  The out-of-time ordered correlator (OTOC) is a measure of scrambling of
quantum information. Scrambling is intuitively considered to be a significant
feature of chaotic systems and thus the OTOC is widely used as a measure of
chaos. For short times exponential growth is related to the classical Lyapunov
exponent, sometimes known as butterfly effect. At long times the OTOC attains
an average equilibrium value with possible oscillations. For fully chaotic
systems the approach to the asymptotic regime is exponential with a rate given
by the classical Ruelle-Pollicott resonances. In this work, we extend this
notion by showing that classical generalized resonances govern the relaxation
to equilibrium of the OTOC in the ubiquitous case of a system with mixed
dynamics, in particular, the standard map.
