diff --git a/src/2_debye_model.md b/src/2_debye_model.md
index ef0df4b97915b91da3c211a2293ec7f1e3b3557f..ae69110bb1d1db97ffb6ef2ee20d1e9a96597db4 100644
--- a/src/2_debye_model.md
+++ b/src/2_debye_model.md
@@ -132,7 +132,7 @@ draw_classic_axes(ax, xlabeloffset=0.3)
 ## Exercises
 
 ### Exercise 1: Debye model: concepts
-1. Describe the concepts of k-space. What momenta are allowed in a 2D system of dimensions LxL?
+1. Describe the concept of k-space. What momenta are allowed in a 2D system of dimensions $L^2$?
 2. The probability to find an atom of a 1D solid that originally had a position $x$ at a displacement $\delta x$ is shown on this plot:
 
 ```python