Value of ∂θ/∂ρ₀ in the (ρ_C, s) plane for infinite D. Particles come from s = ∞, they reach their minimum radii at s = 0, at which the last term of (Eq. (41)) diverges, and they go on with negative s. The white line shows the level curve ∂θ/∂ρ₀ = 0, corresponding to the caustic (overdensity of particles). It is formed by the set of T₁ trajectories (ρ_C > 1). See Fig. 6 for its shape in the physical plane.