In this talk I'll discuss D3-brane probes of singularities in non-trivial F-theory backgrounds. The D3-brane theory can be obtained by deformation of two quite different N=2 SCFTs, and details of the theory can be determined by studying elliptic fibrations over a three-sphere surrounding the singularity. In this picture, string junctions end on knots and D3-brane traversal of the knot complement plays a critical role.

Coauthors: Antonella Grassi, Fabian Ruehle, and Julius Shaneson