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

Comment on "Violation of the Luttinger sum rule within the Hubbard model  on a triangular lattice", by J. Kokalj and P. Prelovsek [Eur. Phys. J. B 63,  431 (2008), arXiv:arXiv:0709.0263]