I'm triying to replicate this paper: https://iopscience.iop.org/article/10.1088/1742-6596/1818/1/012022/pdf
But I don't know how implement this part: Find $d_{k}$ such that
$$\min q_{k}(d) \quad \text{such that} \quad ||d|| \le \quad \delta_{k}$$ where $$_{}() =\frac{1}{2} ‖(_{}) + _{}‖^{2}$$