# LECTURE 3: Models, Over-approximations and Robustness

## LECTURE 3: Models, Over-approximations and Robustness

Eugenio Moggi University of Genova |

Hybrid systems, and related formalisms, have been successfully used to model Cyber- Physical Systems. The usual definition of "reachability", in terms of the reflexive and transitive closure of a transition relation, is unsound for Zeno systems. We propose "safe reachability", which gives an over-approximation of the set of states that are "reachable in finite time" from a set of initial states. Moreover, mathematical models are always a simplification of the system they are meant to describe, and one must be aware of this mismatch, when using these models for system analyses. In safety analysis it is acceptable to use over-approximations of the system behavior, indeed they are the bread and butter of counterexample guided abstraction refinement (CEGAR). We propose a notion of safe (time-bounded) reachability, which is also robust wrt arbitrary small overapproximations, and argue that it is particularly appropriate for safety analysis.