The unimodal Zettl Function is based on the defintion $$f(\mathbf{x}) = (\mathbf{x}_1^2 + \mathbf{x}_2^2 - 2\mathbf{x}_1)^2 + 0.25 \mathbf{x}_1$$ with box-constraints \(\mathbf{x}_i \in [-5, 10], i = 1, 2\).
makeZettlFunction()[smoof_single_objective_function]
H. P. Schwefel, Evolution and Optimum Seeking, John Wiley Sons, 1995.