Positive Polynomials and Projections of Spectrahedra

This article is concerned with different aspects of spectrahedra and their projections, sets that are important in semidefinite optimization. We prove results on the limitations of so-called Lasserre and theta body relaxation methods for semialgebraic sets and varieties. As a special case we obtain the main result of Netzer, Plaumann, and Schweighofer [SIAM J. Optim., 20 (2010), pp. 1944–1955] on nonexposed faces. We also solve the open problems from that work. We further give a unified account of several results on convex hulls of curves and images of polynomial maps. We finally prove a Positivstellensatz for projections of spectrahedra, which exceeds the known results that only work for basic closed semialgebraic sets.