Find ellipsoid that contains intersection of an ellipsoid and a hyperplane I have an $n$-dimensional ellipsoid $E$ and a hyperplane $H$. This hyperplane cuts $E$ into two parts: $E_1$ and $E_2$ (whose disjoint union is $E$). I want to find another ellipsoid $E'$ that has minimal hyper-volume and contains $E_1$.  Is there an efficient algorithm to do this?

My first thought was to formulate it as an optimization problem, but I am having difficulty with formulating it, as I don't know how to formulate the containment ($E_1 \subseteq E'$) constraint.

An approximation for the minimal hyper-volume ellipsoid is also good for my needs.
Your are describing the basic step in the ellipsoid algorithm. Your question might be answered in lecture notes on the algorithm, such as these ones by Goemans.

More generally, given a convex body $K$, the minimum volume ellipsoid containing it is called the Löwner–John ellipsoid. The same name is also given to the maximum volume ellipsoid contained in $K$. See for example these lecture notes of Boyd and Vandenberghe.