A P‐Completeness Result for Visibility Graphs of Simple Polygons

For each vertex of a simple polygon P an integer valued weight is given. We consider the path p₁, p₂, ..., p_k in P which is created according to the following strategy: p₁ is a designated start vertex s and p_i+1 is obtained by choosing the vertex with smallest weight among all vertices visible from p_i and different from p₁, p₂, ..., p_i. If there is no such vertex the path is finished. This path is called geometric lexicographic dead end path. We shall prove the problem of determining whether a distinguished vertex t of P is on the geometric lexicographic dead end path or not to be P‐complete.     

A practical algorithm for decomposing polygonal domains into convex polygons by diagonals

We present algorithms for decomposing a polygon (with holes) into convex polygons by diagonals. The methods are computationally quick, and although the partitions that they produce may not have minimal cardinality they usually have a low number of convex pieces. Thus, the methods are suitable for being used when achieving a modest load on the CPU time is more important than finding optimal decompositions as, for instance, in location problems. Part of the results in this paper are from Fernández (1999), and were presented in Fernández et al. (1998). This work has been supported by the Ministry of Science and Technology of Spain under the research projects BEC2002-01026, SEJ2005-06273/ECON (J. Fernández, B. Tóth and B. Pelegrín) and TIC2003-05982-C05-03 (L. Cánovas), in part financed by the European Regional Development Fund (ERDF).     

Parallel alternating direction multiplier decomposition of convex programs

This paper describes two specializations of the alternating direction method of multipliers: the alternating step method and the epigraphic projection method. The alternating step method applies to monotropic programs, while the epigraphic method applies to general block-separable convex programs, including monotropic programs as a special case. The epigraphic method resembles an earlier parallel method due to Spingarn, but solves a larger number of simpler subproblems at each iteration. This paper gives convergence results for both the alternating step and epigraphic methods, and compares their performance on random dense separable quadratic programs.Some of the research described here was performed at the Massachusetts Institute of Technology and was supported by the Army Research Office under Grant DAAL03-86-K-0171 and the National Science Foundation under Grant ECS-85-19058. This portion of the work was supervised by Dimitri P. Bertsekas, for whose support the author is grateful.     

Banach空间中广义正交分解定理与广义正交可补子空间

本文首先将 Ｈｉｌｂｅｒｔ空间中的Ｒｉｅｓｚ正交分解定理推广到 Ｂａｎａｃｈ空间,得到 Ｂａｎａｃｈ空间广义正交分解定理．然后,利用此定理讨论由Ｊａｍｅｓ Ｒ．Ｃ．［１］引入的Ｂａｎａｃｈ空间中正交概念及 Ｎａｓｈｅｄ Ｍ．Ｚ．［２］引入的 Ｂａｎａｃｈ空间中（广义）正交可补子空间,得到判别子空间广义正交可补的充分必要条件,并由此给出Ｈｉｌｂｅｒｔ空间的一个新特征．     

Parallel searching on m rays

We investigate parallel searching on m concurrent rays. We assume that a target t is located somewhere on one of the rays; we are given a group of m point robots each of which has to reach t. Furthermore, we assume that the robots have no way of communicating over distance. Given a strategy S we are interested in the competitive ratio defined as the ratio of the time needed by the robots to reach t using S and the time needed to reach t if the location of t is known in advance.

If a lower bound on the distance to the target is known, then there is a simple strategy which achieves a competitive ratio of 9—independent of m. We show that 9 is a lower bound on the competitive ratio for two large classes of strategies if m2.

If the minimum distance to the target is not known in advance, we show a lower bound on the competitive ratio of 1+2(k+1)^k+1/k^k where k=logm where log is used to denote the base-2 logarithm. We also give a strategy that obtains this ratio.     

Computing optimal islands

Let S be a bicolored set of n points in the plane. A subset I of S is an island if there is a convex set C such that I=C∩S. We give an O(n³)-time algorithm to compute a monochromatic island of maximum cardinality. Our approach is adapted to optimize similar (decomposable) objective functions. Finally, we use our algorithm to give an O(logn)-approximation for the problem of computing the minimum number of convex polygons that cover a class region.     

