On the Abstract Structure of the Behavioral Approach to Systems Theory

We revisit the behavioral approach to systems theory and make explicit the abstract pattern that governs it. Our end goal is to use that pattern to understand interaction-related phenomena that emerge when systems interact. Rather than thinking of a system as a pair (mathbbU, B), we begin by thinking of it as an injective map from B rightarrow mathbbU. This relative perspective naturally brings about the sought structure, which we summarize in three points. First, the separation of behavioral equations and behavior is developed through two spaces, one of syntax and another of semantics, linked by an interpretation map. Second, the notion of interconnection and variable sharing is shown to be a construction of the same nature as that of gluing topological spaces or taking amalgamated sums of algebraic objects. Third, the notion of interconnection instantiates to both the syntax space and the semantics space, and the interpretation map is shown to preserve the interconnection when going from syntax to semantics. This pattern, in its generality, is made precise by borrowing very basic constructs from the language of categories and functors.