Therefore, one can choose the simultaneous eigenstates ψ ψ of the Hamiltonian H ^ ^ and T ^ ^ R so that for every Bravais lattice vector R , ψ ψ ( r + R ) = T ^ ^ R ψ ψ ( r ) = c ( R ) ψ ψ ( r ) = e i k ⋅ ⋅ R ψ ψ ( r ) So,