A phenomenon in which a system being forced at an irrational period undergoes rational, periodic motion which persists for a finite range of forcing values. It may occur for strong couplings between natural and forcing oscillation frequencies.
The phenomenon can be exemplified in the circle map when, after 
iterations of the map, the new angle differs from the initial value by a rational
number
 |
This is the form of the unperturbed circle map with the map winding number
 |
For 
not a rational
number, the trajectory is quasiperiodic.
