Inverse Homomorphism Research Articles

Inverse homomorphisms of finite groups

Abstract Replacing the equality in the condition of the homomorphism of a multi-based universal algebra with inequality leads to the definition of an inverse homomorphism of a multi-based universal algebra. In this paper methods for constructing nontrivial inverse homomorphisms of finite groups are studied.

Two dimensional fuzzy context-free languages and tiling patterns

Fuzzy context-free languages are powerful compared to fuzzy regular languages as they are generated by fuzzy context-free grammars and fuzzy pushdown automata, which follow an enhanced computational mechanism. A two dimensional language (picture language) is a collection of two dimensional words, which are a rectangular array of symbols made up of finite alphabets. Two dimensional automata can recognize two dimensional languages that could not be recognized by one dimensional automata. In this paper, we introduce two dimensional fuzzy context-free languages generated by the two dimensional fuzzy context-free grammars and accepted by the two dimensional fuzzy pushdown automata in order to deal with the vagueness that arises in two dimensional context-free languages. We can construct a two dimensional fuzzy context free grammar from the given two dimensional fuzzy pushdown automata and vice versa. In addition, we prove that two dimensional fuzzy context-free languages are closed under union, column concatenation, column star, homomorphism, inverse homomorphism, reflection about right-most vertical, reflection about base, conjugation and half-turn and also show that two dimensional fuzzy context-free languages are not closed under matrix homomorphism, quarter-turn and transpose. Further, we have given the applications and the uses of closure properties in the formation of tiling patterns.

On the power of pushing or stationary moves for input-driven pushdown automata

Input-driven pushdown automata (IDPDAs) are pushdown automata where the next action on the pushdown store (push, pop, nothing) is solely governed by the input symbol. Nowadays such devices are usually defined such that every push operation pushes exactly one additional symbol on the pushdown store and, in addition, stationary moves are not allowed so that the devices work in real time. Here, we relax this strong definition and consider IDPDAs that may push more than one symbol in one step (push-IDPDA) or may perform stationary moves (stat-IDPDA). We study the computational power of the extended variants both in the deterministic and nondeterministic case, we investigate several decidability questions for the new automata classes, and we obtain interesting representations by inverse homomorphisms. Namely, every (1) deterministic, (2) real-time deterministic, and (3) nondeterministic context-free language can be characterized as the inverse homomorphic image of a language accepted by a (1) stat-IDPDA, (2) push-IDPDA, and (3) nondeterministic push-IDPDA.

Homomorphisms and inverse homomorphisms on graph-walking automata

Graph-walking automata analyze an input graph by moving between its nodes, following the edges. This paper investigates the effect of node-replacement graph homomorphisms and inverse homomorphisms on recognizability by these automata. For deterministic graph-walking automata, it is shown that the family of graph languages they recognize is closed under inverse homomorphisms: for an n-state automaton, the inverse homomorphic images of the graphs it accepts can be recognized by an automaton with at most kn+1 states, where k is the number of labels of edge end-points in the pre-image graphs. At the same time, it is proved that in the worst case these inverse homomorphic images require a deterministic graph-walking automaton with at least kn states. The upper bound kn+1 also holds for nondeterministic graph-walking automata. The second result is that already for tree-walking automata, both deterministic and nondeterministic, the families they recognize are not closed under injective homomorphisms. Here the proof is based on a new homomorphic characterization of regular tree languages: every regular tree language is representable as h−1(g(all trees)), for some injective node-replacement homomorphisms g and h.

The hardest language for grammars with context operators

In 1973, Greibach (“The hardest context-free language”, SIAM J. Comp., 1973) constructed a context-free language L0 with the property that every context-free language can be reduced to L0 by a homomorphism, thus representing it as an inverse homomorphic image h−1(L0). In this paper, a similar characterization is established for a family of grammars equipped with operators for referring to the left context of any substring, recently defined by Barash and Okhotin (“An extension of context-free grammars with one-sided context specifications”, Inform. Comput., 2014). An essential step of the argument is a new normal form for grammars with context operators, in which every nonterminal symbol defines only strings of odd length in left contexts of even length: the even-odd normal form. The characterization is completed by showing that the language family defined by grammars with context operators is closed under inverse homomorphisms; actually, it is closed under injective nondeterministic finite transductions.

