Given a nite alphabet and a deterministic nite automaton on , the problem of determining whether the language recognized by the automaton contains any pangram is NP-complete. Various other language classes and problems around pangrams are analyzed.
The All Colors Shortest Path problem (ACSP) @cite_5 asks the shortest path in an undirected graph with nodes colored, under the constraint that the paths must visit all colors. The pangram problem for can be regarded as an edge-colored and directed version of All Colors Path problem. Our result shows that the problem is -complete even if the shortestness condition is dropped and just the existence of such paths is asked.
@cite_5
2240749562 2109368894 1520709705 2025904765
All Colors Shortest Path problem defined on an undirected graph aims at finding a shortest, possibly non-simple, path where every color occurs at least once, assuming that each vertex in the graph is associated with a color known in advance. To the best of our knowledge, this paper is the first to define and investigate this problem. Even though the problem is computationally similar to generalized minimum spanning tree, and the generalized traveling salesman problems, allowing for non-simple paths where a node may be visited multiple times makes All Colors Shortest Path problem novel and computationally unique. In this paper we prove that All Colors Shortest Path problem is NP-hard, and does not lend itself to a constant factor approximation. We also propose several heuristic solutions for this problem based on LP-relaxation, simulated annealing, ant colony optimization, and genetic algorithm, and provide extensive simulations for a comparative analysis of them. The heuristics presented are not the standard implementations of the well known heuristic algorithms, but rather sophisticated models tailored for the problem in hand. This fact is acknowledged by the very promising results reported. Coloring the nodes of a graph with a small number of colors is one of the most fundamental problems in theoretical computer science. In this paper, we study graph coloring in a distributed setting. Processors of a distributed system are nodes of an undirected graph G. There is an edge between two nodes whenever the corresponding processors can directly communicate with each other. We assume that distributed coloring algorithms start with an initial m-coloring of G. In the paper, we prove new strong lower bounds for two special kinds of coloring algorithms. For algorithms which run for a single communication round---i.e., every node of the network can only send its initial color to all its neighbors---, we show that the number of colors of the computed coloring has to be at least Ω(Δ2 log2 Δ+ log log m). If such one-round algorithms are iteratively applied to reduce the number of colors step-by-step, we prove a time lower bound of Ω(Δ log2 Δ+ log*m) to obtain an O(Δ)-coloring. The best previous lower bounds for the two types of algorithms are Ω(log log m) and Ω(log*m), respectively. We study the maximum differential graph coloring problem, in which the goal is to find a vertex labeling for a given undirected graph that maximizes the label difference along the edges. This problem has its origin in map coloring, where not all countries are necessarily contiguous. We define the differential chromatic number and establish the equivalence of the maximum differential coloring problem to that of k-Hamiltonian path. As computing the maximum differential coloring is NP-Complete, we describe an exact backtracking algorithm and a spectral-based heuristic. We also discuss lower bounds and upper bounds for the differential chromatic number for several classes of graphs. Graph coloring problems, in which one would like to color the vertices of a given graph with a small number of colors so that no two adjacent vertices receive the same color, arise in many applications, including various scheduling and partitioning problems. In this paper the complexity and performance of algorithms which construct such colorings are investigated. For a graph G , let k( G ) denote the minimum possible number of colors required to color G and, for any graph coloring algorithm A , let A ( G ) denote the number of colors used by A when applied to G . Since the graph coloring problem is known to be “NP-complete,” it is considered unlikely that any efficient algorithm can guarantee A ( G ) = k( G ) for all input graphs. In this paper it is proved that even coming close to khgr;( G ) with a fast algorithm is hard. Specifically, it is shown that if for some constant r d there exists a polynomial-time algorithm A which guarantees A ( G ) ≤ r ·k( G ) + d , then there also exists a polynomial-time algorithm A which guarantees A ( G ) = k( G ).