Update 4_sommerfeld_model.md - typo

src/4_sommerfeld_model.md

@@ -51,9 +51,9 @@ On the one hand, there are important differences, but on the other hand there ar
 
 * The solutions of the equation of motion are still plane waves $ψ ∝ \exp(i\mathbf{k}\mathbf{r})$.
 * The periodic boundary conditions work exactly the same: $k_{x,y,z}=…, \frac{-4\pi}{L}, \frac{-2\pi}{L}, 0, \frac{2\pi}{L}, \frac{4\pi}{L}, …$
-* The density of $k$-states per unit volume in the reciprocal space is the same: $(2π/L)³$.
+* The density of $k$-states per unit volume in the reciprocal space is the same: $(L/2π)³$.
 * The expression for the total energy of the system is *very similar*:  
-  $E_\textrm{total} = 2_s (2π/L)³∫ ε(\mathbf{k}) n_F[ε(\mathbf{k})]d³\mathbf{k}.$
+  $E_\textrm{total} = 2_s (L/2π)³∫ ε(\mathbf{k}) n_F[ε(\mathbf{k})]d³\mathbf{k}.$
 
 With this comparison we have everything to analyze the electron behavior.