Study curvature operators with sectional curvature bounds using convex algebraic geometry.
problem Characterize algebraic curvature operators with sectional curvature bounds.
method Apply convex algebraic geometry techniques, including spectrahedra and hierarchies of inner and outer approximations.
result For , the set of curvature operators is a spectrahedron or a spectrahedral shadow, providing counter-examples to the Helton--Nie Conjecture.