The Trecanni function belongs to the unimodal test functions. It is based
on the formula
$$f(\mathbf(x)) = \mathbf(x)_1^4 - 4 \mathbf(x)_1^3 + 4 \mathbf(x)_1 + \mathbf(x)_2^2.$$
The box-constraints \(\mathbf{x}_i \in [-5, 5], i = 1, 2\) define the
domain of defintion.