The Cycle Space of an Infinite Graph

Finite graph homology may seem trivial, but for infinite graphs things become interesting. We present a new approach that builds the cycle space of a graph not on its finite cycles but on its topological circles, the homeomorphic images of the unit circle in the space formed by the graph together with its ends. Our approach permits the extension to infinite graphs of standard results about finite graph homology – such as Tutte’s generating theorem, Whitney’s duality theorem, MacLane’s planarity criterion, the Tutte/Nash-Williams tree packing theorem – whose infinite versions would otherwise fail. A notion of end degrees motivated by these results opens up new possibilities for an ‘extremal’ branch of infinite graph theory. Numerous open problems are suggested.