Improved sorting networks with O(log n) depth

The sorting network described by Ajtai, KomlOs and Szemeredi was the first to achieve a depth of O(Iog n). The networks introduced here are simplifications and improvements based strongly on their work. While the constants obtained for the depth bound still prevent the construction being of practical value, the structure of the presentation offers a convenient basis for further development

On the complexity of string folding

A fold of a finite string S over a given alphabet is an embedding of S in some fixed infinite grid, such as the square or cubic mesh. The score of a fold is the number of pairs of matching string symbols which are embedded at adjacent grid vertices. Folds of strings and sets of strings in two- and three-dimensional meshes are considered, and the corresponding problems of optimizing the score or achieving a given target score are shown to be NP-hard

The depth of all Boolean functions

It is shown that every Boolean function of n arguments has a circuit of depth n+1 over the basis {f|f:{0,1}^2 -> {0,1}}

Finding the median

An algorithm is described which determines the median of n elements using in the worst case a number of comparison asymptotic to 3n

On log concavity for order-preserving and order-non-reversing maps of partial orders

Stanley used the Aleksandrov-Fenchel inequalities from the theory of nixed volumes to prove the following result. Let P be a partially ordered set with n elements, and let x ∊ P. If Ni* is the number of linear extensions , ⋋ : P + (1 , 2,...,n) satisfying ⋋ (x) = i, then the sequence N*1,…,N*n is log concave (and therefore unimodal). Here the analogous results for both order-preserving and order-non-reversing maps are proved using an explicit injection. Further, if vc is the number of order-preserving maps of P into a chain of length c, then vc is shown to be 1-og concave, and the corresponding result is established for order-non-reversing maps

Partitioning space for range queries

It is shown that, given a set S of n points in R3, one can always find three planes that form an eight-partition of S, that is, a partition where at most n/8 points of S lie in each of the eight open regions. This theorem is used to define a data structure, called an octant tree, for representing any point set in R3. An octant tree for n points occupies O(n) space and can be constructed in polynomial time. With this data structure and its refinements, efficient solutions to various range query problems in 2 and 3 dimensions can be obtained, including (1) half-space queries: find all points of S that lie to one side of any given plane; (2) polyhedron queries: find all points that lie inside (outside) any given polyhedron; and (3) circular queries in R2: for a planar set S, find all points that lie inside (outside) any given circle. The retrieval time for all these queries is T(n)=O(na + m) where a= 0.8988 (or 0.8471 in case (3)) and m is the size of the output. This performance is the best currently known for linear-space data structures which can be deterministically constructed in polynomial time

Combinatorial and computational aspects of multiple weighted voting games

Weighted voting games are ubiquitous mathematical models which are used in economics, political science, neuroscience, threshold logic, reliability theory and distributed systems. They model situations where agents with variable voting weight vote in favour of or against a decision. A coalition of agents is winning if and only if the sum of weights of the coalition exceeds or equals a specified quota. We provide a mathematical and computational characterization of multiple weighted voting games which are an extension of weighted voting games1. We analyse the structure of multiple weighted voting games and some of their combinatorial properties especially with respect to dictatorship, veto power, dummy players and Banzhaf indices. Among other results we extend the concept of amplitude to multiple weighted voting games. An illustrative Mathematica program to compute voting power properties of multiple weighted voting games is also provided

Dense edge-disjoint embedding of binary trees in the mesh

We present an embedding of the complete binary tree with n leaves in the Vn x Vn mesh, for any n = 2exp(2m) where m is a positive integer. The embedding has the following properties: at most two tree nodes (one of which is a leaf) are mapped onto each mesh node, paths of the tree are mapped onto edge-disjoint paths in the mesh (each mesh edge considered as two anti-parallel directed edges) and the maximum distance from a leaf to the root of the tree is Vn + O (log n) mesh steps. This embedding facilitates efficient implementation of many P-RAM algorithms on the mesh, particularly those using the balanced binary tree technique. Such an embedding offers greater flexibility of use and improves the time complexity of these implementations by a constant factor compared with previously described embeddings