Fig. 4

The convergence of the MF trend, MF and OK models of the \(c_1\). In c–h, the left, the middle and the right columns show the values of \(c_{1}^{MF}\), \({\widetilde{c}}_{1}^{MF}\) (see Eq. (9)) and \({\widetilde{c}}_{1}^{OK}\). The points in the DoE are represented as white dots