Universal Witnesses for State Complexity of Basic Operations Combined with Reversal

We study the state complexity of boolean operations, concatenation and star with one or two of the argument languages reversed. We derive tight upper bounds for the symmetric differences and differences of such languages. We prove that the previously discovered bounds for union, intersection, concatenation and star of such languages can all be met by the recently introduced universal witnesses and their variants.Comment: 18 pages, 8 figures. LNCS forma

Nondeterministic State Complexity for Suffix-Free Regular Languages

We investigate the nondeterministic state complexity of basic operations for suffix-free regular languages. The nondeterministic state complexity of an operation is the number of states that are necessary and sufficient in the worst-case for a minimal nondeterministic finite-state automaton that accepts the language obtained from the operation. We consider basic operations (catenation, union, intersection, Kleene star, reversal and complementation) and establish matching upper and lower bounds for each operation. In the case of complementation the upper and lower bounds differ by an additive constant of two.Comment: In Proceedings DCFS 2010, arXiv:1008.127

Advanced Topics on State Complexity of Combined Operations

State complexity is a fundamental topic in formal languages and automata theory. The study of state complexity is also strongly motivated by applications of finite automata in software engineering, programming languages, natural language and speech processing and other practical areas. Since many of these applications use automata of large sizes, it is important to know the number of states of the automata. In this thesis, we firstly discuss the state complexities of individual operations on regular languages, including union, intersection, star, catenation, reversal and so on. The state complexity of an operation on unary languages is usually different from that of the same operation on languages over a larger alphabet. Both kinds of state complexities are reviewed in the thesis. Secondly, we study the exact state complexities of twelve combined operations on regular languages. The state complexities of most of these combined operations are not equal to the compositions of the state complexities of the individual operations which make up these combined operations. We also explore the reason for this difference. Finally, we introduce the concept of estimation and approximation of state complexity. We show close estimates and approximations of the state complexities of six combined operations on regular languages which are good enough to use in practice

Syntactic Complexities of Six Classes of Star-Free Languages

© Otto-von-Guericke-Universit¨at Magdeburg. This is an accepted manuscript. Details about the final published version are available here: http://theo.cs.ovgu.de/jalc/1996-2015/The syntactic complexity of a regular language is the cardinality of its syntactic semi-group. The syntactic complexity of a subclass of regular languages is the maximal syntactic complexity of languages in that subclass, taken as a function of the state complexity n of these languages. We study the syntactic complexity of six subclasses of star-free languages. We find a tight upper bound of (n−1)! for finite/cofinite and re-verse definite languages, and a lower bound of ⌊e·(n−1)!⌋ for definite languages, where e is the base of the natural logarithms. We also find tight upper bounds for languages accepted by monotonic, partially monotonic and “nearly monotonic” automata. All these bounds are significantly lower than the bound nn for arbitrary regular languages. Also, witness languages reaching these bounds require alphabets that grow with n. The syntactic complexity of arbitrary star-free languages remains open.Natural Sciences and Engineering Research Council of Canada [OGP0000871

Syntactic Complexities of Six Classes of Star-Free Languages

© Otto-von-Guericke-Universit¨at Magdeburg. This is an accepted manuscript. Details about the final published version are available here: http://theo.cs.ovgu.de/jalc/1996-2015/The syntactic complexity of a regular language is the cardinality of its syntactic semi-group. The syntactic complexity of a subclass of regular languages is the maximal syntactic complexity of languages in that subclass, taken as a function of the state complexity n of these languages. We study the syntactic complexity of six subclasses of star-free languages. We find a tight upper bound of (n−1)! for finite/cofinite and re-verse definite languages, and a lower bound of ⌊e·(n−1)!⌋ for definite languages, where e is the base of the natural logarithms. We also find tight upper bounds for languages accepted by monotonic, partially monotonic and “nearly monotonic” automata. All these bounds are significantly lower than the bound nn for arbitrary regular languages. Also, witness languages reaching these bounds require alphabets that grow with n. The syntactic complexity of arbitrary star-free languages remains open.Natural Sciences and Engineering Research Council of Canada [OGP0000871

Complexity of Suffix-Free Regular Languages

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jcss.2017.05.011 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/We study various complexity properties of suffix-free regular languages. A sequence (Lk,Lk+1,…) of regular languages in some class, where n is the quotient complexity of Ln, is most complex if its languages Ln meet the complexity upper bounds for all basic measures. It is known that there exist such most complex sequences in several classes of regular languages. In contrast to this, we prove that there does not exist a most complex sequence in the class of suffix-free regular languages. However, we do exhibit two such sequences that together meet upper bounds for all basic measures.Natural Sciences and Engineering Research Council of Canada (NSERC) grant No. OGP000087National Science Centre, Poland project number 2014/15/B/ST6/0061

Problems Related to Shortest Strings in Formal Languages

In formal language theory, studying shortest strings in languages, and variations thereof, can be useful since these strings can serve as small witnesses for properties of the languages, and can also provide bounds for other problems involving languages. For example, the length of the shortest string accepted by a regular language provides a lower bound on the state complexity of the language. In Chapter 1, we introduce some relevant concepts and notation used in automata and language theory, and we show some basic results concerning the connection between the length of the shortest string and the nondeterministic state complexity of a regular language. Chapter 2 examines the effect of the intersection operation on the length of the shortest string in regular languages. A tight worst-case bound is given for the length of the shortest string in the intersection of two regular languages, and loose bounds are given for two variations on the problem. Chapter 3 discusses languages that are defined over a free group instead of a free monoid. We study the length of the shortest string in a regular language that becomes the empty string in the free group, and a variety of bounds are given for different cases. Chapter 4 mentions open problems and some interesting observations that were made while studying two of the problems: finding good bounds on the length of the shortest squarefree string accepted by a deterministic finite automaton, and finding an efficient way to check if a finite set of finite words generates the free monoid. Some of the results in this thesis have appeared in work that the author has participated in \cite{AngPigRamSha,AngShallit}