BPS Spectrum, Indices and Wall Crossing in N=4 Supersymmetric Yang-Mills   Theories
  BPS states in N=4 supersymmetric SU(N) gauge theories in four dimensions can be represented as planar string networks with ends lying on D3-branes. We introduce several protected indices which capture information on the spectrum and various quantum numbers of these states, give their wall crossing formula and describe how using the wall crossing formula we can compute all the indices at all points in the moduli space. 
Ashoke Sen
hep-th math.AG