Lines of equal k₂ (blue) and equal M_core/M_total (red) in Δρ-x_m parameter space for a theoretical three-layer model, see Eqs. (9)–(11) with the fixed parameters q₁ = 0.98π, q₂ = 0.99π and x_c = 0.1. Numbers give the corresponding values of k₂ and M_core/M_total, respectively. The equi-k₂-lines demonstrate the degeneracy of k₂ with respect to the position (x_m) and size (Δρ) of a density discontinuity in the envelope and the ratio of core mass to total mass (a blue line intersects several red lines).