symmetric monoidal (∞,1)-category of spectra
An operad is a gadget used to describe algebraic structures in symmetric monoidal categories. It is
a bunch of abstract operations of arbitrarily many arguments;
equipped with a notion of how to compose these;
An algebra over an operad is a concrete realization of these abstract operations: an object equipped with -ary operations as specified by the operad, subject to the composition relation as specified by the operad.
This way an operad is like a Lawvere theory in that it can be used to describe algebraic structures having finitary operations obeying equational laws. However, unlike Lawvere theories, operads can be defined internal to general symmetric monoidal categories where the tensor product might not be the cartesian product.
The notion of operad (and allied notions such as PROP, club, multicategory and so on) come in many flavors. Originally used in algebraic topology to provide a systematic formalism for describing the internal operations which exist on iterated loop spaces, the basic idea is quite flexible and adaptable to many categorical situations, and the importance of operads continues to grow.
The original definition is due to Peter May and was given in his book (The Geometry of Iterated Loop Spaces). It describes an operad as a collection of operations equipped with a notion of composition and subject to various conditions.
Let be a symmetric monoidal category. A ( permutative or symmetric) operad in consists of objects of indexed over the natural numbers [which we intuitively think of as objects that parametrize the -ary operations of an algebraic theory] equipped with the following extra structure:
[which we think of as the result of plugging the outputs of operations into a -ary operation , to produce a new operation ].
These data are subject to obvious identities such as associativity and unitality of composition, and compatibility of composition with symmetric group actions. For example, the unit laws say that the evident composite
is the identity map, as is
Compatibility with symmetric group actions means that for each element , the composition operation
coequalizes a pair of automorphisms
where acts on the big tensor product on the left by permuting tensor factors in the obvious way. If has suitable colimits, this condition could be expressed in terms of tensor products over .
The associativity condition will be left for others to fill in.
There is an evident generalization of the above where we allow the operad to have several objects – called colours is operad theory. This models algebraic structures where elements of different types may be fed into -ary operations.
Let be a set, called the set of colours . Then a coloured operad is
for each and each -tuple an object ;
for each a morphism in – the identity on ;
for each -tuple and other tuples
the compoosition operation;
subject to the conditions that
An equivalent term for a colored operad in is a symmetric multicategory which is enriched over . Which term is used depends on the author and the point of view being taken. (According to the common terminology of horizontal categorification, one might also call a colored operad an “operad-oid,” but thankfully this seems not to be common. On the other hand one might adapt the terminology backwards and call a category a “colored monoid”.)
An algebra over an operad in is just a semantics for interpreting the as objects of actual -ary operations on an object . That is, an -algebra structure on an object in consists of a collection of maps
which intuitively is a mapping like this:
so that “elements” of are interpreted as as -ary operations on . These data are subject to some natural conditions which implement this idea.
Perhaps the quickest way to define it is to suppose that is symmetric monoidal closed, and work by way of parallel to how representations or modules work. Just as an -module (over a ring ) can be defined as a ring homomorphism
where the hom here is an internal hom of abelian groups, called an endomorphism ring, so there is such a thing as an endomorphism operad attached to any object in a symmetric monoidal closed category, and an -algebra over an operad is the same thing as an operad morphism
to an endomorphism operad (also called a tautological operad).
Now that the clue has been given, the rest is not hard to figure out. The components of the endomorphism operad are defined by
Certainly acts on the right (that is, contravariantly) on the hom-object . And clearly there is a canonical map to play the role of the unit. The operad composition involves an instance of enriched functoriality of iterated tensor products: there is a map
The endomorphism operad composition is obtained by tensoring this last arrow with on the left, and composing the result with ordinary internal hom-composition
A closely related way of defining an -algebra is via the monad attached to an operad, which we will describe below.
Note that this definition still makes sense when lives in any symmetric monoidal -enriched category, not only itself.
We describe here a compact one-sentence definition of operad first worked out by Max Kelly, after a few preliminaries which are important in their own right. The treatment is essentially an exercise in enriched category theory and the formalism of Day convolution. We will work this out fully in the case of ordinary category theory first, that is for categories enriched in ; the case for categories enriched in a complete, cocomplete, symmetric monoidal closed is completely parallel.
More details along these lines are add Towards a doctrine of operads.
Let be the groupoid of finite cardinals with bijections as morphisms – the permutation category. Since is the core groupoid of the category of finite cardinals and functions between them, the coproduct on restricts to a symmetric monoidal product called the cardinal sum on .
Under this symmetric monoidal structure, may be characterized as the free symmetric strict monoidal category on one generator.
By abuse of notation, we will also denote the presheaf category equipped with the monoidal structure induced by the cardinal sum by .
Since is a presheaf category, it is cocomplete, and since the Day convolution is cocontinuous in each of its separate arguments we say that is symmetric monoidally cocomplete.
In addition to the standard coend formula, the Day convolution product on the may be described by the rule:
summing over all partitions of into two parts (each possibly empty).
