1,022 research outputs found
Effectively Open Real Functions
A function f is continuous iff the PRE-image f^{-1}[V] of any open set V is
open again. Dual to this topological property, f is called OPEN iff the IMAGE
f[U] of any open set U is open again. Several classical Open Mapping Theorems
in Analysis provide a variety of sufficient conditions for openness.
By the Main Theorem of Recursive Analysis, computable real functions are
necessarily continuous. In fact they admit a well-known characterization in
terms of the mapping V+->f^{-1}[V] being EFFECTIVE: Given a list of open
rational balls exhausting V, a Turing Machine can generate a corresponding list
for f^{-1}[V]. Analogously, EFFECTIVE OPENNESS requires the mapping U+->f[U] on
open real subsets to be effective.
By effectivizing classical Open Mapping Theorems as well as from application
of Tarski's Quantifier Elimination, the present work reveals several rich
classes of functions to be effectively open.Comment: added section on semi-algebraic functions; to appear in Proc.
http://cca-net.de/cca200
Compact manifolds with computable boundaries
We investigate conditions under which a co-computably enumerable closed set
in a computable metric space is computable and prove that in each locally
computable computable metric space each co-computably enumerable compact
manifold with computable boundary is computable. In fact, we examine the notion
of a semi-computable compact set and we prove a more general result: in any
computable metric space each semi-computable compact manifold with computable
boundary is computable. In particular, each semi-computable compact
(boundaryless) manifold is computable
On the topological aspects of the theory of represented spaces
Represented spaces form the general setting for the study of computability
derived from Turing machines. As such, they are the basic entities for
endeavors such as computable analysis or computable measure theory. The theory
of represented spaces is well-known to exhibit a strong topological flavour. We
present an abstract and very succinct introduction to the field; drawing
heavily on prior work by Escard\'o, Schr\"oder, and others.
Central aspects of the theory are function spaces and various spaces of
subsets derived from other represented spaces, and -- closely linked to these
-- properties of represented spaces such as compactness, overtness and
separation principles. Both the derived spaces and the properties are
introduced by demanding the computability of certain mappings, and it is
demonstrated that typically various interesting mappings induce the same
property.Comment: Earlier versions were titled "Compactness and separation for
represented spaces" and "A new introduction to the theory of represented
spaces
Finding subsets of positive measure
An important theorem of geometric measure theory (first proved by Besicovitch
and Davies for Euclidean space) says that every analytic set of non-zero
-dimensional Hausdorff measure contains a closed subset of
non-zero (and indeed finite) -measure. We investigate the
question how hard it is to find such a set, in terms of the index set
complexity, and in terms of the complexity of the parameter needed to define
such a closed set. Among other results, we show that given a (lightface)
set of reals in Cantor space, there is always a
subset on non-zero -measure definable from
Kleene's . On the other hand, there are sets of reals
where no hyperarithmetic real can define a closed subset of non-zero measure.Comment: This is an extended journal version of the conference paper "The
Strength of the Besicovitch--Davies Theorem". The final publication of that
paper is available at Springer via
http://dx.doi.org/10.1007/978-3-642-13962-8_2
First Order Theories of Some Lattices of Open Sets
We show that the first order theory of the lattice of open sets in some
natural topological spaces is -equivalent to second order arithmetic. We
also show that for many natural computable metric spaces and computable domains
the first order theory of the lattice of effectively open sets is undecidable.
Moreover, for several important spaces (e.g., , , and the
domain ) this theory is -equivalent to first order arithmetic
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