# Mathematics needed for higher dimensional category theory? [closed]

I'm a undergrad(third year, Manchester uni and want to do a PhD) that is thinking of doing a PhD in this area or category theory in general.(Sorry for asking it here, Maths exchange stack didn't help as asked twice last week and then today, I spend like an hour a day worrying about what I'm studying and what I should study). Imagine a chess board and you need to move a piece, how you know you are moving the best piece?, now times that by a thousand.

Just wondering, what branches of Maths should I focus on? As I've been told that topology particular Algebraic topology is the main area for this. A lecturer told me I should focus on getting down Algebraic topology before thinking of doing category theory as most of the examples of category theory are from algebraic topology.

However, another lecturer told me I should be doing logic and particular model theory. I can take a course in it this year, but would mean not doing commutative algebra.

So before you delete this can you please tell me what I should be studying? As it would save me countless hours of worrying if I'm doing the right subjects.

P.S. I can do most of the stuff in Conceptual Mathematics category book. I don't understand topoi through.

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## closed as not a real question by Dan Petersen, Noah Snyder, Ryan Budney, quid, J.C. OttemSep 19 '11 at 8:21

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So you've asked two professors, in-person, and gotten two different answers. And now you've come to ask thousands of virtual mathematicians... –  Steve D Sep 18 '11 at 20:40
I don't think mathematics works like that. There's no way you're going to get anywhere by learning only the bare minimum necessary for your particular field, if such a thing even exists. This is why you're getting such different answers. You're going to have to learn about all of those things to some extent, but probably you will only be able to learn one or two of them in-depth. (There's no point in deciding which ones now.) –  Dan Petersen Sep 18 '11 at 20:53
@simplicity - what do you want to do with this hypothetical PhD of yours? Research or otherwise? If you are looking at higher category theory, I would guess the former, and in that case, one needs to at least absorb the 'vibe' of as much maths as you can. If you don't really love maths, then you may underestimate how much a PhD demands of you. –  David Roberts Sep 18 '11 at 22:19
Sometimes I wonder if higher category theory is really the theme of so much of this new Century mathematics or if category theorists have just done a wide advertisement of their field on the internet... (or perhaps both...) –  Qfwfq Sep 18 '11 at 22:28
Please do not cross-post: math.stackexchange.com/questions/65559/… –  Qiaochu Yuan Sep 18 '11 at 22:38

It seems to me that category theory (whether higher or not) may be the worst part of mathematics to approach with a narrow viewpoint --- just topology, or just algebraic geometry, or just logic, or just any one area. To me, much of the value and beauty of category theory lies in how it exhibits connections and similarities among many areas of mathematics.

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As many other have just said you cannot think to study just some particular subjects ignoring some other areas, expecially if you want to do research. Most of math was born from the observation of some similar phenomena in many different areas: for instance the concept of category itself was born from the observation that in math we deal every time with collections of structures and morphisms preserving those structures, that led to the abstraction of category, similarly I strongly doubt that Grothendieck could invent the concept of (generalized) sheaf if first he hadn't known the many concrete sheaves that appear in topology, differential geometry and algebraic geometry, so it couldn't get to the concept of (Grothendieck's) topos, and without that I'm not so sure that Lawvere could get to the concept of elementary topos while doing his research in logic. This are just some example of as math have evolved thanks to interaction of different areas (for instance, as you can see in the example above, from interaction of geometry and logic).

Just to answer to your comment about analysis there's a professor in Italy who studies higher dimensional category theory for his research in analysis, so analysis need higher category theory.

Of course the best place where you can get a lot of intuition of higher category theory is algebraic topology where higher categories are used to model homotopy types for topological spaces, via $\infty$-groupoids, and directed space, via $(n,r)$-categories where $n,r \in \omega \cup \{\infty\}$ but you can find a lot of higher dimensional category theory in logic and computer science too, I've seen some application in calculability theory and model theory where (higher) category theory is used to model the semantic of theories, in particular type theory (if you're interested in application of higher categorical logic-model theory you can take a look to Makkai's work and also Mike Shulman's work on homotopy type theory). Also in mathematical physics there are a lot of higher category theory as John Baez's work prove.

I suppose above you were referring to Cheng-Lauda "Illustrated guide book", that's a good book if you want to learn many approaches to $n$-categories, but in higher category theory there's a lot of more then just $(n,r)$-categories (like usually Mr.Shulman says), Leinster's "Higher operads, Higher categories" is more complete from this point of view because it presents a lot of stuff like generalized multicategories/operads or $fc$-multicategories. Anyway if you want some references on higher category theory you can find some here.