diff --git a/src/11_nearly_free_electron_model.md b/src/11_nearly_free_electron_model.md
index e37cfdae74fc02308c9d287de26c6689fa8eeaa9..8fa427a1f824ec99f614391d70bfec2b1427e15a 100644
--- a/src/11_nearly_free_electron_model.md
+++ b/src/11_nearly_free_electron_model.md
@@ -108,7 +108,7 @@ Check out section 15.1.1 of the book for the details of this calculation.
 
 Now we expand the definition of $W$:
 
-$$W = \langle \psi_+ | V(x) | \psi_- \rangle = \int_0^{a} dx \left[e^{i\pi x/a}\right]^* V(x) \left[e^{-i\pi x/a}\right] = \int_0^a e^{2\pi i x /a} V(x) = V_1$$
+$$W = \langle \psi_+ | V(x) | \psi_- \rangle = \frac{1}{a}\int_0^{a} dx \left[e^{-i\pi x/a}\right]^* V(x) \left[e^{i\pi x/a}\right] = \frac{1}{a}\int_0^a e^{-2\pi i x /a} V(x) = V_1$$
 
 Here $V_1$ is the first Fourier component of $V(x)$ (using a complex Fourier transform).