Second Order Differential Operators on Graphs

The commutator of a pair of vector fields on a graph is not a vector field in general, but rather a second order differential operator.

We investigate this departure from the classical case of vectors fields on a manifold by examining the geometry of balls of radius two, concentrating on the set of paths of length two connecting a given vertex with the center of the ball.

There is a natural surjection from the space of sections of the second tangent bundle to the space of second order differential operators whose kernel reflects the geometry of these balls.

Using this map we draw several conclusions about second order differential operators including canonical forms, formulas for their adjoints, and a necessary and sufficient condition for a commutator to be a vector field.