Stochastic Formal Methods for Hybrid Systems

TL;DR

This paper introduces a probabilistic framework using formal methods to bound errors in hybrid systems, applicable to critical domains like aerospace and nuclear power, ensuring high reliability through concrete error bounds.

Contribution

It develops a formal theory for stochastic analysis of hybrid systems, providing formulas and applications for error probability bounds with formal verification tools.

Findings

Bounded accumulated errors with high confidence in hybrid systems.

Concrete formulas for error probability estimates.

Validated applications in safety-critical systems.

Abstract

We provide a framework to bound the probability that accumulated errors were never above a given threshold on hybrid systems. Such systems are used for example to model an aircraft or a nuclear power plant on one side and its software on the other side. This report contains simple formulas based on L\'evy's and Markov's inequalities and it presents a formal theory of random variables with a special focus on producing concrete results. We selected four very common applications that fit in our framework and cover the common practices of hybrid systems that evolve for a long time. We compute the number of bits that remain continuously significant in the first two applications with a probability of failure around one against a billion, where worst case analysis considers that no significant bit remains. We are using PVS as such formal tools force explicit statement of all hypotheses and prevent incorrect uses of theorems.