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Let
be a pointed, polyhedral cone. In this paper, we study the cone
of quadratic forms. Understanding the structure of
is important for globally solving NP-hard quadratic programs over
P
. We establish key characteristics of
and construct a separation algorithm for
provided one can optimize with respect to a related cone over the boundary of
P
. This algorithm leads to a nonlinear representation of
and a class of tractable relaxations for
, each of which improves a standard polyhedral-semidefinite relaxation of
. The relaxation technique can further be applied recursively to obtain a hierarchy of relaxations, and for constant recursive depth, the hierarchy is tractable. We apply this theory to two important cases:
P
is the nonnegative orthant, in which case
is the cone of completely positive matrices; and
P
is the homogenized cone of the “box” [0, 1]
n
. Through various results and examples, we demonstrate the strength of the theory for these cases. For example, we achieve for the first time a separation algorithm for 5 × 5 completely positive matrices.