Algebraic operads up to homotopy
  This paper deals with the homotopy theory of differential graded operads. We
endow the Koszul dual category of curved conilpotent cooperads, where the
notion of quasi-isomorphism barely makes sense, with a model category structure
Quillen equivalent to that of operads. This allows us to describe the homotopy
properties of differential graded operads in a simpler and richer way, using
obstruction methods.
