diff --git a/report.tex b/report.tex
index dc8bcc8..1bbf207 100644
--- a/report.tex
+++ b/report.tex
@@ -255,7 +255,7 @@
   \begin{frame}{Towards first order Z4c}{First order reduction}
     We can introduce auxiliary variables to reduce a second order PDE system to a first order one. For example, for the wave equation on flat spacetime:
     \begin{equation}
-      \pdvt[2]{\phi} - \nabla^2 \phi = 0,
+      \pdvt[2]{\phi} - \tensor{\delta}{^i^j} \Partial{i} \Partial{j} \phi = 0,
     \end{equation}
     we can introduce $\pi := \pdvt{\phi}$ and $\psi := \pdv{\phi}{x}$, and rewrite the wave equation as
     \begin{equation}