Exact two-dimensional superconformal R-symmetry and c-extremization
  We uncover a general principle, dubbed c-extremization, which determines the exact R-symmetry of a two-dimensional unitary superconformal field theory with N=(0,2) supersymmetry. To illustrate its utility, we study superconformal theories obtained by twisted compactifications of four-dimensional N=4 super-Yang-Mills on Riemann surfaces, and construct their gravity duals. 
Francesco Benini, Nikolay Bobev
hep-th