Single k-reversible restarting automata are a special version of restarting automata which can be effectively learned from samples. We show that their power lies between GCSL and CSL. We show that their subclasses form an infinite hierarchy of classes of languages with respect to the reversibility level k and we also show that limiting types of allowed rewrites lowers the power of the model. Finally, we study their relation to strictly locally testable restarting automata.One of the fundamental properties of all models of restarting automata is called the error preserving property. This property means that whenever a restarting automaton reduces a word from outside of the language accepted by that automaton ("the word contains an error"), the resulting word is not accepted by the automaton ("it contains still some error"), either.
Proposition 2.2. (Error Preserving Property; [8])Let M = (Σ, Γ, I) be an RRWW-automaton, and let u, v be words over its input alphabet Σ. If u ⊢ * M v holds and u ∈ L(M ), then v ∈ L(M ), either.This proposition is extensively used for proving that some language cannot be accepted by any restarting automaton of a certain type (see [17]).The general RRWW-automata are not suitable for learning from positive samples only. Therefore in [14] a subclass of restarting automata, called strictly locally testable automata, was introduced, that is based on strictly locally testable languages [20,9].
Definition 2.3. ([20, 9])Let k > 0 and let Σ be an alphabet. A language L ⊆ Σ * is strictly k-testable if there exist finite sets A, B, C ⊆ Σ k such that for each w ∈ Σ ≥k , it holds that w ∈ L if and only if P k (w) ∈ A, S k (w) ∈ B, and I k (w) ⊆ C, where P k (w), S k (w), and I k (w) denote the k-length prefix of w, the k-length suffix of w, and the set of all interior subwords of length k of w. The tuple (A, B, C) is called a triple for L. A language L is called strictly locally testable if L is strictly k-testable for some k.The considered restriction proposed in the following definition allows the resulting model to learn from positive samples.
Definition 2.4. ([14])Let k > 0. A rewriting meta-instruction (E ℓ , x → y, E r ) is strictly k-testable, if both E ℓ and E r are strictly k-testable and |x| ≤ k. An accepting meta-instruction (E, Accept) is strictly k-testable if E is strictly k-testable. A restarting automaton M is strictly k-testable (k-SLT-R-automaton), if all its metainstructions are strictly k-testable. A restarting automaton is strictly locally testable (SLT-R-automaton), if it is strictly k-testable for some k.The method Omega * is similar to the approach of [14]. The considered model utilizes the kreversible languages [2, 13] instead of strictly locally testable languages.
Definition 2.5. ([2, 13])The FSA A is deterministic with lookahead k if for any pair p, q ∈ Q such that p = q, if p, q ∈ I or p, q ∈ δ(r, a) for some r ∈ Q, a ∈ Σ, then no word is a k-follower of both p and q.Moreover, the learning method Omega * has the following feature: for all u ∈ Γ + , v ∈ Γ * (Γ being the ...