diff --git a/src/lecture_4.md b/src/lecture_4.md
index eadad1f4b3f9f6b1afc9a8ab4e745150941b4b2f..9728744cee72857b66283ba753205e097619cec5 100644
--- a/src/lecture_4.md
+++ b/src/lecture_4.md
@@ -260,6 +260,7 @@ Also a sanity check: when the energy is close to the bottom of the band, $E = E_
     - formulate equations of motion for electrons or phonons in 1D for systems with more than one degree of freedom per unit cell.
     - solve these equations to arrive at the dispersion relation.
     - derive the group velocity, effective mass, and density of states.
+    - explain what happens with the band structure when the periodicity of the lattice is increased or reduced.
     
 ### More hoppings in LCAO