Recursive Euler and Hamilton paths

Abstract:

We employ recursion theoretic arguments to show that Hamilton paths for locally finite graphs are more difficult to find, in general, than Euler paths. A locally finite graph is recursive if we can effectively decide whether or not any two given vertices are adjacent, and highly recursive if we can effectively find all vertices adjacent to any given vertex. We find that there are recursive planar graphs with Euler or Hamilton paths but no such recursive paths. There are even particularly simple classes of connected, planar, highly recursive graphs for which we can show there is no effective way to decide about the existence of Euler or Hamilton paths. However, we obtain the following contrast: If a highly recursive graph has an Euler path we can effectively find a recursive Euler path; whereas, there is a planar, highly recursive graph with Hamilton paths but no recursive Hamilton path.