Three-frequency resonances in a quasiperiodically driven system

The fine structure of three-frequency resonances is investigated in an exactly solvable model (nonlinear system of coupled ordinary differential equations). In particular the qualitative behavior founded suggest the existence of generic features in the corresponding parameter space organization. In particular we have identified an hierarchical structure of the stability widths which can be adequately described in terms of a generalized Farey sum operation between fractions of real numbers.