Figure 1.

(a) Typical aspect of the logarithm of the cost function F(W,D²,α₀) for a fixed value of α₀. The logarithm is shown here in order to enhance the visual contrast. This cost function has one minimum denoted $(W_{*},D_{*}^2)$ that depends on α₀. (b) Typical aspect of the function $\log \left(F(W_{*},D_{*}^2,{\alpha }_0)\right)$ for varying values of α₀ (W_* and $D_{*}^2$ being calculated for each α₀). This cost function has a single minimum.