Variation of the sausage oscillation period with the wavelength in a plasma slab with α = 6, for different values of δ: the solid line corresponds to δ = 0.5, the triple dot-dashed line corresponds to δ = 0.7, the dashed line to δ = 0.8, and the dot-dashed line to δ = 0.9. The thick straight line shows P = λ/C_Ae, that is the cutoff separating the trapped and leaky regimes. The thinner straight lines show P = λ/C_A0 that correspond to the short wavelength asymptote in each case.