nLab
monadicity theorem

Statement

Theorem

(Beck’s monadicity theorem, tripleability theorem)

A functor $U:D\to C$ is monadic (tripleable) if and only if

1. $U$ has a left adjoint, and
2. $U$ creates coequalizers of $U$-split pairs.

A proof is reproduced at (Borceux, vol 2, theorem 4.4.4).

Here a parallel pair $f,g:a\to b$ in $D$ is $U$-split if the pair $Uf,Ug$ has a split coequalizer in $C$. Specifically, this means that there is a diagram in $C$:

$$Ua\underset{Uf}{\overset{Ug}{\rightrightarrows }}Ub\stackrel{h}{\to }c$$U a \;\underoverset{U f}{U g}{\rightrightarrows}\; U b \;\overset{h}{\rightarrow}\; c

where $h$ has a section $s$ and $Uf$ has a section $t$ such that $Ug\cdot t=s\cdot h$. This implies that the arrow $h$ is necessarily a coequalizer of $Uf$ and $Ug$. To say that $U$ creates coequalizers of $U$-split pairs is to say that for any such $U$-split pair, there exists a coequalizer $e$ of $f,g$ in $D$ which is preserved by $U$, and moreover any fork in $D$ whose image in $C$ is a split coequalizer must itself be a coequalizer (not necessarily split).

An equivalent, and sometimes easier, way to state these conditions is to say that

Theorem

A functor $U:D\to C$ is monadic precisely if

1. $U$ has a left adjoint,
2. $U$ reflects isomorphisms (i.e. it is conservative), and
3. $D$ has, and $U$ preserves, coequalizers of $U$-split pairs.

This is equivalent because a conservative functor reflects any limits or colimits which exist in its domain and which it preserves, while monadic functors are always conservative.

Variations

The crude monadicity theorem

The crude monadicity theorem gives a sufficient, but not necessary, condition for a functor to be monadic. It states that a functor $U:D\to C$ is monadic if

1. $U$ has a left adjoint
2. $U$ reflects isomorphisms
3. $D$ has and $U$ preserves coequalizers of reflexive pairs.

(Recall that a parallel pair $f,g:a\to b$ is reflexive if $f$ and $g$ have a common section.) This sufficient, but not necessary, condition is sometimes easier to verify in practice. In contrast to the crude monadicity theorem, the necessary and sufficient condition above is sometimes called the precise monadicity theorem.

Duskin’s monadicity theorem

Duskin’s monadicity theorem gives a different sufficient, but not necessary, condition which refers only to quotients of congruences. It says that a functor $U:D\to C$ is monadic if

1. $U$ has a left adjoint
2. $D$ and $C$ are finitely complete
3. $U$ creates coequalizers for congruences in $D$ whose images in $C$ have split coequalizers.

We can weaken the hypothesis a bit further to obtain the theorem:

• A right adjoint between finitely complete categories is monadic if it creates quotients for congruences.

As usual, we can also modify it by replacing reflection of limits by reflection of isomorphisms.

• A conservative right adjoint $U:D\to C$ between finitely complete categories is monadic if any congruence in $D$ which has a quotient in $C$ already has a quotient in $D$, and that quotient that is preserved by $U$.

If we view the objects of $D$ as underlying $C$-objects with structure, this says that any congruence in $D$ induces a $D$-structure on its quotient in $C$. As with the crude monadicity theorem, this condition is sometimes easier to verify since quotients of congruences are often better-behaved than arbitrary coequalizers. This is the case in many “algebraic” situations.

Duskin actually gave a slightly more precise version only assuming the categories $C$ and $D$ to have particular finite limits, rather than all of them.

Monadicity over Set

In the case when the base category $C$ is Set, one can further refine the requisite conditions. Linton proved that a functor $U:D\to \mathrm{Set}$ is monadic if and only if

1. $U$ has a left adjoint,
2. $D$ admits kernel pairs and coequalizers,
3. A parallel pair $R\rightrightarrows S$ in $D$ is a kernel pair if and only if its image under $U$ is so, and
4. A morphism $A\to B$ in $D$ is a regular epimorphism if and only if its image under $U$ is so.

