diff --git a/susceptibility1.tex b/susceptibility1.tex
index 47e9fd9..5b8fb19 100644
--- a/susceptibility1.tex
+++ b/susceptibility1.tex
@@ -685,8 +685,9 @@ describes nonlinear interactions that generate responses at the sum and differen
 The second-order susceptibility
 \begin{equation}
   \label{eq:susceptibility}
-  \chi_{2}(\omega_{1}, \omega_{2}) = \frac{S_{xss} (\omega_{1},\omega_{2})}{2S_{ss} (\omega_{1}) S_{ss} (\omega_{2})}
+  \chi_{2}(\omega_{1}, \omega_{2}) = \frac{S_{xss} (\omega_{1},\omega_{2})}{2\sqrt{S_{ss} (\omega_{1}) S_{ss} (\omega_{2})}}
 \end{equation}
+\note{is the denominator right? What is the unit?}
 normalizes the second-order cross-spectrum by the spectral power at the two stimulus frequencies.
 % Applying the Fourier transform this can be rewritten resulting in:
 % \begin{equation}