## Wednesday, December 11, 2013

### Old posts: Max Kelly 2007

With the closing of our offices in via Carloni the web server of the old department was offline for a time and I suspect will not be reliable in future. I have decided to transfer material from the server to this blog or to the Como Category Archive.
The first item is a post made when Max Kelly died in 2007.

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Max Kelly died on the 26th January 2007. I have known Max since, as a graduate student in Canberra of Bernhard and Hanna Neumann, I travelled to Sydney to see him one day about my developing interest in Category Theory. It must have been 1968.

After I returned from Chicago in 1970 to a position at the University of Sydney, I attended with Don Cartwright a course of Max's at the University of New South Wales and then in 1971 Max went on leave, Ross came to Macquarie, we started the Category Seminar. The seminar, first called the Sydney Category Seminar, and now the Australian Category Seminar, with Max and Ross, and with the visitors who came, and the students, was the basis of my research life. I had many struggles with Max over the years. I am very pleased that at the end we were writing a paper together with Richard Wood and Aurelio Carboni.

I was deeply saddened by his death.

I include some of the letters written to the Categories mailing list after his death was announced.
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From: Bill Lawvere
To: categories@mta.ca
Date: sabato 27 gennaio 2007 16.31.58

I am deeply saddened by the loss of Max. In our field he was a rock of reliability and a fountain of imagination. I will miss my lively, warm, kind, and sometimes mischievous friend.

Bill Lawvere
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From: Eduardo J. Dubuc
To: categories@mta.ca
Date: domenica 28 gennaio 2007 17.39.02

I am deeply saddened by the death of Max Kelly. When I saw the subject in Ross posting, and before opening the message, my heart already felt anguish. I am more saddened with his loss that what I have been by the loss of any other member of our category theory community. In fact, I loved Max. I admired his courage, his independence of thought, his lack of hypocrisy, and I loved him simply by the way he was. I am proud that he considered me his friend. For me, our community is not the same without Max.
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From: Richard Wood
To: categories@mta.ca
Date: lunedì 29 gennaio 2007 21.11.43

Max Kelly, a master of coherence

We would like to add to Bill and Eduardo's letters also our feelings of deep sadness at Max's death.

Max Kelly's Last Work
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In due course Max's last work will appear in a four-author paper. While it is not usual for coauthors to divulge who contributed what to a paper the present circumstances seem to warrant such, as an appreciation of Max's extraordinary talents and tenacity. Carboni, Kelly, Walters, and Wood, [CKWW] have for some time been extending the Carboni and Walters notion of cartesian bicategory' to the general case of bicategories that are not necessarily locally ordered.

A cartesian bicategory B ultimately has a tensor product, a pseudofunctor *:BxB--->B that is naively associative and unitary. It is natural to ask whether such (B,*) is a monoidal bicategory, in other words a one-object tricategory in the sense of [Coherence for Tricategories; Gordon, Power, and Street]=[GPS]. Early in 2005 [CKWW] had shown that _if_ a bicategory A with finite products' -x- and 1, in the bilimit sense, has (A,x) a monoidal bicategory then a cartesian bicategory B has (B,*) monoidal. In the course of polishing the paper it came to Max's attention that nobody had _proved_ the

Theorem: A bicategory with finite products is monoidal.

Nobody doubted the truth of this. In fact, experts in higher dimensional category theory said that if it were not true then the definition of tricategory is wrong! But when you consider the rather large amount of data that must be assembled and the many equations (some merely implicit in words such as pseudonatural and modification) that must be verified from the apparently rather weak universal property of finite products in the bilimit sense, it seemed like a rather thankless task to write out the details.

This was to Max a completely unacceptable state of affairs. If nobody doubts the statement then it must be possible to find a good proof! Now Max had no intention of redrawing any of the diagrams in [GPS].

For the last few years Max, with little central vision left as a result of macular degeneration, has been doing Mathematics using an 8-fold magnification monitor. This allowed him to see only a few square centimetres of a page at a time. Many [GPS] diagrams consume an entire page. His proof, that we were privileged to receive in the last few weeks, has _no_ diagrams (though doubtless we will incorporate a few in a publishable version of the paper). Max attributed the key idea in his proof to Ross Street.

Briefly, this is how it goes: For X a finite family of objects in the bicategory A, write A(X) for the bicategory of product cones over X. Thus an object of A(X) consists of an object b of A, together with a family of arrows p_i:b--->X_i, such that for all a, the induced functor A(a,b)--->Pi A(a,X_i) is an equivalence of categories.
Lemma: !:A(X)--->1 is a biequivalence
(Recall that to say B--->1 is a biequivalence is to say that: i) there is an object b in B ii) for any objects c and d in B, there is an arrow f:c--->d iii) for any arrows g,h:c--->d in B, there is a unique 2-cell g--->h. It follows that in a bicategory biequivalent to 1, every arrow is an equivalence and every 2-cell is an isomorphism.)

Next, Max observes that if A has finite products then, for any B, the bicategory [B,A] of pseudofunctors, pseudonatural transformations, and modifications also has finite products, given pointwise' by the products of A. -x- is an object of [A^2,A].
We can use (a x b) x c and a x (b x c) as names for objects in [A^3,A] and applying the Lemma to [A^3,A](a,b,c) deduce the existence of the associator equivalence, pseudonatural in a,b, and c. The associator gives rise to two arrows (abbreviating somewhat) ((ab)c)d ===> a(b(cd)) in [A^4,A](a,b,c,d) and between these we have a unique invertible modification, the pi of [GPS].
The coherence of pi is chiefly the Stasheff non-abelian 4-cocycle condition (again see [GPS]) and for this we need only apply the Lemma to [A^5,A](a,b,c,d,e) to see that the two modifications in question are equal. Of course the other data and equations are handled with similar appeals to the Lemma.

Max was not content to stop here. In his last few days he had been learning the rather subtle definition of _symmetric_ monoidal bicategory and constructed the requisite braiding equivalence and syllepsis isomorphism for a bicategory with finite products. Everything follows from the universal property but Max has shown us _how_ so that we can calculate with these things. His insights show us the way to deal with coherence issues arising from birepresentability generally and weak n-representability when the need arises.

Max's personal copy of [GPS] was autographed by Ross with the words `To Max Kelly, a master of coherence''. Yes, he was.

Aurelio Carboni, Robert Walters, and Richard Wood
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From: George Janelidze
To: categories@mta.ca
Date: lunedì 29 gennaio 2007 12.15.06

Gregory Maxwell Kelly was one of the great mathematicians of our time, so perfect in his research and vision of mathematics, and in every aspect of academic life.

And he was so exceptionally kind to everyone. It is hard to imagine that Max is not with us anymore, and it is a great pain for his family and for all of us, his friends and colleagues...

George Janelidze
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From: Andre Joyal
To: categories@mta.ca
Date: mercoledì 31 gennaio 2007 21.17.42

Letter to Max Dear Max, I feel deeply sad that you have left. Now that you are gone, I realise how much you mean to me.

I regret not telling you that.

I wish to repair that by writing you this letter. If I send it to Imogen and to your friends, it will reach you in some way.

Your work has been a constant source of inspiration for me. It combines beauty, rigor and depth. It is fundamental, I use it every day. It will last forever. You were a great mathematician.

I also want to thank you for inviting me to Australia. I did some of my best work there. You were a great host. I made many friends.

I wish we could meet again.

I will talk with you in my dreams.

Yours, Andre
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