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August 4, 2023

Who Introduced the Term “Categorical Group”?

Posted by John Baez

I’m writing a paper in honor of Hoàng Xuân Sính’s 90th birthday, and I’m running into a lot of questions.

The term “categorical group” is often used to mean a group object in Cat; these days we also call such a thing a strict 2-group. Who first introduced the term “categorical group”, and when? Perhaps it appeared in the French literature under some name like “groupe catégorique”?

Here are some things I know, which don’t answer my question, but might provide clues.

In 1973 Hoàng Xuân Sính finished her thesis with Grothendieck on a more general concept, which she called “Gr-catégories”; these are now also called 2-groups. In 1978 she published this paper:

Strict Gr-categories are the same as categorical groups, but I don’t think she uses a term like “groupe catégorique” anywhere in this paper. In this paper she proves (among other things) that every Gr-category is equivalent to a strict one, and that you can get strict ones from crossed modules.

Earlier, Brown and Spencer wrote a related paper:

𝒢\mathcal{G}-groupoids are the same as categorical groups, but they don’t use the term “categorical group” anywhere. In this paper they prove (among other things) that you can get 𝒢\mathcal{G}-groupoids from crossed modules and vice versa. They write:

This result was, we understand, known to Verdier in 1965; it was then used by Duskin [6] ; it was discovered independently by us in 1972. The work of Verdier and Duskin is unpublished, we have found that Theorem 1 is little known, and so we hope that this account will prove useful.

Reference [6] is to an unpublished paper which I have not located:

  • J. Duskin, Preliminary remarks on groups. Unpublished notes, Tulane University, 1969.

The first actual use of the term “categorical group” that I’ve found comes considerably later:

  • A. Solian, Coherence in categorical groups, Comm. Algebra 9 (1981), 1039-1057.

However, this paper actually deals with with 2-groups, not the strict 2-groups = strict Gr-categories that are nowadays often meant by the term “categorical group”.

Also in 1981 there is a paper in German that uses the term “Kategorie mit Gruppenstruktur”:

but again this is about 2-groups in general, not the strict ones, and I don’t see the term “categorical group” (or some German equivalent) anywhere in this paper.

Posted at August 4, 2023 9:24 AM UTC

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Re: Who Introduced the Term “Categorical Group”?

I’ve always assumed the “gr” in “gr-category” was “group”. Is it?

Posted by: Theo Johnson-Freyd on August 4, 2023 8:34 PM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

Yes. This term was introduced in Hoàng Xuân Sính’s thesis:

In her English summary she tersely introduces the concept thus:

A Gr-category is an AU-category, the objects of which are all invertible, and the base category a groupoid (i.e. all arrows are isomorphisms). Thus a Gr-category is like a group.

Posted by: John Baez on August 5, 2023 11:54 AM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

I have Gavin Wraith’s blessing to pass on the following wonderful anecdote about Alexandru Solian, who wrote the first paper I’ve found that mentions categorical groups. I think such anecdotes deserve not to be forgotten.

In 1974 I was a visiting Professor at SUNY in Buffalo, and I lent my flat in Brighton, UK, to Bucur who was visiting Sussex University as a guest of Prof. Bernard Scott, our head of department. Perhaps as a consequence, I was sent by the British Council on a visit to Romania in 1977, to the universities of Bucharest and Cluj-Napoca, to report on the state of mathematics in Romania. One of the people I met in Bucharest was Prof. Solian. You have to understand that at that time Romania was under the thumb of the Ceaucescu regime. Bucur was not allowed to meet foreigners like myself because his student, Diaconescu, had defected. I was supposed to be escorted by a ‘party member’ wherever I went, to see that I did not get up to mischief. In fact I did talk to Bucur, in an empty lecture theatre, while the ‘party member’s attention was diverted by a friend who was a mathematical, as opposed to a political, member of the mathematics department. But we only talked mathematics. The paranoia was absurd and pointless, of course. Solian, who had no students to defect, was allowed to talk to me. Moreover, he had an elderly mother to care for, which kept him chained to Romania, so his desire for travel and meeting foreign mathematicians could not be satisfied.

In 1978 I was invited to give a talk at the ICM in Helsinki. Solian was there ( his mother had died), in a group of Romanian mathematicians who had been allowed out after Ceaucescu’s daughter, who was studying for a degree in mathematics, had begged her mother to let them go. They were not allowed to take any money. They were issued with meal tickets and had to stay in a group.

I was approached by a French mathematician who asked if I would help Solian to defect, and I said I would. The problem was that telephone calls from Helsinki were monitored by the Russians, who would probably pass on relevant information to the Romanians. He would arrange for a ticket on a boat from Helsinki to Stockholm if I would get Solian onto the boat. The Wednesday afternoon was ‘outing day’.

Solian feigned illness while the rest of his group was escorted on a tour of the islands. I took Solian on a walk in the cemetery, where we were less likely to be spotted. He was very nervous. Eventually I got him onto the boat. I never saw him again.

I later heard that after the boat had sailed Solian had met Foias, Miss Ceaucescu’s thesis adviser, in the lounge. Each of the two men thought the other had been sent to spy on them!

Excuse me burdening you with this tale. The Ceaucescus are gone. Romania is a different place now. In fact my half-brother is married to a Romanian, and their daughter is happily studying in the USA.

— Best wishes, Gavin Wraith

Posted by: John Baez on August 5, 2023 2:49 PM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

I see from Solian’s article mentioned in the post that he is in Birmingham, Alabama by 1981. Does anyone know how he ends up there from Stockholm?

Posted by: David Corfield on August 5, 2023 8:02 PM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

Thank you John for pointing out Solian’s paper, that I was not aware of. However, it seems to me, that what he calls “categorical group” is not precisely what we currently mean for (weak) 2-group.

Indeed, in his definition, the inverse functor (“reciprocity functor” in his terminology) is contravariant.

This is a key point, because this assumption does not force the underlying monoidal category to be a groupoid, as for instance in Example (2) of preordered groups in Solian’s paper.

On the other hand, it is plausible that he has introduced the terminology “categorical group” for such a kind of monoidal structures. This seems to be confirmed by Mac Lane’s MathSciNet review of another paper by Solian, namely

  • A. Solian, A categorical analogue of Z nZ_n, Ann. Sci. Math. Quebec 9 (1985), no.2, 203-219.
Posted by: Beppe Metere on September 3, 2023 5:29 PM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

Beppe wrote:

Indeed, in his definition, the inverse functor (“reciprocity functor” in his terminology) is contravariant.

Oh, I hadn’t noticed that! Then his ‘categorical groups’ are not always weak 2-groups (= Gr-categories), at least not in this paper.

Posted by: John Baez on September 6, 2023 10:44 AM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

Maybe the term “categorical group” has been used to mean (weak) 2-group from the beginning. I mean, the first actual use of the term you have found is by A. Solian, 1981. Why should we suspect that this terminology was used before to mean strict 2-group?

Posted by: Beppe Metere on September 3, 2023 11:06 AM | Permalink | Reply to this

Re: Who Introduced the Term “Categorical Group”?

I wasn’t claiming ‘categorical group’ was used before 1981 to mean strict 2-group, but I thought it’s often been used to mean strict 2-group. Now that I’m looking around more, I’m realizing that may not be true.

Posted by: John Baez on September 3, 2023 3:29 PM | Permalink | Reply to this

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