An Analysis of Abstraction in Problem Solving

A quantitative model of abstraction in problem solving is presented which explains how and to what extent it reduces the amount of search necessary to solve a problem. It is shown that a single level of abstraction can reduce search time by a factor of the square root of the size of the original space. Multiple hierarchical levels of abstraction can reduce the search complexity of a problem from linear in the size of the original problem space to logarithmic

An Analysis of Abstraction in Problem Solving

A quantitative model of abstraction in problem solving is presented which explains how and to what extent it reduces the amount of search necessary to solve a problem. It is shown that a single level of abstraction can reduce search time by a factor of the square root of the size of the original space. Multiple hierarchical levels of abstraction can reduce the search complexity of a problem from linear in the size of the original problem space to logarithmic

A*+BFHS: A Hybrid Heuristic Search Algorithm

We present a new algorithm A*+BFHS for solving problems with unit-cost operators where A* and IDA* fail due to memory limitations and/or the existence of many distinct paths between the same pair of nodes. A*+BFHS is based on A* and breadth-first heuristic search (BFHS). A*+BFHS combines advantages from both algorithms, namely A*'s node ordering, BFHS's memory savings, and both algorithms' duplicate detection. On easy problems, A*+BFHS behaves the same as A*. On hard problems, it is slower than A* but saves a large amount of memory. Compared to BFIDA*, A*+BFHS reduces the search time and/or memory requirement by several times on a variety of planning domains.Comment: 8 pages, 5 figures, 1 tabl

Analyzing the performance of pattern database heuristics

Abstract We introduce a model for predicting the performance of IDA* using pattern database heuristics, as a function of the branching factor of the problem, the solution depth, and the size of the pattern databases. While it is known that the larger the pattern database, the more efficient the search, we provide a quantitative analysis of this relationship. In particular, we show that for a single goal state, the number of nodes expanded by IDA* is a fraction of (log b s + 1)/s of the nodes expanded by a brute-force search, where b is the branching factor, and s is the size of the pattern database. We also show that by taking the maximum of at least two pattern databases, the number of node expansions decreases linearly with s compared to a brute-force search. We compare our theoretical predictions with empirical performance data on Rubik's Cube. Our model is conservative, and overestimates the actual number of node expansions

Silencing of germline-expressed genes by DNA elimination in somatic cells

SummaryChromatin diminution is the programmed elimination of specific DNA sequences during development. It occurs in diverse species, but the function(s) of diminution and the specificity of sequence loss remain largely unknown. Diminution in the nematode Ascaris suum occurs during early embryonic cleavages and leads to the loss of germline genome sequences and the formation of a distinct genome in somatic cells. We found that ∼43 Mb (∼13%) of genome sequence is eliminated in A. suum somatic cells, including ∼12.7 Mb of unique sequence. The eliminated sequences and location of the DNA breaks are the same in all somatic lineages from a single individual and between different individuals. At least 685 genes are eliminated. These genes are preferentially expressed in the germline and during early embryogenesis. We propose that diminution is a mechanism of germline gene regulation that specifically removes a large number of genes involved in gametogenesis and early embryogenesis

Planning Technologies for Interactive Storytelling

Since AI planning was first proposed for the task of narrative generation in interactive storytelling (IS), it has emerged as the dominant approach in this field. This chapter traces the use of planning technologies in this area, considers the core issues involved in the application of planning technologies in IS, and identifies some of the remaining challenges

Search reduction in hierarchical distributed problem solving

Knoblock and Korf have determined that abstraction can reduce search at a single agent from exponential to linear complexity (Knoblock 1991; Korf 1987). We extend their results by showing how concurrent problem solving among multiple agents using abstraction can further reduce search to logarithmic complexity. We empirically validate our formal analysis by showing that it correctly predicts performance for the Towers of Hanoi problem (which meets all of the assumptions of the analysis). Furthermore, a powerful form of abstraction for large multiagent systems is to group agents into teams, and teams of agents into larger teams, to form an organizational pyramid. We apply our analysis to such an organization of agents and demonstrate the results in a delivery task domain. Our predictions about abstraction's benefits can also be met in this more realistic domain, even though assumptions made in our analysis are violated. Our analytical results thus hold the promise for explaining in general terms many experimental observations made in specific distributed AI systems, and we demonstrate this ability with examples from prior research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42828/1/10726_2005_Article_BF01384251.pd

Whipworm genome and dual-species transcriptome analyses provide molecular insights into an intimate host-parasite interaction.

Whipworms are common soil-transmitted helminths that cause debilitating chronic infections in man. These nematodes are only distantly related to Caenorhabditis elegans and have evolved to occupy an unusual niche, tunneling through epithelial cells of the large intestine. We report here the whole-genome sequences of the human-infective Trichuris trichiura and the mouse laboratory model Trichuris muris. On the basis of whole-transcriptome analyses, we identify many genes that are expressed in a sex- or life stage-specific manner and characterize the transcriptional landscape of a morphological region with unique biological adaptations, namely, bacillary band and stichosome, found only in whipworms and related parasites. Using RNA sequencing data from whipworm-infected mice, we describe the regulated T helper 1 (TH1)-like immune response of the chronically infected cecum in unprecedented detail. In silico screening identified numerous new potential drug targets against trichuriasis. Together, these genomes and associated functional data elucidate key aspects of the molecular host-parasite interactions that define chronic whipworm infection