Nonnegative Polynomials and Moment Problems on Algebraic Curves

The cone of nonnegative polynomials is of fundamental importance in real algebraic geometry, but its facial structure is understood in very few cases. We initiate a systematic study of the facial structure of the cone of nonnegative polynomials $\text{pos}$ on a smooth real projective curve $X$. We show that there is a duality between its faces and totally real effective divisors on $X$. This allows us to fully describe the face lattice in case $X$ has genus one. We compute the Carathéodory number of the dual moment cone ${\text{pos}}^{\vee }$ for an elliptic normal curve $X$, which measures the complexity of quadrature rules of measures supported on $X$. Interestingly, the topology of the real locus of $X$ influences the Carathéodory number of ${\text{pos}}^{\vee }$. We apply our results to truncated moment problems on affine cubic curves, where we deduce sharp bounds on the flat extension degree.