On Minimization and Learning of Deterministic ω-Automata in the Presence of Don't Care Words
Pith reviewed 2026-05-24 11:27 UTC · model grok-4.3
The pith
The number of priorities in deterministic parity automata can be minimized efficiently under arbitrary don't care words.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The number of priorities in a deterministic parity automaton can be minimized in polynomial time under an arbitrary don't care set. This is obtained from a general minimization procedure applicable to deterministic parity automata that possess an informative right-congruence. For the subclass of weak deterministic Büchi automata and don't care languages whose right-congruence is trivial, the minimal automaton is unique and can be computed efficiently; it is characterized by the congruence that merges states indistinguishable under the don't care condition. In contrast, when the automata have informative right-congruence, even with trivial don't care congruence, there is no unique minimal wDB
What carries the argument
The general minimization procedure for deterministic parity automata under don't care conditions that reduces the priority count while preserving acceptance behavior.
If this is right
- An efficient algorithm exists to minimize priorities in any deterministic parity automaton given any don't care set.
- The same general result yields an efficient minimization for deterministic parity automata with informative right-congruence without don't cares.
- Weak deterministic Büchi automata admit a unique minimal form under don't care sets with trivial right-congruence, characterized by a specific congruence.
- Minimization of deterministic automata with informative right-congruence under trivial right-congruence don't cares is NP-hard.
- The active learning algorithm for weak deterministic Büchi automata can be extended to handle don't care words with trivial right-congruence.
Where Pith is reading between the lines
- The separation between trivial and informative right-congruences may guide the design of don't care handling in other classes of omega-automata.
- One could investigate whether similar priority minimization techniques apply to nondeterministic parity automata or other acceptance conditions.
- Practical implementations of the learning extension might improve specification mining when some behaviors are unspecified.
Load-bearing premise
The results on uniqueness and efficient computation for weak deterministic Büchi automata hold only when the don't care words induce the trivial right-congruence.
What would settle it
A deterministic parity automaton together with a don't care set for which no polynomial-time procedure produces an equivalent automaton with the fewest priorities.
Figures
read the original abstract
We study minimization problems for deterministic $\omega$-automata in the presence of don't care words. We prove that the number of priorities in deterministic parity automata can be efficiently minimized under an arbitrary set of don't care words. We derive that from a more general result from which one also obtains an efficient minimization algorithm for deterministic parity automata with informative right-congruence (without don't care words). We then analyze languages of don't care words with a trivial right-congruence. For such sets of don't care words it is known that weak deterministic B\"uchi automata (WDBA) have a unique minimal automaton that can be efficiently computed from a given WDBA (Eisinger, Klaedtke 2006). We give a congruence-based characterization of the corresponding minimal WDBA, and show that the don't care minimization results for WDBA do not extend to deterministic $\omega$-automata with informative right-congruence: for this class there is no unique minimal automaton for a given don't care set with trivial right congruence, and the minimization problem is NP-hard. Finally, we extend an active learning algorithm for WDBA (Maler, Pnueli 1995) to the setting with an additional set of don't care words with trivial right-congruence.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims to prove that the number of priorities in deterministic parity automata can be efficiently minimized under an arbitrary set of don't care words. This follows from a more general result that also yields an efficient minimization algorithm for deterministic parity automata with informative right-congruence (without don't cares). For don't care words with trivial right-congruence, it gives a congruence-based characterization of the minimal weak deterministic Büchi automaton (extending Eisinger-Klaedtke 2006) and shows that uniqueness and efficient computability fail for the informative right-congruence case, where minimization is NP-hard. It also extends the Maler-Pnueli active learning algorithm for WDBA to the don't-care setting with trivial right-congruence.
Significance. If the results hold, they advance automata theory by supplying efficient priority-minimization procedures for parity automata under don't cares—an important class for verification and synthesis—and by cleanly delineating the boundary between the tractable (trivial right-congruence) and intractable (informative right-congruence) cases for WDBA. The derivation of multiple positive results from a single general theorem is a strength, as is the explicit extension of the 1995 learning algorithm. These contributions are directly usable in tools that manipulate ω-automata.
minor comments (2)
- [Abstract] Abstract: the statement that minimization is 'efficient' would be strengthened by naming the complexity class (e.g., polynomial time) rather than leaving it implicit.
- The manuscript states proofs and complexity results clearly in the abstract, but the absence of explicit algorithm pseudocode or a high-level complexity analysis in the provided summary text makes independent verification of the claimed polynomial-time bounds more difficult.
Simulated Author's Rebuttal
We thank the referee for the positive evaluation and the recommendation of minor revision. The provided summary accurately captures the paper's contributions on priority minimization for deterministic parity automata under don't care words, the general result, the congruence characterization for trivial right-congruence WDBA, the NP-hardness result for informative right-congruence, and the extension of the Maler-Pnueli learning algorithm.
Circularity Check
No significant circularity; derivation relies on external citations and standard automata properties
full rationale
The paper's central positive claims (efficient minimization of priorities in DPA under don't cares, and for informative right-congruence) are stated as proven results derived from a general theorem, without reducing to self-definitions or fitted inputs. The WDBA uniqueness result is explicitly attributed to the external Eisinger-Klaedtke 2006 paper (different authors), and the learning extension builds on Maler-Pnueli 1995. Negative results (non-uniqueness and NP-hardness for informative right-congruence) are established via explicit counterexamples and hardness arguments. No load-bearing self-citation chains, ansatzes smuggled via citation, or renamings of known results appear in the provided abstract and claims. The derivation chain is self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
axioms (2)
- standard math Right-congruence relations on words induce equivalence classes that determine automaton states and minimality
- standard math Deterministic omega-automata acceptance is defined via parity or Büchi conditions on infinite runs
Reference graph
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