  The geodesics for a sub-Riemannian metric on a three-dimensional contact
manifold $M$ form a 1-parameter family of curves along each contact direction.
However, a collection of such contact curves on $M$, locally equivalent to the
solutions of a fourth-order ODE, are the geodesics of a sub-Riemannian metric
only if a sequence of invariants vanish. The first of these, which was earlier
identified by Fels, determines if the differential equation is variational. The
next two determine if there is a well-defined metric on $M$ and if the given
paths are its geodesics.
