Fig. 1.

Examples of the stationary point where $\nabla \mathrm{OFV}=\overrightarrow{0}$ for the case of two parameter model (i.e., p = 2). Top left: a minimum on the surface, where the curvature is positive in all directions. Top right: a saddle point, marked *, where the curvature is negative in one direction around a point, but positive in the other. Bottom left: a so-called monkey saddle, a degenerate saddle point with reversing curvature (inflection) around a point. Bottom right: a region of non-identifiability, where the curvature is zero in one direction, and all values of θ₁ produce the same, lowest OFV value along a vector