A bulk Hamiltonian in a particular symmetry group is restricted to be a Hermitian matrix with no zero-energy eigenvalues (i.e. so that the spectrum is "gapped" and the system is a bulk insulator) satisfying the symmetry constraints of the group. In the case of d > 0 dimensions, this Hamiltonian is a continuous function H ( k ) of the d parameters in the Bloch momentum vector k → → in the Brillouin zone ; then the symmetry constraints must hold for all k → → .