The cone of Betti diagrams over a hypersurface ring of low embedding dimension

We give a complete description of the cone of Betti diagrams over a standard graded hypersurface ring of the form k[x,y]/〈q〉, where q is a homogeneous quadric. We also provide a finite algorithm for decomposing Betti diagrams, including diagrams of infinite projective dimension, into pure diagrams. Boij–Söderberg theory completely describes the cone of Betti diagrams over a standard graded polynomial ring; our result provides the first example of another graded ring for which the cone of Betti diagrams is entirely understood.