Helmholtz equation in a cube with Dirichlet boundary conditions

Solve

\[ ( \nabla ^ { 2 } + k ^ { 2 } ) u ( x , y , z ) = 0 \quad\tag{40} \]

so that \(u = 0\) whenever any of the coordinates is equal to 0 or L. What are the allowed values of \(k ^ { 2 } ?\)