typo - added differential dx

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 $W$:
 
-$$W = \langle \psi_+ | V(x) | \psi_- \rangle = \frac{1}{a}\int_0^{a} dx \left[e^{i\pi x/a}\right]^* V(x) \left[e^{-i\pi x/a}\right] = \frac{1}{a}\int_0^a e^{-2\pi i x /a} V(x) = V_1$$
+$$W = \langle \psi_+ | V(x) | \psi_- \rangle = \frac{1}{a}\int_0^{a} dx \left[e^{i\pi x/a}\right]^* V(x) \left[e^{-i\pi x/a}\right] = \frac{1}{a}\int_0^a e^{-2\pi i x /a} V(x) dx = V_1$$
 
 Here $V_1$ is the first Fourier component of $V(x)$ (using a complex Fourier transform).