T. Banakh

  1. Compactly convex sets in linear topological spaces.

    Authors: T. Banakh, M. Mitrofanov, O. Ravsky
    Subjects: Functional Analysis
    Abstract

    A convex subset X of a linear topological space is called compactly convex if
    there is a continuous compact-valued map $\Phi:X\to exp(X)$ such that
    $[x,y]\subset\Phi(x)\cup \Phi(y)$ for all $x,y\in X$. We prove that each convex
    subset of the plane is compactly convex. On the other hand, the space $R^3$
    contains a convex set that is not compactly convex. Each compactly convex
    subset $X$ of a linear topological space $L$ has locally compact closure $\bar
    X$ which is metrizable if and only if each compact subset of $X$ is metrizable.

  2. Nonseparably connected complete metric spaces.

    Authors: T. Banakh, M. Vovk, M. R. Wójcik
    Subjects: Metric Geometry
    Abstract

    A topological space is nonseparably connected if it is connected but all of
    its connected separable subspaces are singletons. We show that each connected
    first countable space is the image of a nonseparably connected complete metric
    space under a continuous monotone hereditarily quotient map.

  3. On metric spaces with the properties of de Groot and Nagata in dimension one.

    Authors: T. Banakh, D. Repovs, I. Zarichnyi
    Subjects: Metric Geometry
    Abstract

    A metric space $(X,d)$ has the de Groot property $GP_n$ if for any points
    $x_0,x_1,...,x_{n+2}\in X$ there are positive indices $i,j,k\le n+2$ such that
    $i\ne j$ and $d(x_i,x_j)\le d(x_0,x_k)$. If, in addition, $k\in\{i,j\}$ then
    $X$ is said to have the Nagata property $NP_n$. It is known that a compact
    metrizable space $X$ has dimension $dim(X)\le n$ iff $X$ has an admissible
    $GP_n$-metric iff $X$ has an admissible $NP_n$-metric.

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