On the longest chain of faces of the completely positive and copositive cones

Abstract: We consider a wide class of closed convex cones $K$ in the space of real $n\times n$ symmetric matrices and establish the existence of a chain of faces of $K$, the length of which is maximized at $\frac{n(n+1)}{2} + 1$. Examples of such cones include, but are not limited to, the completely positive and the copositive cones. Using this chain, we prove that the distance to polyhedrality of any closed convex cone $K$ that is sandwiched between the completely positive cone and the doubly nonnegative cone of order $n \ge 2$, as well as its dual, is at least $\frac{n(n+1)}{2} - 2$, which is also the worst-case scenario.