Free-Field Resolutions of the Unitary N=2 Super-Virasoro Representations
  We construct free-field resolutions of unitary representations of the N=2 superconformal algebra. The irreducible representations are singled out from free-field spaces as the cohomology of fermionic screening operators. We construct and evaluate the cohomology of the resolution associated with one fermionic screening (which is related to the representation theory picture of ``gravitational descendants''), and a {\it butterfly} resolution associated with two fermionic screenings. 
B L Feigin and A M Semikhatov