The Hardest LL(k) Language

This paper establishes an analogue of Greibach’s hardest language theorem (“The hardest context-free language”, SIAM J. Comp., 1973, http://dx.doi.org/10.1137/0202025 ) for the classical family of LL([Formula: see text]) languages. The first result is that there is a language [Formula: see text] defined by an LL(1) grammar in the Greibach normal form, to which every language [Formula: see text] defined by an LL(1) grammar in the Greibach normal form can be reduced by a homomorphism, that is, [Formula: see text] if and only if [Formula: see text]. Then it is shown that this statement does not hold for the full class of LL([Formula: see text]) languages. The other hardest language theorem is then established in the following form: there is a language [Formula: see text] defined by an LL(1) grammar in the Greibach normal form, such that, for every language [Formula: see text] defined by an LL([Formula: see text]) grammar, with [Formula: see text], there exists a homomorphism [Formula: see text], for which [Formula: see text] if and only if [Formula: see text] [Formula: see text] [Formula: see text], where [Formula: see text] is a new symbol. The results lead to two robust language families: the closures of the languages defined by LL(1) grammars in the Greibach normal form under inverse homomorphisms and under inverse finite transductions.

Regular string token fuzzy Petri nets

Fuzzy Petri nets are a type of classical Petri nets designed to deal with imprecise and ambiguous data, that have been widely used to represent fuzzy production rules and fuzzy rule-based reasoning. In this paper, we introduce a new model called string token fuzzy Petri nets to generate fuzzy regular languages. Also, we study the equivalences of fuzzy regular grammar and regular string token fuzzy Petri net and establish some closure properties such as union, catenation, kleene closure, reversal, homomorphism and inverse homomorphism of the languages generated by the regular string token fuzzy Petri nets.

Inverse homomorphisms of finite groups

Если в выражении условия гомоморфизма многоосновной универсальной алгебры вместо равенства выполняется неравенство, то говорят о существовании инверсного гомоморфизма многоосновной универсальной алгебры. В работе исследуются методы построения нетривиальных инверсных гомоморфизмов конечных групп.

Linear context-free tree languages and inverse homomorphisms

We prove that the class of linear context-free tree languages is not closed under inverse linear tree homomorphisms. In fact, we prove a stronger result: we encode Dyck words into a linear context-free tree language and prove that its preimage under a certain linear tree homomorphism cannot be generated by any context-free tree grammar. A positive result can still be obtained: the linear monadic context-free tree languages are closed under inverse linear tree homomorphisms.

Operational State Complexity and Decidability of Jumping Finite Automata

We consider jumping finite automata and their operational state complexity and decidability status. Roughly speaking, a jumping automaton is a finite automaton with a non-continuous input. This device has nice relations to semilinear sets and thus to Parikh images of regular sets, which will be exhaustively used in our proofs. In particular, we prove upper bounds on the intersection and complementation. The latter result on the complementation upper bound answers an open problem from [G. J. Lavado, G. Pighizzini, S. Seki: Operational State Complexity of Parikh Equivalence, 2014]. Moreover, we correct an erroneous result on the inverse homomorphism closure. Finally, we also consider the decidability status of standard problems as regularity, disjointness, universality, inclusion, etc. for jumping finite automata.

On the Descriptional Complexity of Operations on Semilinear Sets

We investigate the descriptional complexity of operations on semilinear sets. Roughly speaking, a semilinear set is the finite union of linear sets, which are built by constant and period vectors. The interesting parameters of a semilinear set are: (i) the maximal value that appears in the vectors of periods and constants and (ii) the number of such sets of periods and constants necessary to describe the semilinear set under consideration. More precisely, we prove upper bounds on the union, intersection, complementation, and inverse homomorphism. In particular, our result on the complementation upper bound answers an open problem from [G. J. LAVADO, G. PIGHIZZINI, S. SEKI: Operational State Complexity of Parikh Equivalence, 2014].

