User:TakuyaMurata/Friedrichs' lemma

In mathematics, Friedrich's lemma states that: when a coefficient $$a$$ has a bounded derivative
 * $$\int | a {\partial \over \partial x_j} \left( f * \rho_\epsilon \right) - \left(a {\partial \over \partial x_j} f \right) * \rho_\epsilon |^2 dx \to 0$$ as $$\epsilon \to 0$$.