Sample profiles of semiconvective (left) and exponentially overshooting (right) 3  M _☉ models undergoing core helium burning. Top panels show the radiative, adiabatic, temperature, and Ledoux gradients that determine mixing boundaries and diffusion coefficients. Bottom panels show the resulting diffusion coefficients for energy and chemical transport. In either case, a thin dotted line spanning a single intermediate cell joins the convective and semiconvective/overshoot curves. This is intended merely as a guide for the eye, as diffusion coefficients are defined only at the two boundaries of a cell. In particular, diffusion for this intermediate cell is governed by convection at its interior boundary and semiconvection/overshoot at the exterior. The semiconvective model shown here was run with α _sc = 0.01, the exponentially overshooting model with f _ov = 10 ⁻⁵. The profiles are taken at the points marked in Figure 15.