Unambiguous finite automaton

In automata theory, an unambiguous finite automaton (UFA) is a special kind of a nondeterministic finite automaton (NFA). Each deterministic finite automaton (DFA) is an UFA, but not vice versa. DFA, UFA, and NFA recognize exactly the same class of formal languages. On the one hand, an NFA can be exponentially smaller than an equivalent DFA. On the other hand, some problems are easily solved on DFAs and not on UFAs. For example, given an automaton A, an automaton A' which accepts the complement of A can be computed in linear time when A is a DFA, it is not known whether it can be done in polynomial time for UFA. Hence UFAs are a mix of the worlds of DFA and of NFA; in some cases, they lead to smaller automata than DFA and quicker algorithms than NFA.

Formal definition

An NFA is represented formally by a 5-tuple, A=(Q, Σ, Δ, q₀, F). An UFA is an NFA such that, for each word w = a₁a₂ … a_n, there exists at most one sequence of states r₀,r₁, …, r_n, in Q with the following conditions:

r₀ = q₀
r_i+1 ∈ Δ(r_i, a_i+1), for i = 0, …, n−1
r_n ∈ F.

In words, those conditions state that, if w is accepted by A, there is exactly one accepting path, that is, one path from an initial state to a final state, labelled by w.

Example

Let L be the set of words over the alphabet {a,b} whose nth last letter is an a. The figures show a DFA and a UFA accepting this language for n=2.

Deterministic automaton (DFA) for the language L for n=2

Unambiguous finite automaton (UFA) for the language L for n=2

The minimal DFA accepting L has 2ⁿ states, one for each subset of {1...n}. There is an UFA of n+1 states which accepts L: it guesses the nth last letter, and then verifies that only n-1 letters remain. It is indeed unambiguous as there exists only one nth last letter.

Inclusion and related problems

Three PSPACE-hard problems for general NFA belong to PTIME for DFA and are now considered.

Inclusion

It is decidable in polynomial-time whether an automaton's language is a subset of an automaton of another language.

Sketch of the proof of inclusion
Let A and B be two automata. Let L(A) and L(B) be the languages accepted by those automata. Then L(A)⊆L(B) if and only if L(A∩ B)=L(A), where A∩B denotes the Cartesian product automaton, which can be proven to be also unambiguous. Now, L(A∩B) is a subset of L(A) by construction; hence both set are equal if and only if for each length n∈ℕ, the number of words of length n in L(A∩B) is equal to the number of words of length n in L(A). It can be proved that is sufficient to check each n up to the product of the number of states of A and B. The number of words of length n accepted by an automaton can be computed in polynomial time using dynamic programming, which ends the proof.^[1]

Sketch of the proof of inclusion

Let A and B be two automata. Let L(A) and L(B) be the languages accepted by those automata. Then L(A)⊆L(B) if and only if L(A∩ B)=L(A), where A∩B denotes the Cartesian product automaton, which can be proven to be also unambiguous. Now, L(A∩B) is a subset of L(A) by construction; hence both set are equal if and only if for each length n∈ℕ, the number of words of length n in L(A∩B) is equal to the number of words of length n in L(A). It can be proved that is sufficient to check each n up to the product of the number of states of A and B.

The number of words of length n accepted by an automaton can be computed in polynomial time using dynamic programming, which ends the proof.^[1]

Checking whether an automaton in unambiguous

For a nondeterministic finite automaton A with n states and an m letter alphabet, it is decidable in time O(n²m) whether A is unambiguous.

Sketch of the proof of unambiguity
It suffices to use a fixpoint algorithm to compute the set of pairs of states q and q' such that there exists a word w which leads both to q and to q' . The automaton is unambiguous if and only if there is no such a pair such that both states are accepting. There are at most O(n²) state pairs, and for each pair there are m letters to consider to resume the fixpoint algorithm, hence the computation time.

Sketch of the proof of unambiguity

It suffices to use a fixpoint algorithm to compute the set of pairs of states q and q' such that there exists a word w which leads both to q and to q' . The automaton is unambiguous if and only if there is no such a pair such that both states are accepting. There are at most O(n²) state pairs, and for each pair there are m letters to consider to resume the fixpoint algorithm, hence the computation time.

Some properties

The cartesian product of two UFAs is a UFA.^[2]
The notion of unambiguity extends to finite state transducers and weighted automata. If a finite state transducer T is unambiguous, then each input word is associated by T to at most one output word. If a weighted automaton A is unambiguous, then the set of weight does not need to be a semiring, instead it suffices to consider a monoid. Indeed, there is at most one accepting path.

References

↑ i.e.: given a UFA, does it accept every string at all?
↑ i.e.: given two UFAs, do they accept the same set of strings?

Christof Lödig, Unambiguous Finite Automata, Developments in Language Theory, (2013) pp. 29–30 (Slides)

↑ Christof Lödig, Unambiguous Finite Automata, Slide 8
↑ Christof Lödig, Unambiguous Finite Automata, Slide 8

Automata theory: formal languages and formal grammars

Chomsky hierarchy	Grammars	Languages	Abstract machines

Type-0 — Type-1 — — — — — Type-2 — — Type-3 — —	Unrestricted (no common name) Context-sensitive Positive range concatenation Indexed — Linear context-free rewriting systems Tree-adjoining Context-free Deterministic context-free Visibly pushdown Regular — Non-recursive	Recursively enumerable Decidable Context-sensitive Positive range concatenation^* Indexed^* — Linear context-free rewriting language Tree-adjoining Context-free Deterministic context-free Visibly pushdown Regular Star-free Finite	Turing machine Decider Linear-bounded PTIME Turing Machine Nested stack Thread automaton restricted Tree stack automaton Embedded pushdown Nondeterministic pushdown Deterministic pushdown Visibly pushdown Finite Counter-free (with aperiodic finite monoid) Acyclic finite

Each category of languages, except those marked by a ^*, is a proper subset of the category directly above it. Any language in each category is generated by a grammar and by an automaton in the category in the same line.

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