Period of small oscillations around P as a function of the parameters. There are two oscillation modes, shown in the two panels. In the hatched area, the equilibrium is unstable. The singular points S₁ and S are labelled, and the cutting line of the three-dimensional surface is shown by a grey line (same as Fig. 9). Three-dimensional figures are provided in Appendix C, where the geometry of the stable regions is clearer.