  On the basis of an analysis of the numerical results corresponding to the
half-filled 1D t-t'-V model on some finite lattices, Kokalj and Prelovsek (KP)
have in a recent paper [Phys. Rev. B 78, 153103 (2008), arXiv:arXiv:0803.4468]
concluded that the Luttinger theorem (LT) does not apply for the Mott-Hubbard
(MH) insulating phase of this model (i.e. for V >> t) in the thermodynamic
limit; KP even suggested, incorrectly, that failure of the LT were apparent for
a half-filled finite system consisting of N=26 lattice sites. By employing a
simple model for the self-energy Sigma of a MH state, we show that the
finite-size-scaling approach of the type utilised by KP is not reliable for the
system sizes considered by KP. On the basis of the equivalence of the model
under consideration (at half-filling and for t'/t << 1) and the XXZ spin-chain
Hamiltonian for SU(2) spins, we further show that for V > V_c(t,t') the system
under consideration has a charge-density-wave (CDW) ground state (GS) in the
thermodynamic limit, corresponding to a doubling of the unit cell in comparison
with that specific to the underlying lattice. Although this GS is also
insulating, its spectral gap is due to the broken translational symmetry of the
GS; it is not a correlation-induced MH gap. The LT is therefore a priori valid
for this GS. This fact establishes that the conclusion by KP is indeed
erroneous. Finally, we present a heuristic argument due to Volovik that sheds
light on the mechanism underlying the robustness of the LT. In an appendix, we
present the details of the calculation of the single-particle Green function of
the broken-symmetry GS of the model under consideration by means of
bosonization and in terms of the form factors of a class of soliton-generating
fields pertaining to the quantum sine-Gordon Hamiltonian. [Shortened abstract]
