Spectral networks, finite webs, and the $T_3$ cubic differential

Abstract

We describe spectral networks for cubic differentials on $\mathbf{P}^1$ with exactly three poles all of order two, following observations of Hollands-Neitzke. To do so, we give a precise definition and establish some basic properties of a class of finite webs (corresponding to BPS states in physics) in the cubic differential setting, including their non-vanishing central charge. Turning to the example, we prove that at rational angles the network is totally degenerate; the number of walls and Stokes regions is explicitly computed and depends on an arithmetic criterion. At all other angles, the spectral network is dense in the surface and contains no finite webs, as expected. Physically, this example corresponds to a fundamental building block for a large class of four-dimensional $\mathcal{N}=2$ QFTs with gauge group $\mathrm{SU}(3)$.