diff --git a/src/11_nearly_free_electron_model.md b/src/11_nearly_free_electron_model.md
index 06decabeb13480d0aac4fe8307519642dc9968d6..dc4c11f38c121238a0852b339be0ef2e7d9a1306 100644
--- a/src/11_nearly_free_electron_model.md
+++ b/src/11_nearly_free_electron_model.md
@@ -175,7 +175,7 @@ Let's consider a 1D crystal with a period $a$. Let $k_0$ be any wave number of a
         To answer this question, only consider consider two free electron wavefunctions in the Hamiltonian and ignore all the others. Between what two of free electron wavefunctions does the coupling give significant contribution to the energy levels of the free electron wavefunctions?
 
 #### Exercise 3: the tight binding model vs. the nearly free electron model
-Consider a 1D crystal with a periodic potential given by delta peaks: $$V(x) = -\lambda \sum_{n=-\infty}^{\infty} \delta(x+na),$$ where $\lambda>0$. In this exercise, we will find find the band structure of this crystal in two ways:
+Consider a 1D crystal with a periodic potential given by delta peaks: $$V(x) = -\lambda \sum_{n=-\infty}^{\infty} \delta(x+na),$$ where $\lambda>0$. In this exercise, we will find the band structure of this crystal in two ways:
 
 - By means of the nearly free electron model explained in this lecture.
 - By means of the tight binding model explained in [lecture 7](/7_tight_binding).