  Resonant relaxation is a novel form of two-body relaxation that arises in
nearly Keplerian disks such as protoplanetary disks. Resonant relaxation does
not affect the semimajor axes of the particles, but enhances relaxation of
particle eccentricities and inclinations. The equilibrium state after resonant
relaxation is a Rayleigh distribution, with the mean-square eccentricity and
inclination inversely proportional to mass. The rate of resonant relaxation
depends strongly on the precession rate of the disk. If the precession due to
the disk's self-gravity is small compared to the total precession, then the
relaxation is concentrated near the secular resonance between each pair of
interacting bodies; on the other hand if the precession rate is dominated by
the disk's self-gravity then relaxation occurs through coupling to the
large-scale low-frequency m=1 normal modes of the disk. Depending on the disk
properties, resonant relaxation may be either stronger or weaker than the usual
non-resonant relaxation.
