Using band-structure theory, the authors calculate the O 1s and Cu 2p X-ray-absorption spectra of YBa2Cu3O7. The former spectra are in rather good agreement with experiment, showing that the peaks in the low-energy region are due to the van Hove singularity in the chain band. Using core-exciton theory, they argue that this van Hove singularity can survive the strong Cu 2p-3d Coulomb interaction, giving rise to the 'satellite' in the chain Cu 2p spectrum.
 

Physical Review B, 40 837-40, 1989.