Publications (112)27.46 Total impact

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ABSTRACT: We solve an open problem concerning syntactic complexity: We prove that the cardinality of the syntactic semigroup of a suffixfree language with $n$ left quotients (that is, with state complexity $n$) is at most $(n1)^{n2}+n2$ for $n\ge 7$. Since this bound is known to be reachable, this settles the problem. We also reduce the alphabet of the witness languages reaching this bound to five letters instead of $n+2$, and show that it cannot be any smaller. Finally, we prove that the transition semigroup of a minimal deterministic automaton accepting such a witness language is unique for each $n\ge 7$. 
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ABSTRACT: We solve two open problems concerning syntactic complexity: We prove that the cardinality of the syntactic semigroup of a left ideal or a suffixclosed language with $n$ left quotients (that is, with state complexity $n$) is at most $n^{n1}+n1$, and that of a twosided ideal or a factorclosed language is at most $n^{n2}+(n2)2^{n2}+1$. Since these bounds are known to be reachable, this settles the problems. 
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ABSTRACT: The quotient complexity of a regular language L, which is the same as its state complexity, is the number of left quotients of L. An atom of a nonempty regular language L with n quotients is a nonempty intersection of the n quotients, which can be uncomplemented or complemented. An NFA is atomic if the right language of every state is a union of atoms. We characterize all reduced atomic NFAs of a given language, i.e., those NFAs that have no equivalent states. We prove that, for any language L with quotient complexity n, the quotient complexity of any atom of L with r complemented quotients has an upper bound of 2n − 1 if r = 0 or r = n; for 1 ≤ r ≤ n − 1 the bound is For each n ≥ 2, we exhibit a language with 2n atoms which meet these bounds.International Journal of Foundations of Computer Science 02/2014; 24(07). DOI:10.1142/S0129054113400285 · 0.33 Impact Factor 
Article: Theory of átomata
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ABSTRACT: We show that every regular language defines a unique nondeterministic finite automaton (NFA), which we call “átomaton”, whose states are the “atoms” of the language, that is, nonempty intersections of complemented or uncomplemented left quotients of the language. We describe methods of constructing the átomaton, and prove that it is isomorphic to the reverse automaton of the minimal deterministic finite automaton (DFA) of the reverse language. We study “atomic” NFAs in which the right language of every state is a union of atoms. We generalize Brzozowski's doublereversal method for minimizing a deterministic finite automaton (DFA), showing that the result of applying the subset construction to an NFA is a minimal DFA if and only if the reverse of the NFA is atomic. We prove that Sengoku's claim that his method always finds a minimal NFA is false. 
Article: Large Aperiodic Semigroups
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ABSTRACT: The syntactic complexity of a regular language is the size of its syntactic semigroup. This semigroup is isomorphic to the transition semigroup of the minimal deterministic finite automaton accepting the language, that is, to the semigroup generated by transformations induced by nonempty words on the set of states of the automaton. In this paper we search for the largest syntactic semigroup of a starfree language having $n$ left quotients; equivalently, we look for the largest transition semigroup of an aperiodic finite automaton with $n$ states. We introduce two new aperiodic transition semigroups. The first is generated by transformations that change only one state; we call such transformations and resulting semigroups unitary. In particular, we study complete unitary semigroups which have a special structure, and we show that each maximal unitary semigroup is complete. For $n \ge 4$ there exists a complete unitary semigroup that is larger than any aperiodic semigroup known to date. We then present even larger aperiodic semigroups, generated by transformations that map a nonempty subset of states to a single state; we call such transformations and semigroups semiconstant. In particular, we examine semiconstant tree semigroups which have a structure based on full binary trees. The semiconstant tree semigroups are at present the best candidates for largest aperiodic semigroups. We also prove that $2^n1$ is an upper bound on the state complexity of reversal of starfree languages, and resolve an open problem about a special case of state complexity of concatenation of starfree languages. 
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ABSTRACT: A right ideal is a language L over an alphabet A that satisfies L = LA*. We show that there exists a stream (sequence) (R_n : n \ge 3) of regular right ideal languages, where R_n has n left quotients and is most complex under the following measures of complexity: the state complexities of the left quotients, the number of atoms (intersections of complemented and uncomplemented left quotients), the state complexities of the atoms, the size of the syntactic semigroup, the state complexities of the operations of reversal, star, and product, and the state complexities of all binary boolean operations. In that sense, this stream of right ideals is a universal witness. 
