Contents

### Context

#### Higher algebra

higher algebra

universal algebra

# Contents

## Idea

A free operad is free on a collection of operations.

Given a collection $k$-ary operations-to-be for each $k \in \mathbb{N}$, the free operad on this collection has as $n$-ary operations the collection of all trees with $n$ leaves equipped with a labelling of each vertex $v$ with a $k$-ary operation, for $k$ the incoming edges to $v$.

## Definition

Let $V$ be a symmetric monoidal category.

For $G$ a discrete group, write $V^G$ for the category of objects of $V$ equipped with a $G$-action. For $V$ symmetric monoidal this is again a symmetric monoidal category and the forgetful functor $V^G \to V$ is symmetric monoidal.

###### Definition

The category of collections (Berger-Moerdijk) or $\mathbb{S}$-modules (Getzler-Kapranov) of $V$, or the category of $V$-species, is

$V Coll := \prod_{n \in \mathbb{N}} V^{S_n} \,.$

Notice that both $S_0$ and $S_1$ are the trivial group.

So a $V$-operad $P$ is a special $V$-collection with extra structure relating its components. This gives an evident forgetful functor

$U : V Operad \to V Coll \,.$
###### Definition

The free functor left adjoint to this forgetful functor is the the free operad functor

$F : V Coll \stackrel{\leftarrow}{\to} V Operad : U \,.$

For $C$ a given collection, we call $F(C)$ the operad free on the collection $C$.

###### Remark

This free/forgetful adjunction is used to define the model structure on operads by transfer.

## Properties

### Explicit construction

The free operad functor may more explcitly be described as follows (see (Berger-Moerdijk, section 5.8)).

###### Definition

Let $\mathbb{T} := Core(\Omega_pl)$ be the core of the category of planar rooted trees and non-planar morphisms (so the morphisms need not respect the given planar structure).

Write

• $t_n \in \Omega_n$ for the $n$-corolla (the tree with a single vertex, $n$ inputs and its unique output root);

• for $T$ any tree with $n$-ary root vertex let $\{T_i\}_{i=1}^n$ be the sub-trees such that $T = t_n \circ (T_1, \cdots, T_n)$.

Then every collection $K \in V Coll$ defines a functor $\bar K : \mathbb{T}^{op} \to V$ by the inductive formula

$\bar K (|) \coloneqq I$
$\bar K(t_n(T_1, \cdots, T_n)) \coloneqq K(n) \otimes \bar K(T_1) \otimes \cdots \bar K(T_n) \,.$

Define moreover the functor

$\lambda : \mathbb{T} \to Set$

to be the functor that sends a tree to the set of numberings of its leaves, and let $\bar \lambda : \mathbb{T} \to V$ be given by postcomposition with $S \mapsto \coprod_{s \in S} I$, where on the right we have the coproduct of ${\vert S \vert}$ copies of the tensor unit in the monoidal category $V$.

So for $T$ a tree with $n$ leaves we have

$\bar \lambda(T) \simeq \coprod_{\sigma \in \Sigma_n} I \,,$

where the coproduct ranges over the elements of the symmetric group on $n$ elements.

###### Proposition

The free operad on a collection $K$ is isomorphic to the coend

$\bar K \otimes_{\mathbb{T}} \bar \lambda = \int^{T \in \mathbb{T}} \bar K(T) \otimes \bar \lambda(T) \,.$
###### Remark

The groupoid $\mathbb{T}$ is equivalent to the disjoint union over isomorphism classes of planar trees of the one-object groupoids with morphisms the given automorphism group

$\mathbb{T} \simeq \coprod_{[T] \in \pi_0\mathbb{T}} \mathbf{B} Aut(T) \,.$

Therefore the above coend is equivalently

$\bar K \otimes_{\mathbb{T}} \bar \lambda = \coprod_{[T] \in \pi_0\mathbb{T}} \bar K(T) \otimes_{Aut(T)} \bar \lambda(T) \,.$

## Examples

Let $K$ be the collection with $K(0) = \emptyset$ and $K(n) = I$ for $n \gt 0$. The corresponding free operad has as $n$-ary operations all rooted trees with $n$ leaves. And composition of operations is given by grafting of trees.

Riemann surfaces operad (TO BE EXPANDED)

Deligne-Mumford opeard (TO BE EXPANDED)

little discs operad, framed little discs operad (TO BE EXPANDED) – See Deligne conjecture

A brief remark on free operads is in (1.12) of

• Ezra Getzler, Mikhail Kapranov, Cyclic operads and cyclic homology, Conf. Proc. Lect. Notes Geom. Topology IV, Int. Press Camb. (1995), 167–201.

A detailed discussion is in Part II, chapter I, section 1.9 of

• Martin Markl, S. Shnider, Jim Stasheff, Operads in Algebra, Topology and Physics, Surveys and Monographs of the Amer. Math. Soc. 96 (2002).

and in section 3 of

The coend-description is given in section 5.8 of