Matching parameter p as a function of the Sérsic index n (solid line) determined by numerically solving Eq. (15) so that the total luminosity of the profile ν_a(r) in Eq. (13) coincides with L of the true profile in Eq. (1). As explained in Appendix A, p(n) vanishes for n → 1 and then increases for decreasing n < 1 (see Table B.1). The dashed line gives the transition radius (in units of R_e) as defined in Eq. (16); for n < 1, s_t increases for decreasing n, remaining small, however.