Dear colleagues

I am sending you a review I have witten on the book "Brownian Agents
and Active Particles" by Frank Schweitzer.

Best wishes

Enrico Scalas
--
F. Schweitzer
Brownian Agents and Active Particles
Collective Dynamics in the Natural and Social Sciences
With a Foreword by J. Doyne Farmer

Springer Verlag, 2003
Hard cover, 64.15 euro

Review by Enrico Scalas

The field discussed by the book of Frank Schweitzer has been recently
popularized by a novel of Michael Crichton: "Prey". If you want to
know more about flocks and swarms and you are interested not only in
science fiction, but also in science, the work of Frank Schweitzer is
the right place to start with. The popularization of a rather esoteric
scientific field, as the one discussed by Schweitzer, is a clear sign of
its increasing relevance.

As usual, Crichton's book has a list of references and, as usual, almost
only research performed in the United States is quoted. You will find many
clues on this US tendency to completely overlook the work done elsewhere
in the world also in the book by Schweitzer. Schweitzer's bibliography
does justice to the huge efforts taking place in Germany and in Europe.
Even if it is very difficult to give proper credits dealing with such a
large range of issues as Schweitzer does, his bibliography is to be
praised. His book is about Brownian agents, a smart generalization of
Brownian particles including internal states. Brownian agents can be
effectively used as phenomenological models for many natural and social
phenomena including track formation in biological systems, movement and
trail formation of humans, evolutionary optimization strategies, urban
growth, quantitative sociodynamics, spatial opinion structures in social
systems.

Schweitzer's approach is gradual. The first four chapters are devoted to
introducing more and more complexities and subtleties in the Brownian
agent models, and the focus is on the models themselves rather than on the
systems. Reading and understanding these chapters may be a difficult
time-consuming task, but the reward is high. Starting from chapter five
(on tracks and trail formation in biological systems) and ending with
chapter ten (on opinion formation), the reader can amuse him/herself in
dealing with models of real systems and devote his/her attention to the
more relevant issues for his/her research.

This book contains some gems. My favorite one is in chapter nine: the
discussion of a spatial dynamic model for the labor market introduced by
the well-known US economist Paul Krugman where "workers are assumed to
move toward locations that offer them higher real wages". Schweitzer shows
not only that Krugman's model is nothing else that an instance of a
selection equation of the Fisher-Eigen type, but also, using the formalism
developed previously, he can easily generalize it and
question the economic meaning of the assumptions leading to Krugman's
equations.

A limit of this book is that the comparison between theoretical results
and available empirical data is not always discussed. In many cases,
however, not many empirical data are available or of good quality. To this
respect, this book can become a stimulus for further empirical research in
the fields outlined.

Finally, as in many contemporary books, there are various misprints
scattered throughout the chapters. However, these are minor and do not
hamper the understanding of the text.

I can recommend this book to all those working in the field of
complex systems. They will find a detailed survey of the Brownian agent
method and they might get good hints for further research in some of the
fascinating fields herein discussed.