diff --git a/report.tex b/report.tex
index cd301a7..a30296f 100644
--- a/report.tex
+++ b/report.tex
@@ -245,6 +245,7 @@
     \begin{equation}
       \pdvt{u^I} + \tensor{A}{^i^I_J} \pdv{u^J}{x^i} = S^I,
     \end{equation}
+    where both $A$ and $S$ can depend on $u^I$ but not their derivatives. For any unit covector $\xi_i$, the \emph{Principal symbol} matrix is defined as $P(\xi) := \tensor{A}{^i^I_J} \xi_i$.
   \end{frame}
 
   \begin{frame}{Existing first order formulations}