BOOK REVIEWS a mathematically unsophisticated audience, and so mathematical arguments for the most part proceed formally or are kept, on an intuitive level. On occasion, however, the unavoidable amount of imprecision which such an approach entails is compounded through carelessness. For example, the adjective "continuous" is used in a number of objectionable ways, not the least of which is the definition of a "continuous signal" as "any function of a continuous variable." In a somewhat different category is the incorrect assertion made in a footnote on p. 48 to the effect that, a necessary condition for a differential equation to uniquely specify a system is that the system lie nonanticipative. Despite these critical comments, a reader armed with the appropriate mathematical caveats will find much of value in this book, which is likely to be successful as an introductory text on modern system theory.

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Clusters of galaxies may contain radio sources if they still experience successive mergers at present. This has been confirmed by radio observations that about 30% of nearby clusters possess radio halos. We present a theoretical prediction of radio cluster counts using a semi-analytic approach which incorporates the empirical correlation between radio power and dynamical mass of clusters, and the cluster mass function described by the Press-Schechter formalism. The total population of radio clusters over the whole sky and their redshift distribution are given.

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