Big list of comonads

Question

The concept of a monad is very well established, and there are very many examples of monads pertaining almost all areas of mathematics.

The dual concept, a comonad, is less popular.

What are examples of comonads, in different categories, and different fields of math?

Answer 1

Here are the examples of comonads that I personally find most helpful. First from topology:

• The universal covering is an idempotent comonad on (suitably nice) pointed topological spaces. The functor takes a pointed space $(X,x_0)$ and gives the space of homotopy classes of paths starting at $x_0$. The counit forgets the path and just keeps the endpoint, the comultiplication is an isomorphism. Co-Kleisli morphisms are continuous functions that "depend on the path", such as the complex logarithm. Coalgebras are simply connected spaces. (This comonad appeared in the comments to another answer.)
• Similar to the example above, the rooted tree comonad on the category of pointed directed multigraphs can be seen as a "discrete universal covering". The functor takes a pointed graph $(X,x_0)$ and gives the graph whose vertices are paths on the graph starting at $x_0$, and which have a unique edge between them if and only if they differ by an edge in $X$. Again this comonad is idempotent, and its unit forgets the path and only keeps the endpoint. Co-Kleisli morphisms are path-dependent incidence-preserving maps. Coalgebras are rooted trees.

The following two are used in theoretical computer science.

• The reader comonad on the category of sets. Fix a set $A$ of "extra data". The functor maps a set $X$ to the set $X\times A$, "adding the extra data". The comultiplication copies the extra data, and the counit forgets it. Co-Kleisli morphisms are functions that have access to these extra data. Coalgebras are sets equipped with a "default choice" of the data, a function $X\to A$.
• The stream comonad on sets. Fix a monoid $N$, that we can think of "time". The functor maps a set $X$ to the set of maps $N\to X$, or "sequences" or "trajectories" or "histories". The counit forgets the history and just keeps the present state, the comultiplication looks at the history of the history (which is the history except the latest states...and so on). Co-Kleisli morphisms are maps that may depend on the history, and coalgebras are dynamical systems.

These four examples are taken from my notes (arXiv:1912.10642), sections 5.3 and 5.4 - see there for all the details. I don't claim that I have invented any one of these myself. (In the future I'd like to add these examples, and maybe some more, to the nLab. If anyone wants to help, I'd appreciate that.)

Answer 2

Given a topology on a set $X$, let $2^X$ be the poset of subsets of $X$ ordered by inclusion. Then the interior operator for the topology is a comonad on $2^X$. In fact the topologies on $X$ correspond precisely to the finite-limit-preserving comonads on $2^X$. The coalgebras of the comonad are precisely the open sets.

Given a topological space $X$, define a bundle on $X$ to be a topological space $Y$ and a continuous map $f:Y\to X$. The category of bundles is the overcategory $\mathbf{Top}/X$. Say that a bundle is étalé if the map $f$ is a local homeomorphism. Then the étalé bundles form a coreflective subcategory of $\mathbf{Top}/X$, meaning that there is an étalification comonad on $\mathbf{Top}/X$. The coalgebras of the comonad are precisely the étalé bundles, which correspond to sheaves on $X$.

The first example is a special case of the second, in the sense that if we view a subset of $X$ with its inclusion map as a bundle then its étalification is precisely its interior. Note also that in the first example the coalgebras form a locale, and in the second they form a topos.

Answer 3

The category $\mathrm{Mfd}$ of finite dimensional manifolds sits fully faithfully inside the larger category $\mathrm{LocProMfd}$ of locally pro-finite dimensional manifolds, which basically extends manifolds by spaces locally modeled on the Fréchet space $\mathbb{R}^\mathbb{N}$. Take a finite dimensional base manifold $\Sigma$ and consider the category $\mathrm{LocProMfd}_{\downarrow \Sigma}$ of fibered locally pro-finite dimensional manifolds over $\Sigma$. The functor $$ J^\infty_\Sigma \colon \mathrm{LocProMfd}_{\downarrow \Sigma} \to \mathrm{LocProMfd}_{\downarrow \Sigma} , \quad (F\to M) \mapsto (J^\infty_\Sigma(F) \to M) , $$ that assigns to a fibered manifold the bundle of jets of its sections over $\Sigma$ is a comonad. The fiber of the infinite order jet bundle of a finite dimensional fibered manifold is already infinite dimensional. So there is no way to escape this enlargement of $\mathrm{Mfd}$, or something like it. This comonadicity observation is due to Michal Marvan, first recorded in a conference proceedings note in 1986 and elaborated in his 1989 PhD thesis at Moscow State University.

Together with Urs Schreiber (arXiv:1701.06238) we have checked that the jet functor both survives in the much more general setting of synthetic differential geometry and maintains its comonad property for some basic category theoretic reasons (it is a base change comonad of another functor). Check the arXiv preprint for a detailed discussion and precise references to Marvan's original observations.

