The 4d Superconformal Index from q-deformed 2d Yang-Mills
  We identify the 2d topological theory underlying the N=2 4d superconformal index with an explicit model: q-deformed 2d Yang-Mills. By this route we are able to evaluate the index of some strongly-coupled 4d SCFTs, such as Gaiotto's T_N theories. 
Abhijit Gadde, Leonardo Rastelli, Shlomo S. Razamat, Wenbin Yan
hep-th