Derived length for arbitrary topological spaces.
Let be a Hausdorff space and let be one of the hyperspaces , , or ( a positive integer) with the Vietoris topology. We study the following disconnectedness properties for : extremal disconnectedness, being a -space, -space or weak -space and hereditary disconnectedness. Our main result states: if is Hausdorff and is a closed subset such that (a) both and are totally disconnected, (b) the quotient is hereditarily disconnected, then is hereditarily disconnected. We also...