diff --git a/src/powerSpectrum.hpp b/src/powerSpectrum.hpp
index 58b3c30..ad0b0a9 100644
--- a/src/powerSpectrum.hpp
+++ b/src/powerSpectrum.hpp
@@ -23,6 +23,7 @@ namespace Cosmology {
   extern double Gamma0;
 
   void updateCosmology();
+  double tophatFilter(double u);
   double powerSpectrum(double k, double normPower);
   double computePowSpecNorm(double sigma8);
   double computeVariance(double powNorm,  double topHatRad1);