Resumen de Extremal balleans
A ballean (or coarse space) is a set endowed with a coarse structure.A ballean $X$ is called normal if any two asymptotically disjoint subsets of $X$ are asymptotically separated.We say that a ballean $X$ is ultranormal (extremely normal) if any two unbounded subsets of $X$ are not asymptotically disjoint (every unbounded subset of $X$ is large).Every maximal ballean is extremely normal and every extremely normal ballean is ultranormal, but the converse statements do not hold.A normal ballean is ultranormal if and only if the Higson$^{\prime}$s corona of $X$ is a singleton.A discrete ballean $X$ is ultranormal if and only if $X$ is maximal.We construct a series of concrete balleans with extremal properties.
