Abstract. Hybrid systems combine discrete and continuous dynamics. We introduce a semantics for such systems consisting of a coalgebra together with a monoid action. The coalgebra captures the (discrete) operations on a state space that can be used by a client (like in the semantics of ordinary (non-temporal) object-oriented systems). The monoid action captures the in uence of time on the state space, where the monoids that we consider are the natural numbers monoid (N; 0; +) of discrete time, and the positive reals monoid (R 0 ; 0; +) of real time. Based on this semantics we develop a hybrid speci cation formalism with timed method applications: it involves expressions like s:meth@ , with the following meaning: in state s let the state evolve for units of time (according to the monoid action), and then apply the (coalgebraic) method meth. In this formalism we specify various (elementary) hybrid systems, investigate their correctness, and display their behaviour in simulations. We further de ne a suitable notion of homomorphism between our hybrid models (of coalgebras plus monoid actions), in such a way that minimal realizations (of the speci ed behaviour) appear as terminal models. We identify the terminal models of our example speci cations, and give general constructions. This leads to an investigation of various topics related to terminality: bisimilarity, behaviour-realization adjunctions, re nement (with a coinductive proof method for correctness) and inheritance. In a nal section we brie y discuss non-homogeneous hybrid systems (with continuous inputs).
Around the same time, Jacobs REF suggested an object oriented coalgebraic framework where hybrid systems are regarded as coalgebras equipped with a monoid action: coalgebras define the discrete transitions, and monoid actions the continuous evolutions.
Object-Oriented Hybrid Systems of Coalgebras plus Monoid Actions
Algebraic Methodology and Software Technology (AMAST
Computer Science