Probabilistic matching of planar regions

We analyze a probabilistic algorithm for matching shapes modeled by planar regions under translations and rigid motions (rotation and translation). Given shapes A and B, the algorithm computes a transformation t such that with high probability the area of overlap of t(A) and B is close to maximal. In the case of polygons, we give a time bound that does not depend significantly on the number of vertices, but on perimeter and area of the shapes and, in the case of rigid motions, also on the diameter.     

Parallel pivotal algorithm for solving the linear complementarity problem

We propose a parallel implementation of the classical Lemke's algorithm for solving the linear complementarity problem. The algorithm is designed for a loosely coupled network of computers which is characterized by relatively high communication costs. We provide an accurate prediction of speedup based on a simple operation count. The algorithm produces speedup nearp, wherep is the number of processors, when tested on large problems as demonstrated by computational results on the CRYSTAL token-ring multicomputer and the Sequent Balance 21000 multiprocessor.This material is based on research supported by National Science Foundation Grants DCR-84-20963 and DCR-850-21228 and by Air Force Office of Scientific Research Grants AFSOR-86-0172 and AFSOR-86-0255 while the author was at the University of Wisconsin, Madison, Wisconsin.     

Orthogonal double covers of Cayley graphs

Let X and G be graphs, such that G is isomorphic to a subgraph of X.An orthogonal double cover (ODC) of X by G is a collection of subgraphs of X, all isomorphic with G, such that (i) every edge of X occurs in exactly two members of and (ii) and share an edge if and only if x and y are adjacent in X. The main question is: given the pair (X,G), is there an ODC of X by G? An obvious necessary condition is that X is regular.A technique to construct ODCs for Cayley graphs is introduced. It is shown that for all (X,G) where X is a 3-regular Cayley graph on an abelian group there is an ODC, a few well known exceptions apart.     

求解刚性常微分方程的并行广义Rosenbrock方法

本文构造了求解刚性常微分方程的并行广义Rosenbrock方法（PEROWs），分析了方法的收敛性和数值稳定性。通过用Powell方法优化方法的稳定域，构造了二级四阶并行格式PEROW4，并证明该方法是A－稳定的。新方法比同级的并行Rosenbrock方法MPROW3及PRM3均高一阶，因而在计算精度上处于优势。此外，PEROW4能使得各处理机上的负载基本均衡，从而达到非常理想的加速比和并行效率。     

Traversing a Set of Points with a Minimum Number of Turns

Given a finite set of points S in ℝ ^d , consider visiting the points in S with a polygonal path which makes a minimum number of turns, or equivalently, has the minimum number of segments (links). We call this minimization problem the minimum link spanning path problem. This natural problem has appeared several times in the literature under different variants. The simplest one is that in which the allowed paths are axis-aligned. Let L(S) be the minimum number of links of an axis-aligned path for S, and let G _n ^d be an n×…×n grid in ℤ ^d . Kranakis et al. (Ars Comb. 38:177–192, 1994) showed that L(G _n ²)=2n−1 and and conjectured that, for all d≥3, We prove the conjecture for d=3 by showing the lower bound for L(G _n ³). For d=4, we prove that For general d, we give new estimates on L(G _n ^d ) that are very close to the conjectured value. The new lower bound of improves previous result by Collins and Moret (Inf. Process. Lett. 68:317–319, 1998), while the new upper bound of differs from the conjectured value only in the lower order terms. For arbitrary point sets, we include an exact bound on the minimum number of links needed in an axis-aligned path traversing any planar n-point set. We obtain similar tight estimates (within 1) in any number of dimensions d. For the general problem of traversing an arbitrary set of points in ℝ ^d with an axis-aligned spanning path having a minimum number of links, we present a constant ratio (depending on the dimension d) approximation algorithm. Work by A. Dumitrescu was partially supported by NSF CAREER grant CCF-0444188. Work by F. Hurtado was partially supported by projects MECMTM2006-01267 and Gen. Cat. 2005SGR00692. Work by P. Valtr was partially supported by the project 1M0545 of the Ministry of Education of the Czech Republic.     

