following the content of the title I am writing here to ask some suggestions concerning a talk I will be presenting at my university in a week or two. The main topic I chose is the fundamental groupoid version of the well-known Van Kampen's theorem, using its formulation in terms of suitable colimits and then showing the fundamental group version as a corollary and some examples (e.g. the fundamental group of a wedge of spaces in which the basepoint has got a contractible neighbourhood in each of the summand).

I would appreciate any advice on how to enrich this talk, for instance some further applications, or generalizations (I have thought about the simplicial sets case, but unfortunately it will take too long to treat that interesting topic).

The main reference I am considering is May's "Concise course in Algebraic Topology", and pay attention to the fact that I will be given approximately 75 minutes to speak.

• You could look at R. Brown's "Topology and groupoids" book. – Bruno Stonek Apr 23 '14 at 13:07
• Check Kosniowski, this one I think – MattAllegro Apr 24 '14 at 18:53

A main point I would make is that connected unions of non connected spaces are commonplace, and an example is given on groupoids page which you are welcome to use, of course.

See also arxiv:1404.0556 for a correction to a proof in my book about the Phragmen-Brouwer Property, with a useful new result on pushouts of groupoids.

The virtue of the use of groupoids is not to give a "nice" proof of a result about fundamental groups, as in some expositions, but to be able to deal with many more examples, of a type usually ignored, as trying to use covering space methods gets unwieldy.

I'll add that in a talk I gave in Chicago in 2012 I called the fact that the usual van Kampen theorem for the fundamental group does not compute the fundamental group of the circle, THE basic example in topology, is a real anomaly. This talk, with others, is available on my preprint page.

Edit 25 April: I found the groupoid SvKT strange since in algebraic topology you usually get exact sequences which do not give complete information; yet here one computes $\pi_1(X,A)$ as a colimit which completely determines any group $\pi_1(X,a)$, though to find that you have to do some combinatorial work. A reason for the success seems to be that groupoids have structure in dimensions $0$ and $1$, allowing the modelling of gluing $1$-types.

So the question was: can this principle be extended to higher dimensions, using new algebraic objects with structure in dimensions $0, \ldots,n$, allowing for the fact that identifications in low dimensions have homotopical effect in high dimensions? Thus the $1$-dimensional theorem pointed to potential higher dimensional results, in which the invariants one might want, such as homotopy groups, were part of a much larger structure which was more computable, thus allowing some new computations of old invariants.

September 3, 2014 I give a link here to a presentation I gave at the IHP, Paris, to a workshop on Homotopy Type Theory, on "The intuitions for cubical sets in nonabelian algebraic topology". The talk is also about the notion of composition for homotopically defined structures, and its relations to the origins of algebraic topology. This talk develops some themes in my 2012 Chicago talk.

• Thanks for the help, I am really pleased to receive an answer from you! I will have a look at your book, even though to completely understand higher-dimensional analogues I have to go through n-categories, which is the next goal I have settled for myself! – Edoardo Lanari Apr 25 '14 at 10:24
• Good! What I have found fun is the pentoil trick, which you will find on my talk at Liverpool, What is and what should be `Higher dimensional group theory'? Dec 2009. I have travelled many places carrying a copper tubing pentoil and trefoil, and a nice length of rope, and brought members of the audience out to help with taking the loop off the knot. The knots themselves were made by a University workshop. And of course the relations at a crossing are really an application of SvKT. See also pages.bangor.ac.uk/~mas010/outofline/motion.html#motion . – Ronnie Brown Apr 25 '14 at 14:48