Top panels: left – sample M(R) relations; right – corresponding M(R̂) relations. Red curve and corresponding observations: example of a training datum, consisting on 20 points from the M(R) and M(R̂), selected assuming random normal distribution with the mean value equal to true values and standard deviations σ_M = 0.1 M_⊙, σ_R = 1.0 km and σ_R̂ = 1.0 km. The configurations with the same M and R have, in general, different R̂. Bottom panels: left – schematic of a model EOS, composed in the low-density part from the realistic crust of the SLy4 EOS (Haensel & Pichon 1994; Douchin & Haensel 2001) and piecewise relativistic polytropes (Tooper 1965), right – mass-radius M(R) relations generated using the piecewise relativistic polytrope model (thin solid grey curves) and astrophysical models of NS sequences, based on the following EOSs: the SLy4 EOS (solid blue curve), the APR EOS (dashed red curve) and the BSK20 EOS (dash-dotted green curve).