34207468
Scientific Discovery and Simplicity of Method
3-Benzoyl-3-thiocyanatopropionic acid, alkyl esters are prepared by the reaction of a 3-benzoyl-3-halopropionic acid alkyl ester with an alkali metal thiocyanate. The products so produced exhibit antitubercular activity.
36481911
Retrospective on "Causality in Device Behavior"
I first encountered causality in the late 1940s, while seeking to formalize the concept of political power (causation of the behavior of one person by another). This concept turned out to be closely related to the concept of identification in simultaneous equation models, then being investigated by the Cowles Commission for Research in Economics. I published in 1952-54 several papers [ 13,14 ] introducing a definition of causal ordering and exploring both the mathematical properties and the underlying semantics of the formalism. When, years later, I read the early papers on qualitative reasoning in AI, it seemed to me that causal ordering, and related ideas for applying the method of comparative statics (with which I had also had experience in my work in economics) provided a more powerful method than those being proposed.
5520100
The Theory of Problem Solving
It is now about fifteen years since the first computer programs were written and tested that used the method of heuristic search to solve problems. Dozens of such programs, some designed for specific task domains, others claiming various degrees of generality, have now been described in the literature, and many experiments with their performance have been reported and analysed. It is an appropriate time to ask what has been learned from these experiments about the general theory of problem solving, and to try to summarize the present state of that theory.
17536925
Building the Teraflops/Petabytes Production Supercomputing Center
Building the Teraflops/Petabytes Production Supercomputing Center Horst D. Simon, William T. C. Kramer, and Robert F. Lucas National Energy Research Scientific Computing Center (NERSC), Mail Stop 50B-4230, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 {hdsimon, wtkramer, rflucas}@lbl.gov Abstract. In just one decade, the 1990s, supercomputer centers have undergone two fundamental transitions which require rethinking their operation and their role in high performance computing. The first transition in the early to mid-1990s resulted from a technology change in high performance computing architecture. Highly parallel distributed memory machines built from commodity parts increased the operational complexity of the supercomputer center, and required the introduction of intellectual services as equally important components of the center. The second transition is happening in the late 1990s as centers are introducing loosely coupled clusters of SMPs as their premier high performance computing platforms, while dealing with an ever-increasing volume of data. In addition, increasing network bandwidth enables new modes of use of a supercomputer center, in particular, computational grid applications. In this paper we describe what steps NERSC is taking to address these issues and stay at the leading edge of supercomputing centers. Introduction In just one decade, the 1990s, supercomputer centers have undergone two fundamental transitions which require a rethinking of the basic tenets of their operation and their role in the high performance computing (HPC) world. The first transition in the early to mid 1990s was a result of a technology change in high performance computing architecture. The introduction of highly parallel distributed memory machines built from commodity parts increased the operational complexity of the supercomputer center, and required the introduction of intellectual services as equally important components of the center. We have only recently completed this transition and developed the tools necessary to bring the revolution of the mid-1990s to a successful conclusion. Now three new developments are appearing which will again force us to step up to new challenges in supercomputer center management: (1) yet another change in the architecture of supercomputing platforms, (2) the increasing importance of managing large volumes of scientific data, and (3) the deployment of a new generation of high-speed wide-area
16243913
Experiments with a Heuristic Compiler
This report describes some experiments in constructing a compiler that makes use of heuristic problem~solving techniques such as those incorporated in the General Problem Solver (GPS) [1]. The experiments were aimed at the dual objectives of throwing light on some of the problems of constructing more powerful programming languages and compilers, and of testing whether the task of writing a computer program can be regarded as a "problem" in the sense in which that term is used in GPS. The present paper is concerned primarily with the second objective--with analyzing some of the problem-solving processes that are involved in writing computer programs. At the present stage of their development, no claims will be made for the heuristic programming procedures described here as practical approaches to the construction of compilers. Their interest lies in what they teach us about the nature of the programming task.
15281282
Fast multilevel implementation of recursive spectral bisection for partitioning unstructured problems
SUMMARY If problems involving unstructured meshes are to be solved efficiently on distributed-memory parallel computers, the meshes must be partitioned and distributed across processors in a way that balances the computational load and minimizes communication. The recursive spectral bisection method (RSB) has been shown to be very effective for such partitioning problems compared to alternative methods, but RSB in its simplest form is expensive. Here a multilevel version of RSB is introduced that attains about an order-of-magnitude improvement in run time on typical examples. 1. INTRODUCTION Unstructured meshes are used in several large-scale scientific and engineering problems, including finite-volume methods for computational fluid dynamics and finite-element methods for structural analysis. If unstructured problems such as these are to be solved on distributed-memory parallel computers, their data structures must be partitioned and distributed across processors; if they are to be solved efficiently, the partitioning must niaximize load balance and minimize interprocessor communication. Recently, the recursive spectral bisection method (RSB)[l] has been shown to be very effective for such partitioning problems compared to alternative methods. Unfortunately, RSB in its simplest form is expensive. We shall describe a multilevel version of RSB that attains about im order-of-magnitude improvement in run time on typical examples.
17442679
Inner Product Spaces for Bayesian Networks
Bayesian networks have become one of the major models used for statistical inference. We study the question whether the decisions computed by a Bayesian network can be represented within a low-dimensional inner product space. We focus on two-label classification tasks over the Boolean domain. As main results we establish upper and lower bounds on the dimension of the inner product space for Bayesian networks with an explicitly given (full or reduced) parameter collection. In particular, these bounds are tight up to a factor of 2. For some nontrivial cases of Bayesian networks we even determine the exact values of this dimension. We further consider logistic autoregressive Bayesian networks and show that every sufficiently expressive inner product space must have dimension at least Ω(n2), where n is the number of network nodes. We also derive the bound 2Ω(n) for an artificial variant of this network, thereby demonstrating the limits of our approach and raising an interesting open question. As a major technical contribution, this work reveals combinatorial and algebraic structures within Bayesian networks such that known methods for the derivation of lower bounds on the dimension of inner product spaces can be brought into play.