According to the yoga of presheaf categories and Day convolution, given a symmetric monoidally cocomplete category , a symmetric monoidal functor
extends uniquely up to isomorphism to a symmetric monoidal cocontinuous functor
taking a presheaf to the weighted colimit .
It follows from the earlier remark and the above that we may describe universally up to equivalence as the free symmetric monoidally cocomplete category on a single generator.
Recall that we can describe as follows: First note that the functor
so also gives a functor by currying through the second coordinate.
Then we define to be the object representing the functor
whenever is representable.
In general, weighted colimits may be described explicitly by coend formulas; here
where denotes the tensoring of a set with an object , that is the coproduct of an -indexed set of copies of . The coend here indicates a coequalizer.
where one of the parallel arrows involves right actions of symmetric groups on the , and the other involves left actions of on objects . In other words, the coend in this instance may be described as a sum of tensor products:
The aforementioned universal property of with its convolution product may be more explicitly described as follows: given a symmetric monoidally cocomplete category and an object therein, there exists up to isomorphism a unique symmetric monoidal cocontinuous functor which sends the presheaf representable by the cardinal , , to .
Explicitly, this functor takes a presheaf to the following object of :
When is the symmetric monoidally cocomplete category and is a set, this formula
is the value at of what Joyal calls the analytic functor associated to a species , which has been proposed as the categorification of the theory of exponential generating functions. The fact that is symmetric monoidal (cocontinuous) means that there is a canonical isomorphism
In other words, behaves like a categorified version of Fourier transform, taking convolution products to ordinary (pointwise) products.
For symmetric monoidally cocomplete categories , let denote the category of symmetric monoidal cocontinuous functors . The universal property of means that we have an equivalence
Consequently, we have an equivalence
Since symmetric monoidal cocontinuous functors are stable under composition, the category on the left carries a monoidal product given by endofunctor composition. By transport of structure across the equivalence, we induce a monoidal product on given by endofunctor composition called the substitution product of species. The substitution product of species is denoted .
In detail: a species induces a symmetric monoidal cocontinuous functor
The -fold Day tensor power of is given (in the language of species) by the formula
where we sum over all ways of breaking up a finite set into blocks, some possibly empty. Thus we have an explicit description of the substitution product,
and it is clear from our discussion above that substitution is a monoidal product. The monoidal unit is a functor where is terminal if , else is initial.
We are at last ready for the one-sentence definition:
We can get different flavors of operad by considering different notions of monoidal category. For instance, for the theory of monoidal categories, the discrete category plays the role of the free (strict) monoidal category on one generator, and the free monoidally cocomplete category on one generator. Similarly, for braided monoidal categories, we have the braid category , and is the free braided monoidally cocomplete category on one generator. Again, for cartesian categories, we have (the opposite of finite sets and functions) as the free cartesian category on one generator, and is the free cartesian monoidally cocomplete category on one generator. In each of these cases we get a corresponding notion of operad by following the above treatment mutatis mutandis: nonpermutative operads, braided operads, cartesian operads (better known as Lawvere theories). These are all special cases of the notion of generalized multicategory.
All of the above carries over to the enriched setting, where we work over a complete, cocomplete symmetric monoidal closed base category . Here ordinary categories (like ) are viewed as -enriched by a simple change of base: change from hom-sets to hom-objects by applying the change of base functor
that takes a set to the -fold coproduct , where is the monoidal unit of . These can also be defined in the framework of generalized multicategories.
Each -based operad gives rise to a monad on . Specifically, the monoidal category acts on in such a way as to give an actegory structure, and therefore an operad or -monoid gives rise to a monad on .
Here are the details. There is a functor
which sends a set to the functor
taking to if , else to . This functor is full and faithful; conceptually, it treats a set as giving a set of 0-ary operations or constants indexed by itself. Notice that the composite
factors through the inclusion (conceptually, when one applies a formal operation to constants, the result is again a constant). This gives an action
for an actegory structure; as it is the restriction of the substitution product along the inclusion in the second argument, we again denote it , by abuse of notation. Given and a set , we have
and given , we also have coherent natural isomorphisms , .
The monad associated with an operad is the functor taking to , equipped with natural transformations
which provide with the structure of a monad.
This definition of the associated monad carries over with ease to the enriched case, and to variants such as nonpermutative operads, braided operads, and cartesian operads (Lawvere theories).
Notice that an algebra for the operad is a set equipped with a structure map which makes a module over the monoid in the monoidal category .
See also related discussion at club.
Ambient categories of relevance in practice are
There is also a canonical notion of free operad.
For a function of sets, there is an evident pullback functor
If the symmetric monoidal category that the operads under consideration are enriched in carries the structure of a monoidal model category, then under suitable conditions there is also the structure of a model category on the category of -operads. This is important for the notion of homotopy algebra over an operad, such as - and -algebras.
operad / (∞,1)-operad
The definition is originally due to
An earlier version was the notion of analyser (known usually by French version analyseur), introduced in
V.A. Smirnov, Simplicial and operad methods in algebraic topology