Introduction

Study of time series has a history much older than modem system theory. Probability theorists, statisticians and economitricians have all contributed to our understanding of time series over the past several decades, as is evidenced by many well established books. One may wonder what system theory can add to this well established field and doubt if any new perspective or insight can be gained by this relative newcomer to the field. The history of science shows us, however, that a given problem can fruitfully be examined by different disciplines, partly because when it is viewed from new perspectives, implications of alternative assumptions are explored by researchers with different backgrounds or interests, and partly because new techniques developed elsewhere are imported to explore areas left untouched by the discipline in which the problem originated. Although a latecomer to the field of time series analysis, system theory has introduced a set of viewpoints, concepts and tools sufficiently different from the traditional ones, and these have proved effective in dealing with vector-valued time-indexed data.