Ordered groupoid quotients and congruences on inverse semigroups

We introduce a preorder on an inverse semigroup S associated to any normal inverse subsemigroup N, that lies between the natural partial order and Green’s $${\mathcal {J}}$$ –relation. The corresponding equivalence relation $$\simeq _N$$ is not necessarily a congruence on S, but the quotient set does inherit a natural ordered groupoid structure. We show that this construction permits the factorisation of any inverse semigroup homomorphism into a composition of a quotient map and a star-injective functor, and that this decomposition implies a classification of congruences on S. We give an application to the congruence and certain normal inverse subsemigroups associate to an inverse monoid presentation.

On the Effect of the IO-Substitution on the Parikh Image of Semilinear Full AFLs

Back in the 1980's, the class of mildly context-sensitive formalisms was introduced so as to capture the syntax of natural languages. While the languages generated by such formalisms are constrained by the constant-growth property, the most well-known and used ones--like tree-adjoining grammars or multiple context-free grammars--generate languages which verify the stronger property of being semilinear. In (Bourreau et al. 2012), the operation of IO-substitution was created so as to exhibit mildly-context sensitive classes of languages which are not semilinear. In the present article, we extend the notion of semilinearity, and characterize the Parikh image of the languages in IO(L), the closure of a class L of semilinear languages under IO-substitution, as universally-semilinear. Based on this result and on the work of Fischer on macro-grammars, we then show that IO(L) is not closed under inverse homomorphism when L is closed under inverse homomorphism, and encompasses the class of regular languages. This result proves that IO(MCFL) is not a full AFL, where $$\mathbf {MCFL}$$MCFL denotes the class of multiple context-free languages, closing an open question in Bourreau et al. (2012). More importantly, our proof gives an insight into the relation between the non-closure under inverse homomorphism of $$\mathbf {IO(MCFL)}$$IO(MCFL) and how IO-substitution breaks semilinearity.

On languages of automaton counter machines

A class of formal languages (ACML) acceptable by automaton counter machines is considered. This class is shown to be close with respect to the operations of union, regular intersection, concatenation, infinite iteration, homomorphism, and inverse homomorphism. It follows from here that this class is a full abstract family of languages [7] with all the properties following from this. Furthermore, the ACML is close with respect to intersection and substitution but is not closed with respect to complement and reverse. For the ACML class, the problems of emptiness and recognition of words of a language given by an automaton counter machine are decidable, but the problems of inclusion and equivalence of languages are undecidable. A comparison with other classes of languages (regular, context-free, context-sensitive, and Petri-net languages) is performed.

Closure properties of hyper-minimized automata

Two deterministic finite automata are almost equivalent if they disagree in acceptance only for finitely many inputs. An automaton A is hyper-minimized if no automaton with fewer states is almost equivalent to A . A regular language L is canonical if the minimal automaton accepting L is hyper-minimized. The asymptotic state complexity s ∗ (L ) of a regular language L is the number of states of a hyper-minimized automaton for a language finitely different from L . In this paper we show that: (1) the class of canonical regular languages is not closed under: intersection, union, concatenation, Kleene closure, difference, symmetric difference, reversal, homomorphism, and inverse homomorphism; (2) for any regular languages L 1 and L 2 the asymptotic state complexity of their sum L 1 ∪ L 2 , intersection L 1 ∩ L 2 , difference L 1 − L 2 , and symmetric difference L 1 ⊕ L 2 can be bounded by s ∗ (L 1 )·s ∗ (L 2 ). This bound is tight in binary case and in unary case can be met in infinitely many cases. (3) For any regular language L the asymptotic state complexity of its reversal L R can be bounded by 2s ∗ (L ) . This bound is tight in binary case. (4) The asymptotic state complexity of Kleene closure and concatenation cannot be bounded. Namely, for every k ≥ 3, there exist languages K , L , and M such that s ∗ (K ) = s ∗ (L ) = s ∗ (M ) = 1 and s ∗ (K ∗ ) = s ∗ (L ·M ) = k . These are answers to open problems formulated by Badr et al. [RAIRO-Theor. Inf. Appl. 43 (2009) 69–94].

