  We theoretically examine the momentum dependence of resonant inelastic x-ray
scattering (RIXS) spectrum for one-dimensional and two-dimensional cuprates
based on the single-band Hubbard model with realistic parameter values. The
spectrum is calculated by using the numerical diagonalization technique for
finite-size clusters. We focus on excitations across the Mott gap and clarify
spectral features coming from the excitations as well as the physics behind
them. Good agreement between the theoretical and existing experimental results
clearly demonstrates that the RIXS is a potential tool to study the
momentum-dependent charge excitations in strongly correlated electron systems.