Answer 4

The so-called game comonads have been recently studied in the context of finite model theory. The main references are the following:

• Samson Abramsky, Anuj Dawar, and Pengming Wang. "The pebbling comonad in finite model theory." 2017 32nd Annual ACM/IEEE Symposium on Logic in Computer Science (LICS). IEEE, 2017. (see also arXiv:1704.05124)
• Samson Abramsky and Nihil Shah. "Relating Structure and Power: Comonadic semantics for computational resources." International Workshop on Coalgebraic Methods in Computer Science. Springer, Cham, 2018. (see also arXiv:1806.09031)

Game comonads are an active area of research. In nutshell, some of the well-known model comparison can be turned into a comonad. This is done by representing the states of the game semantically. For example, the Ehrenfeucht–Fraïssé comonad $\mathbb E_k$ is a comonad on the category of relational structures. The universe of $\mathbb E_k(A)$ consists of non-empty words of length ${\leq}k$, where the alphabet is taken to be the universe of $A$. A word $[a_1,a_2,\dots,a_n]$ represents spoiler's moves in $A$ in the one-way Ehrenfeucht–Fraïssé game.

Answer 5

Another comonad used in CS is known as Store. It maps $X$ to $(A\to X)\times A$. The comultiplication takes $(f,a)$ to $((f,-),a)$, and the counit takes $(f,a)$ to $f(a)$.

Answer 6

In computer science a class of comonads emerges from sets of datastructures with distinguished positions, often equipped with some kind of notion of a neighbourhood of the distinguished position (often called a "focus").

From this perspective a certain functor $C:\mathit{Set}\rightarrow\mathit{Set}$, taking a set to the set of grids of set elements, together with a scheme for applying certain types of locally specified rule over the entire grid, can be considered a comonad. In other words, comonads give a perspective on cellular automata.

See A Categorical Outlook on Cellular Automata by Capobianco and Uustalu for details.

Answer 7

For any monoid $(M,e,*)$ in $\mathsf{Set}$ there is a corresponding comonad $y^M$ on $\mathsf{Set}$. It sends a set $A$ to the set of morphisms into $A$ from $M$, $$ A\mapsto A^M. $$ Note that $y=y^1$ is the identity endofunctor on $\mathsf{Set}$.

Under the Yoneda embedding, the counit $y^M\to y$ corresponds to the monoid unit $e\colon 1\to M$, and the comultiplication $y^M\to (y^M)^M\cong y^{M^2}$ corresponds to the monoid multiplication $*\colon M^2\to M$.

As an example, this class of comonads includes the stream comonad (mentioned above), using the monoid of $(\mathbb{N},0,+)$ of natural numbers under addition.

Here are three more polynomial comonads for any set $S$:

• Store comonad (mentioned above), the functor $F(y)= Sy^S$.
• Linear comonad, the functor $F(y)=Sy$, with projection and diagonal.

Answer 8

Maybe this is too obvious, but every adjunction gives a comonad. If $(F,G)$ is a pair of adjoint functors, then $F \circ G$ defines a comonad, just as $G \circ F$ defines a monad.

Answer 9

An important example in homotopy theory is given by operadic twisting. This was introduced by T. Willwacher in "M. Kontsevich’s graph complex and the Grothendieck–Teichmüller Lie algebra" (Inventiones Math. 200 (2015), 671–760), and its comonadicity was proved by V. Dolgushev and T. Willwacher in Operadic twisting – With an application to Deligne's conjecture (J. Pure Appl. Alg. 219 (2015), Issue 5, 1349-1428).

Essentially, for a dg operad $\mathcal{O}$ equipped with a morphism from the operad $\mathrm{Lie}$ (or $\mathrm{Lie}_\infty$), one can construct a new operad $\mathrm{Tw}(\mathcal{O})$. It is done in two steps. First, one considers the operad $\mathrm{MC}(\mathcal{O})$ whose algebras are $\mathcal{O}$-algebras with a given solution to the Maurer--Cartan equation $$d\alpha+\frac12[\alpha,\alpha]=0.$$ Then one "twists" the differential $d_{\mathrm{MC}}$ of that latter operad by adding the "commutator with the commutator" (the operadic commutator with the unary operation $\ell_1^\alpha=[\alpha,-]$). It turns out that this construction is a functor and, moreover, a comonad, and furthermore, coalgebras over this comonad are precisely operads $\mathcal{O}$ for which algebra structures can be twisted by solutions to the Maurer--Cartan equation. This construction has been used in a variety of contexts (from algebraic topology and deformation theory to, more recently, stochastic PDEs).

Answer 10

Let $\Lambda$ be the ring of symmetric functions in infinitely many variables. There is a biring structure on $\Lambda$ defined by the coaddition $$\Delta^+: \Lambda \to \Lambda \otimes \Lambda$$ $$f(x_1, x_2, \ldots) \mapsto f(x_1,y_1, x_2, y_2, \ldots)$$ and the comultiplication $$\Delta^\times: \Lambda \to \Lambda \otimes \Lambda$$ $$f(x_1, x_2, \ldots) \mapsto f(\ldots, x_iy_j, \ldots)$$ (where we identify $\Lambda \otimes \Lambda$ with functions of two infinite sets of variables $(x_i)$ and $(y_i)$ which are symmetric in both the $x$'s and the $y$'s). This defines a lift of the hom-functor $\text{Hom}(\Lambda, -)$ to a functor $\text{CRing}\to \text{CRing}$; this is the "ring of Witt vectors" functor. Moreover, the operation of plethysm on $\Lambda$ defines a comonad structure on this functor. Coalgebras for this comonad are Grothendieck's $\lambda$-rings.