This is a question about a naming convention. The Barr-Beck theorem (or simply Barr-Beck) is used a lot in descent theory over the past 30 years, almost invariably without a reference, like folklore.
To make precise which theorem I am talking about: According to one source the Barr-Beck monadicity theorem gives necessary and sufficient conditions for a category to be equivalent to a category of algebras over a monad.
There are occasionally references, e.g., to the Wikipedia page on "Beck's monadicity theorem" which attributes the theorem to Beck's thesis in 1967 under the direction of Eilenberg.
This makes me wonder how Barr is related to Barr-Beck. So I did some further research and found out that there is indeed a Barr-Beck paper with a Barr-Beck theorem. But it is about triple cohomology of algebras. This seems to be different. So here are my questions:
Is the Barr-Beck theorem different from Beck's theorem? If so, how? If
not, how did Barr's name get attached to it?
By the way, the second Beck is Jon Beck while the first is Jonathan Mock Beck with a different author number in MathSciNet. So out of curiosity: Are Jon and Jonathan Beck one and the same person?
Best Answer
It is well-attested in the category theory literature (e.g. in Mac Lane's Categories for the Working Mathematician, Chapter VI) that the well-known theorem giving necessary and sufficient conditions for monadicity of a functor is due to Jon Beck. Indeed, most category theorists I know call this "Beck's monadicity theorem".
So why do others so often call it the "Barr-Beck theorem" nowadays?
As Tim points out in the comments above, there are many variants of the monadicity theorem which give sufficient, but not generally necessary, conditions for a functor to be monadic. Several of these variants may be found in the Exercises to Section VI.7 of Mac Lane (which section is titled "Beck's Theorem"). Exercise 7 reads as follows:
(A functor $G \colon A \to B$ is said by Mac Lane to be CTT when it is has a left adjoint, it preserves and reflects all coequalizers which exist, and the category $A$ has coequalizers of all parallel pairs of arrows $f,g$ such that the pair $Gf,Gg$ has a coequalizer in $B$. Note that these conditions are much stronger than those appearing in Beck's monadicity theorem. Indeed, the composite of two CTT functors is CTT, whereas in general the composite of two monadic functors is need not be monadic.)
Note the attribution to Barr-Beck. Now, this particular theorem was cited by Deligne in Section 4.1 of his paper Catégories tannakiennes in the Grothendieck Festschrift as "Le théorème de Barr-Beck".
My theory is that the name "Barr-Beck theorem" was popularised in certain circles by Deligne's usage here, and that over time (how long?) its usage shifted in these circles to refer to Beck's precise monadicity theorem. I fear that this incorrect usage has now been set in stone by its appearance in the works of modern influential authors such as Lurie.