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Gerald Kuba

Publications and source records attributed to Gerald Kuba.

14 recordsLinked to original sources

Many coarse topologies on the real line

Let c denote the cardinality of the continuum. Let L denote the family of all Hausdorff topologies on the real line coarser than the natural topology. We construct 2^c pairwise non-homeomorphic completely normal topologies in L among which 2^c are Baire and 2^c are of first category. We also construct c pairwise non-homeomorphic completely metrizable topologies in L. Furthermore, we investigate complete lattices of topologies in L and construct extremely long chains of homeomorphic topologies in L.

math.GN

Counting continua

For infinite cardinals $\kappa,\lambda$ let $C(\kappa,\lambda)$ denote the class of all compact Hausdorff spaces of weight $\kappa$ and size $\lambda$. So $C(\kappa,\lambda)=\emptyset$ if $\kappa>\lambda$ or $\lambda>2^\kappa$. If F is a class of pairwise non-homeomorphic spaces in $C(\kappa,\lambda)$ then F is a set of size not greater than $2^\kappa$. For every infinite cardinal $\kappa$ we construct $2^\kappa$ pairwise non-embeddable pathwise connected spaces in $C(\kappa,\lambda)$ for $\lambda=\max\{2^{\aleph_0},\kappa\}$ and for $\lambda=\exp\log(\kappa^+)$. (If $\kappa$ is a strong limit then $\exp\log(\kappa^+)=2^\kappa$.) Additionally, for all infinite cardinals $\kappa,\mu$ with $\mu\leq\kappa$ we construct $2^\kappa$ pairwise non-embeddable connected spaces in $C(\kappa,\kappa^\mu)$. Furthermore, for $\kappa=\lambda=2^\theta$ with arbitrary $\theta$ and for certain other pairs $\kappa,\lambda$ we construct $2^{\kappa}$ pairwise non-embeddable connected, linearly ordered spaces $X\in C(\kappa,\lambda)$ such that $Y\in C(\kappa,\lambda)$ whenever $Y$ is an infinite compact and connected subspace of $X$. On the other hand we prove that there is no space $X$ with this property if $\lambda$ is of countable cofinality and either $\kappa=\lambda$ or $\lambda$ is a strong limit.

math.GN

On real functions with graphs either connected or locally connected

Let S denote the family of all subspaces of the plane that are graphs of functions from the real line R to itself. We prove that S has two subfamilies G,H of spaces such that the cardinality of G is c (the cardinality of the continuum) and the cardinality of H is 2^c, every space in the family G is completely metrizable, each element of H is a dense subset of the plane and the elements of the union of G and H are pairwise non-embeddable (i.p. pairwise non-homeomorphic) subspaces of the plane. On the other hand, the family S contains precisely countably infinitely many locally connected spaces up to homeomorphism, and if X,Y are such spaces then X is embeddable into Y. Furthermore, if T is a topology on the set R finer than the Euclidean topology and the space (R,T) is separable and locally connected then the space is locally compact and homeomorphic to some space in S. In a very natural way we establish a complete classification of all these refinements T of the real line.

math.GN

Many non-embeddable infinite groups

Let K be a set of infinite cardinals such that the cardinality of K is the first strong limit cardinal greater than uncountably many strong limit cardinals. We construct a family of pairwise non-embeddable groups which contains 2^k groups of order k for every cardinal number k in K. (In particular, in this family small groups are never embeddable in large groups.)

math.GR

On compact subsets of the plane

We construct a family F of compact and pathwise connected subsets of the Euclidean plane such that (i) the cardinality of F is that of the continuum (and hence extremely large) and (ii) if X,Y are distinct spaces in F then there never exists a topological embedding from X into Y.

math.GN

Transfinite dimensions

Let H be a Hilbert space and let F be the family of all countable subsets of an orthonormal basis of H. We show that if F is infinite then F is equipollent with every linear basis of the vector space H. In doing so we also present a short proof of the Erdös-Kaplansky theorem more natural and much easier than the one by Bourbaki.

math.GM

On the distribution of reducible polynomials

Let Y_n(t) denote the set of all polynomials over the ring Z which are reducible over the field Q and of degree n>1 and of height not greater than t. We show that the true order of magnitude of |Y_n(t)| equals t^2 log t in the special case n=2 and it equals t^n for each n>2. We also determine the true order of magnitude of the size of certain interesting subsets of Y_n(t).

math.NT

Cantor sets and fields of reals

Our main result is a construction of four families C_1,C_2,B_1,B_2 which are equipollent with the power set of the real line R and satisfy the following properties. (i) The members of the families are proper subfields of R whose algebraic closures equal the field C. (ii) Each field in C_1vC_2 contains a Cantor set. (iii) Each field in B_1vB_2 is a Bernstein set. (iv) All fields in C_1vB_1 are isomorphic. (v) If K,L are fields in C_2vB_2 then K is isomorphic to a subfield of L only in the trivial case K=L.

math.AC

Counting overweight spaces

Let c=2^aleph0 denote the cardinality of the continuum and let a,b,k be infinite cardinal numbers with a 2^a.

math.GN

Scattered and paracompact order topologies

We show that (in ZFC) every infinite set S can be equipped with 2^|S| complete metrics which generate mutually non-homeomorphic scattered order topologies on S. Furthermore, we show that (in ZFC) every uncountable set S can be equipped with 2^|S| mutually non-homeomorphic scattered and compact order topologies. (This would be unprovable in ZFC for countably infinite S.) In both enumeration theorems the cardinality 2^|S| is optimal.

math.GN

On the variety of Euclidean point sets

We construct a continuum of non-homeomorphic compact subspaces of the real line R without singleton components. Thus from the purely topological point of view the real line contains not only more closed sets than open sets but also more closures of open sets than open sets. On the other hand, we show that this discrepancy vanishes either if the topological point of view is sharpened in the metrical or in the order-theoretical direction, or if R is replaced with R^n for dimension n>1. Furthermore, we track down a continuum of topological types of closed and totally disconnected subsets of R. In doing so we also track down a continuum of metrical types of infinite, discrete subsets of the unit interval [0,1]. (As a consequence, any countably infinite discrete space has a continuum of non-homeomorphic metrizable compactifications.)

math.GN

On decompositions of the real line

Let X_t be a totally disconnected subset of the real line R for each t in R. We construct a partition {Y_t | t in R} of R into nowhere dense Lebesgue null sets Y_t such that for every t in R there exists an increasing homeomorphism from X_t onto Y_t. In particular, the real line can be partitioned into 2^{aleph_0} Cantor sets and also into 2^{aleph_0} mutually non-homeomorphic compact subspaces. Furthermore we prove that for every cardinal number k with 2 \leq k \leq 2^{aleph_0} the real line (as well as the Baire space R\Q) can be partitioned into exactly k homeomorphic Bernstein sets and also into exactly k mutually non-homeomorphic Bernstein sets. We also investigate partitions of R into Marczewski sets, including the possibility that they are Luzin sets or Sierpinski sets.

math.GN