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ABSTRACT: The quotient complexity of a regular language L is the number of left quotients of L, which is the same as the state complexity of L. Suppose that L and L' are binary regular languages with quotient complexities m and n, and that the transition semigroups of the minimal deterministic automata accepting L and L' are the symmetric groups S_m and S_n of degrees m and n, respectively. Denote by o any binary boolean operation that is not a constant and not a function of one argument only. For m,n >= 2 with (m,n) not in {(2,2),(3,4),(4,3),(4,4)} we prove that the quotient complexity of LoL' is mn if and only either (a) m is not equal to n or (b) m=n and the bases (ordered pairs of generators) of S_m and S_n are not conjugate. For (m,n)\in {(2,2),(3,4),(4,3),(4,4)} we give examples to show that this need not hold. In proving these results we generalize the notion of uniform minimality to direct products of automata. We also establish a nontrivial connection between complexity of boolean operations and group theory. 
Article: Maximally Atomic Languages
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ABSTRACT: We explore the relationship between the transition semigroup of the minimal deterministic finite automaton (DFA) of a regular language, and the state/quotient complexities of the language's atoms. The atoms of a language are nonempty intersections of complemented and uncomplemented quotients of the language. Tight upper bounds on the number of atoms of a language and on the quotient complexities of atoms are known. We introduce a new class of regular languages called the \emph{maximally atomic languages}  languages with the maximal number of atoms, for which every atom has the maximal possible quotient complexity. We establish several relationships between transition semigroups and atoms; in particular, we give necessary and sufficient conditions a transition semigroup must meet for the associated language to be maximally atomic. Our methods of proof give new insights into the structures of minimal DFAs of atoms.08/2013; 151. DOI:10.4204/EPTCS.151.10 
Article: Maximal Syntactic Complexity of Regular Languages Implies Maximal Quotient Complexities of Atoms
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ABSTRACT: We relate two measures of complexity of regular languages. The first is syntactic complexity, that is, the cardinality of the syntactic semigroup of the language. That semigroup is isomorphic to the semigroup of transformations of states induced by nonempty words in the minimal deterministic finite automaton accepting the language. If the language has n left quotients (its minimal automaton has n states), then its syntactic complexity is at most n^n and this bound is tight. The second measure consists of the quotient (state) complexities of the atoms of the language, where atoms are nonempty intersections of complemented and uncomplemented quotients. A regular language has at most 2^n atoms and this bound is tight. The maximal quotient complexity of any atom with r complemented quotients is 2^n1, if r=0 or r=n, and 1+\sum_{k=1}^{r} \sum_{h=k+1}^{k+nr} \binom{h}{n} \binom{k}{h}, otherwise. We prove that if a language has maximal syntactic complexity, then it has 2^n atoms and each atom has maximal quotient complexity, but the converse is false. 
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ABSTRACT: We examine the NFA minimization problem in terms of atomic NFA's, that is, NFA's in which the right language of every state is a union of atoms, where the atoms of a regular language are nonempty intersections of complemented and uncomplemented left quotients of the language. We characterize all reduced atomic NFA's of a given language, that is, those NFA's that have no equivalent states. Using atomic NFA's, we formalize Sengoku's approach to NFA minimization and prove that his method fails to find all minimal NFA's. We also formulate the KamedaWeiner NFA minimization in terms of quotients and atoms. 
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ABSTRACT: The syntactic complexity of a regular language is the cardinality of its syntactic semigroup. The syntactic complexity of a subclass of the class of regular languages is the maximal syntactic complexity of languages in that class, taken as a function of the state complexity n of these languages. We study the syntactic complexity of R and Jtrivial regular languages, and prove that n! and floor of [e(n1)!] are tight upper bounds for these languages, respectively. We also prove that 2^{n1} is the tight upper bound on the state complexity of reversal of Jtrivial regular languages.International Journal of Foundations of Computer Science 08/2012; 25(07). DOI:10.1007/9783642393105_16 · 0.33 Impact Factor 
Conference Paper: In search of most complex regular languages
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ABSTRACT: Regular languages that are most complex under common complexity measures are studied. In particular, certain ternary languages Un(a,b,c), n ≥ 3, over the alphabet {a,b,c} are examined. It is proved that the state complexity bounds that hold for arbitrary regular languages are also met by the languages Un(a,b,c) for union, intersection, difference, symmetric difference, product (concatenation) and star. Maximal bounds are also met by Un(a,b,c) for the number of atoms, the quotient complexity of atoms, the size of the syntactic semigroup, reversal, and 22 combined operations, 5 of which require slightly modified versions. The language Un(a,b,c,d) is an extension of Un(a,b,c), obtained by adding an identity input to the minimal DFA of Un(a,b,c). The witness Un(a,b,c,d) and its modified versions work for 14 more combined operations. Thus Un(a,b,c) and Un(a,b,c,d) appear to be universal witnesses for alphabets of size 3 and 4, respectively.Proceedings of the 17th international conference on Implementation and Application of Automata; 07/2012 
Article: Universal Witnesses for State Complexity of Boolean Operations and Concatenation Combined with Star
