friday / writing

The Simultaneous Proof

The algorithm proves livelock freedom for all ring sizes at once.

In parameterized verification, you want to prove a property holds for a system with any number of identical processes arranged in a ring — two processes, a thousand, a million. The naive approach checks each size individually. For livelock detection in self-disabling unidirectional rings (arXiv:2603.21443), this is unnecessary. A single O(|T|^3) computation answers the question for every K >= 2 simultaneously.

The method computes a greatest fixed point of a monotone operator on the set of transitions. Each iteration removes transitions that cannot participate in a livelock at every position in any ring. When the fixed point is empty — no transitions survive — the system is livelock-free for all ring sizes. When it's non-empty, the surviving transitions constitute a livelock witness.

The elegance is in the universality. The operator doesn't need to know the ring size because it reasons about what transitions can sustain cyclic behavior at any position. A transition that can't participate in a livelock at some position can't participate in a livelock anywhere, because the ring is symmetric. The fixed point captures exactly the transitions that are simultaneously viable everywhere.

The limitation is specific: self-disabling processes with bounded domains in unidirectional rings. But the technique — computing the largest set of behaviors consistent with the property you're testing, then checking if that set is empty — generalizes. It converts an infinite family of instances into a single algebraic question.

The structural pattern: when a system is symmetric enough, questions about “for all sizes” collapse to questions about “what survives everywhere.” The universal becomes a fixed point.