A Complete Uniform Substitution Calculus for Differential Dynamic Logic

TL;DR

This paper presents a simplified, uniform substitution-based proof calculus for differential dynamic logic, enabling more straightforward implementations and internalization of differential invariants and derivatives.

Contribution

It introduces a uniform substitution calculus for differential dynamic logic, replacing schema-based axioms with a finite set of axioms and internalizing differential reasoning.

Findings

The calculus is sound and relatively complete.

It simplifies implementation by using uniform substitutions.

Differential forms enable internal reasoning about differential invariants.

Abstract

This article introduces a relatively complete proof calculus for differential dynamic logic (dL) that is entirely based on uniform substitution, a proof rule that substitutes a formula for a predicate symbol everywhere. Uniform substitutions make it possible to use axioms instead of axiom schemata, thereby substantially simplifying implementations. Instead of subtle schema variables and soundness-critical side conditions on the occurrence patterns of logical variables to restrict infinitely many axiom schema instances to sound ones, the resulting calculus adopts only a finite number of ordinary dL formulas as axioms, which uniform substitutions instantiate soundly. The static semantics of differential dynamic logic and the soundness-critical restrictions it imposes on proof steps is captured exclusively in uniform substitutions and variable renamings as opposed to being spread in delicate ways across the prover implementation. In addition to sound uniform substitutions, this article introduces differential forms for differential dynamic logic that make it possible to internalize differential invariants, differential substitutions, and derivatives as first-class axioms to reason about differential equations axiomatically. The resulting axiomatization of differential dynamic logic is proved to be sound and relatively complete.