connected components separately. Then, we study incentives for the Web users to form connected components. In particular, we show that there exists an optimal linking strategy that benefits a user with links inside its Web community and on the other hand inappropriate links penalize the Web users and their Web communities. \u

    A scaling analysis of a cat and mouse Markov chain

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    Motivated by an original on-line page-ranking algorithm, starting from an arbitrary Markov chain (Cn)(C_n) on a discrete state space S{\cal S}, a Markov chain (Cn,Mn)(C_n,M_n) on the product space S2{\cal S}^2, the cat and mouse Markov chain, is constructed. The first coordinate of this Markov chain behaves like the original Markov chain and the second component changes only when both coordinates are equal. The asymptotic properties of this Markov chain are investigated. A representation of its invariant measure is in particular obtained. When the state space is infinite it is shown that this Markov chain is in fact null recurrent if the initial Markov chain (Cn)(C_n) is positive recurrent and reversible. In this context, the scaling properties of the location of the second component, the mouse, are investigated in various situations: simple random walks in Z\mathbb{Z} and Z2\mathbb{Z}^2, reflected simple random walk in N\mathbb{N} and also in a continuous time setting. For several of these processes, a time scaling with rapid growth gives an interesting asymptotic behavior related to limit results for occupation times and rare events of Markov processes.\u

    Asymptotic analysis for personalized Web search

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    Personalized PageRank is used in Web search as an importance measure for Web documents. The goal of this paper is to characterize the tail behavior of the PageRank distribution in the Web and other complex networks characterized by power laws. To this end, we model the PageRank as a solution of a stochastic equation R=di=1NAiRi+BR\stackrel{d}{=}\sum_{i=1}^NA_iR_i+B, where RiR_i's are distributed as RR. This equation is inspired by the original definition of the PageRank. In particular, NN models the number of incoming links of a page, and BB stays for the user preference. Assuming that NN or BB are heavy-tailed, we employ the theory of regular variation to obtain the asymptotic behavior of RR under quite general assumptions on the involved random variables. Our theoretical predictions show a good agreement with experimental data

    Smallest singular value of sparse random matrices

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    We extend probability estimates on the smallest singular value of random matrices with independent entries to a class of sparse random matrices. We show that one can relax a previously used condition of uniform boundedness of the variances from below. This allows us to consider matrices with null entries or, more generally, with entries having small variances. Our results do not assume identical distribution of the entries of a random matrix and help to clarify the role of the variances of the entries. We also show that it is enough to require boundedness from above of the rr-th moment, r>2r > 2, of the corresponding entries.Comment: 25 pages, a condition on one parameter was added in the statement of Theorem 1.3 and Lemma 6.2, results unchange