(a) Collapse time as a function of the initial conditions for underdense perturbations with ﹩\delta _{0}=-1﹩. The contours of constant collapse time, expressed by ﹩a^{-1}_{c}﹩, are displayed for the ZA. The thin (thick) contours are spaced by 0.1 (0.5) in ﹩a^{-1}_{c}﹩, with the innermost contour being ﹩a^{-1}_{c}=0﹩. Initial perturbations in the central region do not collapse. (b) Same as (a) except that the DTA is used. (c) Same as (a) except that the LTA is used. (d) Signs of ﹩\lambda ^{0}_{i}﹩ corresponding to each region of the parameter space of initial conditions, with ﹩\delta _{0}=-1﹩. Spherical, planar, and cylindrically symmetric underdense perturbations are marked with a cross, a plus sign, and an asterisk, respectively. The upper parts of these graphs, which correspond to ﹩0\leq \alpha _{0}\leq \pi ﹩, have ﹩\lambda ^{0}_{3}> \lambda ^{0}_{2}> \lambda ^{0}_{1}﹩. The lower one covers the same values of ﹩\lambda ^{0}_{i}﹩, through a permutation of the indices.