Bounded Satisfiability for PCTL

TL;DR

This paper introduces a method for checking the satisfiability of PCTL formulas by searching for bounded, implementable models using SMT solving, aiding practical system design verification.

Contribution

It proposes a bounded satisfiability checking procedure for PCTL formulas, reducing the problem to SMT and enabling practical model verification.

Findings

Applicable to sanity checking in system design

Can determine the existence of small, implementable models

Implemented techniques show practical utility

Abstract

While model checking PCTL for Markov chains is decidable in polynomial-time, the decidability of PCTL satisfiability, as well as its finite model property, are long standing open problems. While general satisfiability is an intriguing challenge from a purely theoretical point of view, we argue that general solutions would not be of interest to practitioners: such solutions could be too big to be implementable or even infinite. Inspired by bounded synthesis techniques, we turn to the more applied problem of seeking models of a bounded size: we restrict our search to implementable -- and therefore reasonably simple -- models. We propose a procedure to decide whether or not a given PCTL formula has an implementable model by reducing it to an SMT problem. We have implemented our techniques and found that they can be applied to the practical problem of sanity checking -- a procedure that allows a system designer to check whether their formula has an unexpectedly small model.