An equality of Monge-Ampère measures

Abstract: Let $u$ and $v$ be two plurisubharmonic functions in the domain of definition of the Monge-Amp`ere operator on a domain $\Omega\subset {\bf C}^n$. We prove that if $u=v$ on a plurifinely open set $U\subset \Omega$ that is Borel measurable, then $(dd^cu)^n|_U=(dd^cv)^n|_U$. This result was proved by Bedford and Taylor in the case where $u$ and $v$ are locally bounded, and by El Kadiri and Wiegerinck when $u$ and $v$ are finite, and by Hai and Hiep when $U$ is of the form $U=\bigcup_{j=1}^m{\varphi_j>\psi_j}$, where $\varphi_j$, $\psi_j$, $j=1,...,m$, are plurisubharmonic functions on $\Omega$.