28 Menger's theorem
We now discuss a theorem that is closely related to Hall's theorem and has far-reaching practical applications. This theorem, due to K. Menger, concerns the number of paths connecting two given vertices v and w in a graph G. We may ask for the maximum number of paths from v to w, no two of which have an edge in common - such paths are called edge-disjoint paths. Alternatively, we may ask for the maximum number of paths from v to w, no two of which have a vertex in common, except, of course, v and w - these are called vertex-disjoint paths. For example, in the graph of Fig.28.1, there are four edge-disjoint paths and two vertex-disjoint ones.