This paper is one of a series in which the ideas of category theory are
applied to problems of system theory. As with the three principal
earlier papers, [1-3], the emphasis is on study of the realization
problem, or the problem of associating with an input-output description
of a system an internal description with something analogous to a
state-space. In this paper, several sorts of machines will be discussed,
which arrange themselves in the following hierarchy: Input process
Machine Output process (Tree automaton) Machine State-behavior Machine I
Adjoint Machine .(Sequential Machine) ., I Decomposable Machine (Linear
System, Group Machine) Each member of the hierarchy includes members
below it; examples are included in parentheaes, and each example is at
its lowest possible point in the hierarchy. There are contrived examples
of output process machines and IV state-behavior machines which are not
adjoint machines [3], but as yet, no examples with the accepted
stature of linear systems [4], group machines [5, 6], sequential
machines [7, Ch. 2], and tree automata [7, Ch. 4].