For example, in the case of two non-resonant modes, a situation generally encountered in RR Lyrae variables, the temporal evolution of the amplitudes A 1 and A 2 of the two normal modes 1 and 2 is governed by the following set of ordinary differential equations d A 1 d t = κ κ 1 A 1 + ( Q 11 A 1 2 + Q 12 A 2 2 ) A 1 d A 2 d t = κ κ 2 A 2 + ( Q 21 A 1 2 + Q 22 A 2 2 ) A 2 where the Q ij are the nonresonant coupling coefficients.