A necessary and sufficient condition for the invertibility of adapted perturbations of identity on the Wiener space

Abstract

Let $(W,H,μ)$ be the classical Wiener space, assume that $U=I_W+u$ is an adapted perturbation of identity satisfying the Girsanov identity. Then, $U$ is invertible if and only if the kinetic energy of $u$ is equal to the relative entropy of the measure induced with the action of $U$ on the Wiener measure $μ$, in other words $U$ is invertible if and only if $$ \half \int_W|u|_H^2dμ=\int_W \frac{dUμ}{dμ}\log\frac{dUμ}{dμ}dμ. $$