  Using the first-order series expansion of the function G(k;mu), in powers of
mu' = mu - U/2, pertaining to the insulating ground state of a single-band
Hubbard Hamiltonian at half-filling, Kokalj and Prelovsek have in a recent
paper [Eur. Phys. J. B 63, 431 (2008), arXiv:arXiv:0709.0263] reported
breakdown of the Luttinger theorem for the specific case where the lattice on
which the Hubbard Hamiltonian is defined is a two-dimensional triangular
lattice, for which the ground state is not invariant under particle-hole
transformation. Here G(k;mu) is the single-particle Green function G(k;e)
evaluated at e = mu, the zero-temperature limit of the chemical potential
corresponding to half-filling, and U the on-site interaction energy. In this
Comment we demonstrate that unless mu' = 0 (to be strictly distinguished from
mu' small but non-vanishing), any finite-order series expansion for G(k;mu) in
powers of mu' in general falsely signals breakdown of the Luttinger theorem.
The violation of this theorem as asserted by Kokalj and Prelovsek is therefore
an artifact of their first-order calculation.
