Semantic characterizations of navigational XPath

We give semantic characterizations of the expressive powerof navigational XPath (also called Core XPath) in terms offirst order logic. XPath can be used to specify sets of nodesand to specify sets of paths in a document tree. We considerboth uses. For sets of nodes, we show that first orderlogic in two variables is equally expressive as XPath. Forpaths, we show that XPath can be defined using four simpleconnectives, which together yield the class of first orderdefinable relations which are safe for bisimulation. Furthermore,we give a characterization of the XPath expressiblepaths in terms of conjunctive queries.