The Radial Part of Brownian Motion on a Manifold: A Semimartingale Property

The usual Ito formula fails to apply for $r(X)$ when $r$ is a distance function and $X$ a Brownian motion on a general manifold, since $r$ fails to be differentiable on the cut-locus. It is shown that the discrepancy between the two sides of Ito's formula forms a monotonic random process (and hence is of locally bounded variation). In particular, $r(X)$ is a semimartingale.