  The concept of boolean autonomous deterministic regular asynchronous system
has its origin in switching theory, the theory of modeling the switching
circuits from the digital electrical engineering. The attribute boolean vaguely
refers to the Boole algebra with two elements; autonomous means that there is
no input; determinism means the existence of a unique state function; and
regular indicates the existence of a function
$\Phi:\{0,1\}^{n}\rightarrow\{0,1\}^{n},\Phi=(\Phi_{1},...,\Phi_{n})$ whose
coordinates iterate independently on each other. Time is discrete or
continuous.
  The flows are these that result by analogy with the dynamical systems.
  The 'nice' discrete time and real time functions that the (boolean)
asynchronous systems work with are called signals and periodicity is a very
important feature in Nature.
  In the first two Chapters we give the most important concepts concerning the
signals and periodicity. The periodicity properties are used to characterize
the eventually constant signals in Chapter 3 and the constant signals in
Chapter 4. Chapters 5,...,8 are dedicated to the eventually periodic points,
eventually periodic signals, periodic points and periodic signals. Chapter 9
shows constructions that, given an (eventually) periodic point, by changing
some values of the signal, change the periodicity properties of the point.
  The book addresses to researchers in systems theory and computer science, but
it is also interesting to those that study periodicity itself. From this last
perspective, the binary signals may be thought of as functions with finitely
many values.
