Minimal values of the parameter σ with respect to parameters n and λ for which the trapping phenomenon occurs. Their limitations with respect to the parameter n is set by considered physically plausible values of σ_causal and by the requirement of the existence of polytropic configurations (rings at the end of the lines; using depicted scope visible up to λ = 5 × 10⁻⁴). Borderlines for common values of n for cases λ < 10⁻⁵ differ only negligibly from the case λ = 0.