Clifford semigroups as functors and their cohomology

We prove that the category of Clifford semigroups and prehomomorphisms \(\mathcal{CSP}\) is isomorphic to a certain subcategory of the category of diagrams over groups. Under this isomorphism, Clifford semigroups are identified with certain functors. As an application of the isomorphism theorem, we show that the category with objects commutative inverse semigroups having the same semilattice of idempotents and with morphisms, the inverse semigroup homomorphisms that fix the semilattice, imbeds into a category of right modules over a certain ring. Also we find a very close relationship between the cohomology groups of a commutative inverse monoid and the cohomology groups of the colimit group of the functor giving the monoid.

Decidability of operation problems for T0L languages and subclasses

We investigate the decidability of the operation problem for T0L languages and subclasses. Fix an operation on formal languages. Given languages from the family considered (0L languages, T0L languages, or their propagating variants), is the application of this operation to the given languages still a language that belongs to the same language family? Observe, that all the Lindenmayer language families in question are anti-AFLs, that is, they are not closed under homomorphisms, inverse homomorphisms, intersection with regular languages, union, concatenation, and Kleene closure. Besides these classical operations we also consider intersection and substitution, since the language families under consideration are not closed under these operations, too. We show that for all of the above mentioned language operations, except for the Kleene closure, the corresponding operation problems of 0L and T0L languages and their propagating variants are not even semidecidable. The situation changes for unary 0L languages. In this case we prove that the operation problems with respect to Kleene star, complementation, and intersection with regular sets are decidable.

Algebraic properties on the cuts of lattice-valued regular languages

In this paper, the algebraic operations on the cuts of lattice-valued regular languages are studied. Some sufficient conditions are given to guarantee the family of the cuts of lattice-valued regular languages to be closed under such algebraic operations as union, intersection, complement, quotient, homomorphism, inverse homomorphism, concatennation, reversal, etc.

Finite turns and the regular closure of linear context-free languages

Turn bounded pushdown automata with different conditions for beginning a new turn are investigated. Their relationships with closures of the linear context-free languages under regular operations are studied. For example, automata with an unbounded number of turns that have to empty their pushdown store up to the initial symbol in order to start a new turn are characterized by the regular closure of the linear languages. Automata that additionally have to re-enter the initial state are (almost) characterized by the Kleene star closure of the linear languages. For both a bounded and an unbounded number of turns, requiring to empty the pushdown store is a strictly stronger condition than requiring to re-enter the initial state. Several new language families are obtained which form a double-stranded hierarchy. Closure properties of these families under AFL operations are derived. The regular closure of the linear languages share the strong closure properties of the context-free languages, i.e., the family is a full AFL. Interestingly, three natural new language families are not closed under intersection with regular languages and inverse homomorphism. Finally, an algorithm is presented parsing languages from the new families in quadratic time.

Partial *-Automorphisms, Normalizers, and Submodules in Monotone Complete C*-Algebras

AbstractFor monotone complete C*-algebras A ⊂ B with A contained in B as a monotone closed C*-subalgebra, the relation X = AsA gives a bijection between the set of all monotone closed linear subspaces X of B such that AX + XA ⊂ X and XX* + X*X ⊂ A and a set of certain partial isometries s in the “normalizer” of A in B, and similarly for the map s ⟼ Ad s between the latter set and a set of certain “partial *-automorphisms” of A. We introduce natural inverse semigroup structures in the set of such X's and the set of partial *-automorphisms of A, modulo a certain relation, so that the composition of these maps induces an inverse semigroup homomorphism between them. For a large enough B the homomorphism becomes surjective and all the partial *-automorphisms of A are realized via partial isometries in B. In particular, the inverse semigroup associated with a type II1 von Neumann factor, modulo the outer automorphism group, can be viewed as the fundamental group of the factor. We also consider the C*-algebra version of these results.