I want to find the smallest sphere which encapsulate a set of points $x_i \in \mathbb{R}^d$.
I can formulate is as
$$ \arg \min_{a \in \mathbb{R}^d, r} r \quad\quad \text{s.t.} \quad || x_i - a|| \leq r $$
Can it be formulated as a linear programming problem?
If not, is there a reasonable approximation?