Fixes

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