115 research outputs found
A survey of clones on infinite sets
A clone on a set X is a set of finitary operations on X which contains all
projections and which is moreover closed under functional composition. Ordering
all clones on X by inclusion, one obtains a complete algebraic lattice, called
the clone lattice. We summarize what we know about the clone lattice on an
infinite base set X and formulate what we consider the most important open
problems.Comment: 37 page
Clones with finitely many relative R-classes
For each clone C on a set A there is an associated equivalence relation
analogous to Green's R-relation, which relates two operations on A iff each one
is a substitution instance of the other using operations from C. We study the
clones for which there are only finitely many relative R-classes.Comment: 41 pages; proofs improved, examples adde
The number of clones determined by disjunctions of unary relations
We consider finitary relations (also known as crosses) that are definable via
finite disjunctions of unary relations, i.e. subsets, taken from a fixed finite
parameter set . We prove that whenever contains at least one
non-empty relation distinct from the full carrier set, there is a countably
infinite number of polymorphism clones determined by relations that are
disjunctively definable from . Finally, we extend our result to
finitely related polymorphism clones and countably infinite sets .Comment: manuscript to be published in Theory of Computing System
Clones on infinite sets
A clone on a set X is a set of finitary functions on X which contains the
projections and which is closed under composition. The set of all clones on X
forms a complete algebraic lattice Cl(X). We obtain several results on the
structure of Cl(X) for infinite X. In the first chapter we prove the
combinatorial result that if X is linearly ordered, then the median functions
of different arity defined by that order all generate the same clone. The
second chapter deals with clones containing the almost unary functions, that
is, all functions whose value is determined by one of its variables up to a
small set. We show that on X of regular cardinality, the set of such clones is
always a countably infinite descending chain. The third chapter generalizes a
result due to L. Heindorf from the countable to all uncountable X of regular
cardinality, resulting in an explicit list of all clones containing the
permutations but not all unary functions of X. Moreover, all maximal submonoids
of the full transformation monoid which contain the permutations of X are
determined, on all infinite X; this is an extension of a theorem by G. Gavrilov
for countable base sets.Comment: 70 pages; Dissertation written at the Vienna University of Technology
under the supervision of Martin Goldstern; essentially consists of the
author's papers "The clone generated by the median functions", "Clones
containing all almost unary functions, "Maximal clones on uncountable sets
that include all permutations" which are all available from arXi
-preclones and the Galois connection -, Part I
We consider -operations in which each argument is
assigned a signum representing a "property" such as being
order-preserving or order-reversing with respect to a fixed partial order on
. The set of such properties is assumed to have a monoid structure
reflecting the behaviour of these properties under the composition of
-operations (e.g., order-reversing composed with order-reversing is
order-preserving). The collection of all -operations with prescribed
properties for their signed arguments is not a clone (since it is not closed
under arbitrary identification of arguments), but it is a preclone with special
properties, which leads to the notion of -preclone. We introduce
-relations , -relational clones, and a
preservation property (), and
we consider the induced Galois connection
-. The -preclones and -relational
clones turn out to be exactly the closed sets of this Galois connection. We
also establish some basic facts about the structure of the lattice of all
-preclones on .Comment: 31 page
Algebraic recognizability of regular tree languages
We propose a new algebraic framework to discuss and classify recognizable
tree languages, and to characterize interesting classes of such languages. Our
algebraic tool, called preclones, encompasses the classical notion of syntactic
Sigma-algebra or minimal tree automaton, but adds new expressivity to it. The
main result in this paper is a variety theorem \`{a} la Eilenberg, but we also
discuss important examples of logically defined classes of recognizable tree
languages, whose characterization and decidability was established in recent
papers (by Benedikt and S\'{e}goufin, and by Bojanczyk and Walukiewicz) and can
be naturally formulated in terms of pseudovarieties of preclones. Finally, this
paper constitutes the foundation for another paper by the same authors, where
first-order definable tree languages receive an algebraic characterization
Clones with finitely many relative R-classes
For each clone C on a set A there is an associated equivalence relation
analogous to Green's R-relation, which relates two operations on A iff each one
is a substitution instance of the other using operations from C. We study the
clones for which there are only finitely many relative R-classes.Comment: 41 pages; proofs improved, examples adde
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