I am trying to solve this problem, but I do not understand what is meant bythe following expression being minimised
$$ D = P_{k} + P_{j} $$
I am trying to solve this problem, but I do not understand what is meant bythe following expression being minimised
$$ D = P_{k} + P_{j} $$
A value is minimised when it becomes as small as it can.
I expect that your $P_k$ and $P_j$ are points. Then $\|P_k-P_j\|$ is the distance between any two points. To minimise $\|P_k-P_j\|$ is to find the two points that are closest to one another out of all the other possible pairs of points.