deletions | additions       diff --git a/section_Constitutive_relationships_Retardation_factor__.tex b/section_Constitutive_relationships_Retardation_factor__.tex  index 146657d..3073495 100644  --- a/section_Constitutive_relationships_Retardation_factor__.tex  +++ b/section_Constitutive_relationships_Retardation_factor__.tex 
...
\section{Partial differential equation}  Mass conservation law, assuming that $v$ $\vec{v}$  and $D$ are functions of $u$. \begin{align}  \frac{\partial u}{\partial t} = - \nabla \cdot \left( \vec{v} u \right) + \nabla \cdot \left( D \nabla u \right) \\