I did not understand how the equation below is obtained:
where $\pi^{\prime}(x_k)=argmax_{u_k \in \mathcal{U}}q_{\pi}(x_k, u_k)$.
Note: the source of the above equation is slide 22, at the follwing link: https://groups.uni-paderborn.de/lea/share/lehre/reinforcementlearning/lecture_slides/built/Lecture03.pdf