On-line construction of the upper envelope of triangles and surface patches in three dimensions

In this paper, we describe a randomized incremental algorithm for computing the upper envelope (i.e., the pointwise maximum) of a set of n triangles in three dimensions. This algorithm is an on-line algorithm. It is structure-sensitive: the expected cost of inserting the n-th triangle is O(log nΣ_r=1ⁿτ(r)/r²) and depends on the expected size τ(r) of an intermediate result for r triangles. Since τ(r) can be Θ(r²(r)) in the worst case, this cost is bounded in the worst case by O(n(n) log n). (The expected behaviour is analyzed by averaging over all possible orderings of the input.) The main new characteristics is the use of a two-level history graph. (The history graph is an auxiliary data structure maintained by randomized incremental algorithms.) Our algorithm is fairly simple and appears to be efficient in practice. It extends to surfaces and surface patches of fixed maximum algebraic degree.     

An elementary algorithm for reporting intersections of red/blue curve segments

Let E_r and E_b be two sets of x-monotone and non-intersecting curve segments, E=E_rE_b and |E|=n. We give a new sweep-line algorithm that reports the k intersecting pairs of segments of E. Our algorithm uses only three simple predicates that allow to decide if two segments intersect, if a point is left or right to another point, and if a point is above, below or on a segment. These three predicates seem to be the simplest predicates that lead to subquadratic algorithms. Our algorithm is almost optimal in this restricted model of computation. Its time complexity is O(nlogn+kloglogn) and it requires O(n) space.     

I/O-efficient algorithms for computing planar geometric spanners

We present I/O-efficient algorithms for computing planar Steiner spanners for point sets and sets of polygonal obstacles in the plane.     

Minimum weight pseudo-triangulations

We consider the problem of computing a minimum weight pseudo-triangulation of a set of n points in the plane. We first present an -time algorithm that produces a pseudo-triangulation of weight which is shown to be asymptotically worst-case optimal, i.e., there exists a point set for which every pseudo-triangulation has weight , where is the weight of a minimum weight spanning tree of . We also present a constant factor approximation algorithm running in cubic time. In the process we give an algorithm that produces a minimum weight pseudo-triangulation of a simple polygon.     

Link distance and shortest path problems in the plane

This paper describes algorithms to compute Voronoi diagrams, shortest path maps, the Hausdorff distance, and the Fréchet distance in the plane with polygonal obstacles. The underlying distance measures for these algorithms are either shortest path distances or link distances. The link distance between a pair of points is the minimum number of edges needed to connect the two points with a polygonal path that avoids a set of obstacles. The motivation for minimizing the number of edges on a path comes from robotic motions and wireless communications because turns are more difficult in these settings than straight movements.Link-based Voronoi diagrams are different from traditional Voronoi diagrams because a query point in the interior of a Voronoi face can have multiple nearest sites. Our site-based Voronoi diagram ensures that all points in a face have the same set of nearest sites. Our distance-based Voronoi diagram ensures that all points in a face have the same distance to a nearest site.The shortest path maps in this paper support queries from any source point on a fixed line segment. This is a middle-ground approach because traditional shortest path maps typically support queries from either a fixed point or from all possible points in the plane.The Hausdorff distance and Fréchet distance are fundamental similarity metrics for shape matching. This paper shows how to compute new variations of these metrics using shortest paths or link-based paths that avoid polygonal obstacles in the plane.     

The point in polygon problem for arbitrary polygons

A detailed discussion of the point in polygon problem for arbitrary polygons is given. Two concepts for solving this problem are known in literature: the even–odd rule and the winding number, the former leading to ray-crossing, the latter to angle summation algorithms. First we show by mathematical means that both concepts are very closely related, thereby developing a first version of an algorithm for determining the winding number. Then we examine how to accelerate this algorithm and how to handle special cases. Furthermore we compare these algorithms with those found in literature and discuss the results.