There are other versions of this theorem, including generalizations to monadicity over presheaf categories, which can be viewed as analogues of Giraud's theorem.

Strict monadicity

The version of the monadicity theorem given in Categories Work uses an evil notion of “creation of limits” and concludes that the comparison functor is an isomorphism of categories, rather than merely an equivalence. But the versions mentioned above can be found in the exercises.

Note however that if $U:D\to C$ is an amnestic isofibration, then $U$ is monadic iff it is strictly monadic. For an application of this observation, see for example the discussion of algebraically free monads at free monad.

Examples and Applications

Groups over sets

We will use Duskin’s variant to prove that the forgetful functor $U:$Grp$\to$Set is monadic. Of course, this is also easy to show by explicit computation, but it serves as a useful example of how to use a monadicity theorem. We first need it to have a left adjoint: this is easy to show by a direct construction of free groups, but we could also invoke the adjoint functor theorem. It is also easy to show that it is conservative (a bijective group homomorphism is a group isomorphism), so it remains to consider congruences.

Since limits in $\mathrm{Grp}$ are created in $\mathrm{Set}$, a congruence in $\mathrm{Grp}$ on a group $G$ is an equivalence relation on $G$ which is also a subgroup of $G\times G$. This latter condition means that if $g_1\sim g_2$ and $h_1\sim h_2$, then also $g_1^{-1}\sim g_2^{-1}$ and $g_1{h}_1\sim g_2{h}_2$. Since $g\sim g$ for all $g$, it follows that $g\sim h$ if and only if $1\sim h{g}^{-1}$, so $\sim$ is determined by the subset $H\subseteq G$ of those $h\in G$ such that $1\sim h$. This $H$ is clearly a subgroup of $G$, and moreover a normal subgroup, since if $h\in H$ and $g\in G$ we have $1=g^{-1}g\sim g^{-1}hg$, so $g^{-1}hg\in H$. Conversely, it is easy to construct a congruence from any normal subgroup, so the two notions are equivalent. It remains only to observe that the quotient of a group by a normal subgroup is, in fact, a quotient of its associated congruence in $\mathrm{Grp}$, which is preserved by $U$. Thus, by Duskin’s monadicity theorem, $U$ is monadic.

Categories over computads

The monadicity theorem becomes more important when the base category $C$ is more complicated and harder to work with explicitly, and when the objects of $D$ are not obviously defined as “objects of $C$ with extra structure.” For instance, the category of strict 2-categories is monadic over the category of 2-globular sets, essentially by definition, but it is much less trivial to show that it is also monadic over the category of 2-computads. This latter fact can, however, be proven using the monadicity theorem.

Monadic descent

The monadicity theorem also plays a central role in monadic descent.

In $(\mathrm{\infty },1)$-categories

There is a version of the monadicity theorem for (∞,1)-monads in section 3.4 of

• Jacob Lurie, Noncommutative algebra (arXiv)

Related concepts

• monadic functor, comonadic functor

References

Canonical textbook references include

• Section 4 in volume 2 of Francis Borceux, Handbook of categorical algebra , in 3 vols.

• Section VI.7 of Categories Work.

• Chapter 3 of Michael Barr, Charles Wells, Triples, toposes, and theories , Grundlehren der math. Wissenschaften 278, Springer-Verlag 1983, ftp, web, pdf

Other references include:

• descent, FGA explained, SGA 1, Bénabou-Roubaud's theorem

• Jean Bénabou, Jacques Roubaud, Monades et descente , C. R. Acad. Sc. Paris, t. 270 (12 Janvier 1970), Serie A, 96–98

• Duško Pavlović, Categorical interpolation: descent and the Beck-Chevalley condition without direct images, Category theory Como 1990, pp. 306–325, Lecture Notes in Mathematics 1488, Springer 1991

• Pierre Deligne, Catégories Tannakiennes , Grothendieck Festschrift, vol. II, Birkhäuser Progress in Math. 87 (1990) pp. 111-195.

• wikipedia: monadicity theorem

• John Bourke, Two dimensional monadicity, arxiv/1212.5123