Algebras over infinity-operads

We develop a notion of an algebra over an infinity-operad with values in infinity-categories which is completely intrinsic to the formalism of dendroidal sets. Its definition involves the notion of a coCartesian fibration of dendroidal sets and extends Lurie's definition of a coCartesian fibration of simplicial sets. We show how, for a dendroidal set X, the coCartesian fibrations over X fit together to form an infinity-category coCart(X). Using a generalization of the Grothendieck construction, we prove that coCart(X) is equivalent to the infinity-category of algebras in infinity-categories over the simplicial operad associated to X. This equivalence can be restricted to give an equivalence between algebras taking values in infinity-groupoids (or equivalently, spaces) and the infinity-category of so-called left fibrations over X.