Grammar-based Representation and Identification of Dynamical Systems

In this paper we propose a novel approach to identify dynamical systems. The method estimates the model structure and the parameters of the model simultaneously, automating the critical decisions involved in identification such as model structure and complexity selection. In order to solve the combined model structure and model parameter estimation problem, a new representation of dynamical systems is proposed. The proposed representation is based on Tree Adjoining Grammar, a formalism that was developed from linguistic considerations. Using the proposed representation, the identification problem can be interpreted as a multi-objective optimization problem and we propose a Evolutionary Algorithm-based approach to solve the problem. A benchmark example is used to demonstrate the proposed approach. The results were found to be comparable to that obtained by state-of-the-art non-linear system identification methods, without making use of knowledge of the system description.Comment: Submitted to European Control Conference (ECC) 201

Transition from the macrospin to chaotic behaviour by a spin-torque driven magnetization precession of a square nanoelement

We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steady-state precession of a thin magnetic nanoelement exhibit a complicate transition from the quasi-macrospin to the chaotic behaviour with the increasing element size. For nanoelement parameters typical for those used experimentally we have found that the macrospin approximation becomes invalid already for very small nanoelement sizes (~ 30 nm), in contrast to the previously reported results (Li and Zhang, Phys. Rev. B, vol. B68, 024404-1 (2003))Comment: Submitted to Phys. Rev.

Micromagnetic simulations of the magnetization precession induced by a spin polarized current in a point contact geometry

This paper is devoted to numerical simulations of the magnetization dynamics driven by a spin-polarized current in extended ferromagnetic multilayers when a point-contact setup is used. We present (i) detailed analysis of methodological problems arising by such simulations and (ii) physical results obtained on a system similar to that studied in Rippard et al., Phys. Rev. Lett., v. 92, 027201 (2004). We demonstrate that the usage of a standard Slonczewski formalism for the phenomenological treatment of a spin-induced torque leads to a qualitative disagreement between simulation results and experimental observations and discuss possible reasons for this discrepancy.Comment: Invited paper on MMM2005 (San Jose); accepted for publication in J. Applied Physic

On the mechanical simulation of habit-forming and learning

This paper discusses digital techniques by which habit-forming and learning may be simulated. After classifying the types of simulation mechanisms it discusses types of habit-forming and learning to be simulated, focusing attention upon reinforcement. It uses the language of computer programming to describe the flow of control, and the language of mathematical probability to analyze the effect of various reinforcement functions on the asymptotic behavior of simulating programs. It shows further, again in programming terms, how the “delayed random selector” part of the simulating process may be “factored out” as a separate unit applicable either to habit-forming or learning, which latter are distinguished by whether the reinforcements are applied immediately or upon “comparison with a goal.”Several reinforcement models are considered, including the “linear asymptotic” model used extensively by Bush and Mosteller, two simple “absorbing boundary” models, and a “nonlinear asymptotic” model currently being investigated by Bush, Galanter, and Luce. A sketch is given of the Harris-Bellman-Shapiro analysis of the linear asymptotic model. Contrasted with this, a complete analysis is given of the simpler absorbing boundary model, with explicit proof of eventual absorption, and formulae for probability of absorption in n trials, and the expected number of steps to absorption. Finally, a special example is given of the second absorbing boundary model to show how its structure differs from the others

Mother Jones: Ireland to North America to Ireland

Although we don\u27t hear her name so often anymore, Mother Jones was one of the great figures of the early twentieth century. She and her family were refugees from the Famine, and I want to argue here that her early life in Ireland, Canada, and the United States molded her, made her the great crusader for social justice and tribune of the working class that she became as an old woman. Freedoms just another word for nothing left to lose, Kris Kristofferson has written, words that well describe the life of Mother Jones

Adapting to Computer Science

Although I am not an engineer who adapted himself to computer science but a mathematician who did so, I am familiar enough with the development, concepts, and activities of this new discipline to venture an opinion of what must be adapted to in it. Computer and Information Science is known as Informatics on the European continent. It was born as a distinct discipline barely a generation ago. As a fresh young discipline, it is an effervescent mixture of formal theory, empirical applications, and pragmatic design. Mathematics was just such an effervescent mixture in western culture from the renaissance to the middle of the twentieth century. It was then that the dynamic effect of high speed, electronic, general purpose computers accelerated the generalization of the meaning of the word computation This caused the early computer science to recruit not only mathematicians but also philosophers (especially logicians), linguists, psychologists, even economists, as well as physicists, and a variety of engineers. Thus we are, perforce, discussing the changes and adaptations of individuals to disciplines, and especially of people in one discipline to another. As we all know, the very word discipline indicates that there is an initial special effort by an individual to force himself or herself to change. The change involves adaptation of one\u27s perceptions to a special way of viewing certain aspects of the - world, and also one\u27s behavior in order to produce special results. For example we are familiar with the enormous prosthetic devices that physicists have added to their natural sensors and perceptors in order to perceive minute particles and to smash atoms in order to do so (at, we might add, enormous expense, and enormous stretching of computational activity). We are also familiar with the enormously intricate prosthetic devices mathematicians added to their computational effectors, the general symbol manipulators, called computers

Sister Carrie---Theodore Dreiser, New York: Doubleday, Page & Co., 1900

Facing the naturalistic, nonjudgmental rendering in Sister Carrie of the stresses of survival in Chicago and New York was seen by some as scandalous. Nonetheless, Theodore Dreiser’s first novel eventually became an American classic and has been published in countless editions. The Heritage edition (1937) includes illustrations by Reginald Marsh (1898– 1954), including one in which the main character, a country girl on a train bound for Chicago, is approached by a salesman whose mistress she will eventually become