The root functor

In this paper we show that any $\infty$-operad is equivalent to the localization of a discrete $\Sigma$-free operad, working in the formalism of dendroidal sets. The key point is defining the root functor of a dendroidal set $X$, a functor from the dendroidal nerve of a discrete operad $\mathbf{\Omega}/X$ into $X$, which we show to be an operadic weak equivalence after localizing $\mathbf{\Omega}/X$. This extends an analogous result for $\infty$-categories due to Joyal: when $X$ is a simplicial set, $\mathbf{\Omega}/X$ is its category of elements, and the root functor is the last vertex map. As an application, we deduce that the $\infty$-category of algebras over an $\infty$-operad is equivalent to that of locally constant algebras over its discrete resolution.