Shimon (Moni) Shahar

Hitting sets when the VC-dimension is small

We present an approximation algorithm for the hitting set problem when the VC-dimension of the set system is small. Our algorithm uses a linear programming relaxation to compute a probability measure for which @?-nets are always hitting sets (see Corollary 15.6 in Pach and Agarwal [Combinatorial Geometry, J. Wiley, New York, 1995]). The comparable algorithm of Bronnimann and Goodrich [Almost optimal set covers in finite VC-dimension, Discrete Comput. Geom. 14 (1995) 463] computes such a probability measure by an iterative reweighting technique. The running time of our algorithm is comparable with theirs, and the approximation ratio is smaller by a constant factor. We also show how our algorithm can be parallelized and extended to the minimum cost hitting set problem.

On approximating a geometric prize-collecting traveling salesman problem with time windows

We study a scheduling problem in which jobs have locations. For example, consider a repairman that is supposed to visit customers at their homes. Each customer is given a time window during which the repairman is allowed to arrive. The goal is to find a schedule that visits as many homes as possible. We refer to this problem as the prize-collecting traveling salesman problem with time windows (TW-TSP).We consider two versions of TW-TSP. In the first version, jobs are located on a line, have release times and deadlines but no processing times. We present a geometric interpretation of TW-TSP on a line that generalizes the longest monotone subsequence problem. We present an O(logn) approximation algorithm for this case, where n denotes the number of jobs. This algorithm can be extended to deal with non-unit job profits.The second version deals with a general case of asymmetric distances between locations. We define a density parameter that, loosely speaking, bounds the number of zig-zags between locations within a time window. We present a dynamic programming algorithm that finds a tour that visits at least OPT/density locations during their time windows. This algorithm can be extended to deal with non-unit job profits and processing times.

Algorithms for capacitated rectangle stabbing and lot sizing with joint set-up costs

In the rectangle stabbing problem, we are given a set of axis parallel rectangles and a set of horizontal and vertical lines, and our goal is to find a minimum size subset of lines that intersect all the rectangles. In this article, we study the capacitated version of this problem in which the input includes an integral capacity for each line. The capacity of a line bounds the number of rectangles that the line can cover. We consider two versions of this problem. In the first, one is allowed to use only a single copy of each line (hard capacities), and in the second, one is allowed to use multiple copies of every line, but the multiplicities are counted in the size (or weight) of the solution (soft capacities). We present an exact polynomial-time algorithm for the weighted one dimensional case with hard capacities that can be extended to the one dimensional weighted case with soft capacities. This algorithm is also extended to solve a certain capacitated multi-item lot-sizing inventory problem with joint set-up costs. For the case of d-dimensional rectangle stabbing with soft capacities, we present a 3d-approximation algorithm for the unweighted case. For d-dimensional rectangle stabbing problem with hard capacities, we present a bi-criteria algorithm that computes 4d-approximate solutions that use at most two copies of every line. Finally, we present hardness results for rectangle stabbing when the dimension is part of the input and for a two-dimensional weighted version with hard capacities.

On Approximating a Geometric Prize-Collecting Traveling Salesman Problem with Time Windows

We study a scheduling problem in which jobs have locations. For example, consider a repairman that is supposed to visit customers at their homes. Each customer is given a time window during which the repairman is allowed to arrive. The goal is to find a schedule that visits as many homes as possible. We refer to this problem as the Prize-Collecting Traveling Salesman Problem with time windows (TW-TSP).

Approximation algorithms for capacitated rectangle stabbing

In the rectangle stabbing problem we are given a set of axis parallel rectangles and a set of horizontal and vertical lines, and our goal is to find a minimum size subset of lines that intersect all the rectangles. We study the capacitated version of this problem in which the input includes an integral capacity for each line that bounds the number of rectangles that the line can cover. We consider two versions of this problem. In the first, one is allowed to use only a single copy of each line (hard capacities), and in the second, one is allowed to use multiple copies of every line provided that multiplicities are counted in the size of the solution (soft capacities). For the case of d-dimensional rectangle stabbing with soft capacities, we present a 6d-approximation algorithm and a 2-approximation algorithm when d = 1. For the case of hard capacities, we present a bi-criteria algorithm that computes 16d-approximate solutions that use at most two copies of every line. For the one dimensional case, an 8-approximation algorithm for hard capacities is presented.

Real-Time video streaming in multi-hop wireless static ad hoc networks

We deal with the problem of streaming multiple video streams between pairs of nodes in a multi-hop wireless ad hoc network. The nodes are static, know their locations, and are synchronized (via GPS). We introduce a new interference model that uses variable interference radiuses. We present an algorithm for computing a frequency assignment and a schedule whose goal is to maximize throughput over all the video streams. In addition, we developed a localized flow-control mechanism to stabilize the queue lengths. We simulated traffic scheduled by the algorithm using OMNET++/MixiM (i.e., physical SINR interference model with 802.11g) to test whether the computed throughput is achieved. The results of the simulation show that the computed solution is sinr-feasible and achieves predictable stable throughputs.

Scheduling of a smart antenna: capacitated coloring of unit circular-arc graphs

We consider scheduling problems that are motivated by an optimization of the transmission schedule of a smart antenna. In these problems we are given a set of messages and a conflict graph that specifies which messages cannot be transmitted concurrently. In our model the conflict graph is a unit circular-arc graph. Two variants of the problem are considered: C-MBL and NU-C-MBL. In C-MBL, the messages have unit demands, whereas in NU-C-MBL demands are arbitrary. We present an optimal algorithm for C-MBL and a 3-approximation algorithm for NU-C-MBL.

Capacitated Arc Stabbing

In the Capacitated Arc Stabbing problem (CAS) we are given a set of arcs and a set of points on a circle. We say that a point p covers, or stabs, an arc A if p is contained in A. Each point has a weight and a capacity that determines the number of arcs it may cover. The goal is to find a minimum weight set of points that stabs all the arcs. CAS models a periodic multi-item lot sizing problem in which we are given a set of production requests each with its own periodic release time and deadline. Production takes place in batches of bounded capacity: each time unit t is associated with a capacity c(t) and weight w(t), where c(t) bounds the number of requests that can be manufactured at time t, and w(t) is a fixed cost for manufacturing any positive number of requests up to c(t) at time t. The goal is to find a minimum weight periodic schedule. We present a polynomial time algorithm for CAS that is based on a non-trivial reduction to Capacitated Interval Stabbing. Our approach applies to both hard and soft capacities. We also consider two variants of CAS in which some arcs may remain uncovered: in the partial variant there is a covering requirement g, and the goal is to find a minimum weight set of points that covers at least g arcs; and in the prize collecting variant each arc has a penalty that must be paid if this arc is not covered.