Cluster abundance ratio ﹩n( z=0) / n( z=0.5) ﹩ vs. σ₈ for clusters with virial mass ﹩M_{v}\geq 3\times 10^{14}﹩ h⁻¹ M_⊙ (richness 1) and ﹩M_{v}\geq 6\times 10^{14}﹩ h⁻¹ M_⊙ (richness 2), from the Press‐Schechter relation, eq. (1) (represented by dots for all values of Ω from 0.2 to 1 [bottom to top] for each σ₈). The lines are the best‐fit empirical relations (eqs. [9a] and [9b], with ﹩\Omega \simeq 0.5﹩).