Small l instead of big L

Sommerfeld model

If we impose periodic boundary conditions, ψ(x,y,z)=ψ(x+l,y+L,z+L), it follows that the allowed wavenumbers are given by

If we impose periodic boundary conditions, ψ(x,y,z)=ψ(x+L,y+L,z+L), it follows that the allowed wavenumbers are given by