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ABSTRACT: We study the state complexity of boolean operations and product (concatenation, catenation) combined with star. We derive tight upper bounds for the symmetric differences and differences of two languages, one or both of which are starred, and for the product of two starred languages. We prove that the previously discovered bounds for the union and the intersection of languages with one or two starred arguments, for the product of two languages one of which is starred, and for the star of the product of two languages can all be met by the recently introduced universal witnesses and their variants. 
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ABSTRACT: 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. 
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ABSTRACT: We study the syntactic complexity of finite/cofinite, definite and reverse definite languages. The syntactic complexity of a class of languages is defined as the maximal size of syntactic semigroups of languages from the class, taken as a function of the state complexity n of the languages. We prove that (n1)! is a tight upper bound for finite/cofinite languages and that it can be reached only if the alphabet size is greater than or equal to (n1)!(n2)!. We prove that the bound is also (n1)! for reverse definite languages, but the minimal alphabet size is (n1)!2(n2)!. We show that \lfloor e\cdot (n1)!\rfloor is a lower bound on the syntactic complexity of definite languages, and conjecture that this is also an upper bound, and that the alphabet size required to meet this bound is \floor{e \cdot (n1)!}  \floor{e \cdot (n2)!}. We prove the conjecture for n\le 4. 
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ABSTRACT: An atom of a regular language L with n (left) quotients is a nonempty intersection of uncomplemented or complemented quotients of L, where each of the n quotients appears in a term of the intersection. The quotient complexity of L, which is the same as the state complexity of L, is the number of quotients of L. We prove that, for any language L with quotient complexity n, the quotient complexity of any atom of L with r complemented quotients has an upper bound of 2^n1 if r=0 or r=n, and 1+\sum_{k=1}^{r} \sum_{h=k+1}^{k+nr} C_{h}^{n} \cdot C_{k}^{h} otherwise, where C_j^i is the binomial coefficient. For each n\ge 1, we exhibit a language whose atoms meet these bounds. 
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ABSTRACT: A ternary algebra is a De Morgan algebra (that is, a distributive lattice with 0 and 1 and a complement operation that satisfies De Morgan's laws) with an additional constant Φ satisfying , , and . We provide a characterization of finite ternary algebras in terms of "subsetpair algebras," whose elements are pairs (X, Y) of subsets of a given base set ℰ, which have the property X ∪ Y = ℰ, and whose operations are based on common set operations.International Journal of Algebra and Computation 11/2011; 07(06). DOI:10.1142/S0218196797000319 · 0.44 Impact Factor 
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ABSTRACT: The syntactic complexity of a regular language is the cardinality of its syntactic semigroup. 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 of these languages. We study the syntactic complexity of starfree regular languages, that is, languages that can be constructed from finite languages using union, complement and concatenation. We find tight upper bounds on the syntactic complexity of languages accepted by monotonic and partially monotonic automata. We introduce "nearly monotonic" automata, which accept starfree languages, and find a tight upper bound on the syntactic complexity of languages accepted by such automata. We conjecture that this bound is also an upper bound on the syntactic complexity of starfree languages. 
Conference Paper: Theory of átomata
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ABSTRACT: We show that every regular language defines a unique nondeterministic finite automaton (NFA), which we call "átomaton", whose states are the "atoms" of the language, that is, nonempty intersections of complemented or uncomplemented left quotients of the language. We describe methods of constructing the átomaton, and prove that it is isomorphic to the normal automaton of Sengoku, and to an automaton of Matz and Potthoff. We study "atomic" NFA's in which the right language of every state is a union of atoms. We generalize Brzozowski's doublereversal method for minimizing a deterministic finite automaton (DFA), showing that the result of applying the subset construction to an NFA is a minimal DFA if and only if the reverse of the NFA is atomic.Proceedings of the 15th international conference on Developments in language theory; 07/2011 
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ABSTRACT: The syntactic complexity of a regular language is the cardinality of its syntactic semigroup. The syntactic complexity of a subclass of the class of regular languages is the maximal syntactic complexity of languages in that class, taken as a function of the state complexity $n$ of these languages. We study the syntactic complexity of prefix, suffix, bifix, and factorfree regular languages. We prove that $n^{n2}$ is a tight upper bound for prefixfree regular languages. We present properties of the syntactic semigroups of suffix, bifix, and factorfree regular languages, conjecture tight upper bounds on their size to be $(n1)^{n2}+(n2)$, $(n1)^{n3} + (n2)^{n3} + (n3)2^{n3}$, and $(n1)^{n3} + (n3)2^{n3} + 1$, respectively, and exhibit languages with these syntactic complexities.Theoretical Computer Science 03/2011; DOI:10.1016/j.tcs.2012.04.011 · 0.52 Impact Factor
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977  Citations  
27.46  Total Impact Points  
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Institutions

1976–2014

University of Waterloo
 David R. Cheriton School of Computer Science
Waterloo, Ontario, Canada


2000–2005

University of Toronto
 Department of Computer Science
Toronto, Ontario, Canada


1980

University of California, Berkeley
 Computer Science Division
Berkeley, CA, United States
