arXiv subjects

Liliana Cojocaru

Publications and source records attributed to Liliana Cojocaru.

3 recordsLinked to original sources

arXiv2024

On Some Complexity Results for Even Linear Languages

We deal with a normal form for context-free grammars, called Dyck normal form. This normal form is a syntactical restriction of the Chomsky normal form, in which the two nonterminals occurring on the right-hand side of a rule are paired nonterminals. This pairwise property, along with several other terminal rewriting conditions, makes it possible to define a homomorphism from Dyck words to words generated by a grammar in Dyck normal form. We prove that for each context-free language L, there exist an integer K and a homomorphism phi such that L=phi(D'_K), where D'_K is a subset of D_K and D_K is the one-sided Dyck language over K letters. As an application we give an alternative proof of the inclusion of the class of even linear languages in AC1.

cs.FL↗

arXiv2022

On the Complexity of Coordinated Table Selective Substitution Systems

We investigate computational resources used by Turing machines (TMs) and alternating Turing machines (ATMs) to accept languages generated by coordinated table selective substitution systems with two components. We prove that the class of languages generated by real-time (RL; 0S)-systems, an alternative device to generate lambda-free labeled marked Petri nets languages, can be accepted by nondeterministic TMs in O(log n) space and O(nlog n) time. Consequently, this proper sub-class of Petri nets languages (known also as L-languages) is included in NSPACE(log n). The class of languages generated by (RL; RB)-systems for which the nonterminal alphabet of the RL-grammar is composed of only one symbol and the nonterminal alphabet of the RB-grammar is composed of two symbols, can be accepted by ATMs in O(log n) time and space. Consequently, this proper subclass of one-counter languages generated by one-counter machines with only one control state is included in U_E*-uniform NC1, hence in SPACE(log n).

cs.FL↗

arXiv2015

Around Context-Free Grammars -- a Normal Form, a Representation Theorem, and a Regular Approximation

We introduce a normal form for context-free grammars, called Dyck normal form. This is a syntactical restriction of the Chomsky normal form, in which the two nonterminals occurring on the right-hand side of a rule are paired nonterminals. This pairwise property allows to define a homomorphism from Dyck words to words generated by a grammar in Dyck normal form. We prove that for each context-free language L, there exist an integer K and a homomorphism h such that L=h(D'_K), where D'_K is a subset of the one-sided Dyck language over K letters. Through a transition-like diagram for a context-free grammar in Dyck normal form, we effectively build a regular language R that satisfies the Chomsky-Schutzenberger theorem. Using graphical approaches we refine R such that the Chomsky-Schutzenberger theorem still holds. Based on this readjustment we sketch a transition diagram for a regular grammar that generates a regular superset approximation for the initial context-free language.

cs.FL↗