The study of the connections between mathematical automata and for- mal
logic is as old as theoretical computer science itself. In the founding
paper of the subject, published in 1936, Turing showed how to describe
the behavior of a universal computing machine with a formula of first-
order predicate logic, and thereby concluded that there is no algorithm
for deciding the validity of sentences in this logic. Research on the
log- ical aspects of the theory of finite-state automata, which is the
subject of this book, began in the early 1960's with the work of J.
Richard Biichi on monadic second-order logic. Biichi's investigations
were extended in several directions. One of these, explored by
McNaughton and Papert in their 1971 monograph Counter-free Automata, was
the characterization of automata that admit first-order behavioral
descriptions, in terms of the semigroup- theoretic approach to automata
that had recently been developed in the work of Krohn and Rhodes and of
Schiitzenberger. In the more than twenty years that have passed since
the appearance of McNaughton and Papert's book, the underlying semigroup
theory has grown enor- mously, permitting a considerable extension of
their results. During the same period, however, fundamental
investigations in the theory of finite automata by and large fell out of
fashion in the theoretical com- puter science community, which moved to
other concerns.