Schematic representation of the interior structures of an sdB star, displaying the logarithm of the diffusion coefficient for mixing during the core He-burning stage. Different levels of mixing are indicated by the grayscale bar on the right-hand y-axis. The panels show the results of the sign-change numerical method (a), the PM scheme (b), and the CPM scheme (c). Gray denotes convective core regions, with the hatched area in panel (c) indicating semi-convection in the transitional zone between convective and radiative regions.