Many atoms per unit cell exercise 3
File in which the problem appears
Many atomns per unit cell, exercise 3, subquestion 2 solutions
Problematic sentence
It looks like the indices of the right-hand-side matrix are swapped. Also, in the second row, it says κ_1(u_{n,3} - u_{n,2}) where I think it should be κ_1(u_{1,n} - u_{2,n}). Also, in the same row in that matrix, there is a + sign too many.
Correct version
κ_1(u_{1,n} - u_{2,n})