Title: Algebraic operads up to homotopy,
Abstract: 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.