154 research outputs found
Open sets satisfying systems of congruences
A famous result of Hausdorff states that a sphere with countably many points
removed can be partitioned into three pieces A,B,C such that A is congruent to
B (i.e., there is an isometry of the sphere which sends A to B), B is congruent
to C, and A is congruent to (B union C); this result was the precursor of the
Banach-Tarski paradox. Later, R. Robinson characterized the systems of
congruences like this which could be realized by partitions of the (entire)
sphere with rotations witnessing the congruences. The pieces involved were
nonmeasurable. In the present paper, we consider the problem of which systems
of congruences can be satisfied using open subsets of the sphere (or related
spaces); of course, these open sets cannot form a partition of the sphere, but
they can be required to cover "most of" the sphere in the sense that their
union is dense. Various versions of the problem arise, depending on whether one
uses all isometries of the sphere or restricts oneself to a free group of
rotations (the latter version generalizes to many other suitable spaces), or
whether one omits the requirement that the open sets have dense union, and so
on. While some cases of these problems are solved by simple geometrical
dissections, others involve complicated iterative constructions and/or results
from the theory of free groups. Many interesting questions remain open.Comment: 44 page
Translating the Cantor set by a random
We determine the constructive dimension of points in random translates of the
Cantor set. The Cantor set "cancels randomness" in the sense that some of its
members, when added to Martin-Lof random reals, identify a point with lower
constructive dimension than the random itself. In particular, we find the
Hausdorff dimension of the set of points in a Cantor set translate with a given
constructive dimension
Narrow coverings of omega-product spaces
Results of Sierpinski and others have shown that certain finite-dimensional
product sets can be written as unions of subsets, each of which is "narrow" in
a corresponding direction; that is, each line in that direction intersects the
subset in a small set. For example, if the set (omega \times omega) is
partitioned into two pieces along the diagonal, then one piece meets every
horizontal line in a finite set, and the other piece meets each vertical line
in a finite set. Such partitions or coverings can exist only when the sets
forming the product are of limited size.
This paper considers such coverings for products of infinitely many sets
(usually a product of omega copies of the same cardinal kappa). In this case, a
covering of the product by narrow sets, one for each coordinate direction, will
exist no matter how large the factor sets are. But if one restricts the sets
used in the covering (for instance, requiring them to be Borel in a product
topology), then the existence of narrow coverings is related to a number of
large cardinal properties: partition cardinals, the free subset problem,
nonregular ultrafilters, and so on.
One result given here is a relative consistency proof for a hypothesis used
by S. Mrowka to construct a counterexample in the dimension theory of metric
spaces
Finite left-distributive algebras and embedding algebras\endtitle
We consider algebras with one binary operation and one generator
({\it monogenic}) and satisfying the left distributive law . One can define a sequence of finite
left-distributive algebras , and then take a limit to get an infinite
monogenic left-distributive algebra~. Results of Laver and Steel
assuming a strong large cardinal axiom imply that is free; it is
open whether the freeness of can be proved without the large
cardinal assumption, or even in Peano arithmetic. The main result of this paper
is the equivalence of this problem with the existence of a certain algebra of
increasing functions on natural numbers, called an {\it embedding algebra}.
Using this and results of the first author, we conclude that the freeness of
is unprovable in primitive recursive arithmetic
On disjoint Borel uniformizations
Larman showed that any closed subset of the plane with uncountable vertical
cross-sections has aleph_1 disjoint Borel uniformizing sets. Here we show that
Larman's result is best possible: there exist closed sets with uncountable
cross-sections which do not have more than aleph_1 disjoint Borel
uniformizations, even if the continuum is much larger than aleph_1. This
negatively answers some questions of Mauldin. The proof is based on a result of
Stern, stating that certain Borel sets cannot be written as a small union of
low-level Borel sets. The proof of the latter result uses Steel's method of
forcing with tagged trees; a full presentation of this method, written in terms
of Baire category rather than forcing, is given here
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