Two-dimensional, multimodal test function. The defintion is given by
$$f(\mathbf{x}) = \sin(\mathbf{x}_1 + \mathbf{x}_2) + (\mathbf{x}_1 - \mathbf{x}_2)^2 - 1.5 \mathbf{x}_1 + 2.5 \mathbf{x}_2 + 1$$
subject to \(\mathbf{x}_1 \in [-1.5, 4], \mathbf{x}_2 \in [-3, 3]\).