  We have investigated the absorption spectrum of multilayer graphene in high
magnetic fields. The low energy part of the spectrum of electrons in graphene
is well described by the relativistic Dirac equation with a linear dispersion
relation. However, at higher energies (>500 meV) a deviation from the ideal
behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation
from linearity is 40 meV. This result is in good agreement with the theoretical
model, which includes trigonal warping of the Fermi surface and higher-order
band corrections. Polarization-resolved measurements show no observable
electron-hole asymmetry.
