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11 votes
1 answer
415 views

Why can we take the colimit over the category of elements?

I'm trying to understand J. P. Murre's Tata notes on Grothendieck's theory of the fundamental group. For a Galois category $\mathcal C$ (which I'm taking to be locally small) with fundamental functor $...
themathandlanguagetutor's user avatar
5 votes
2 answers
581 views

Can we define fundamental groups functorially for non-pointed path connected topological spaces?

Let $\text{ppTop}$ denote the category of pointed and path connected topological spaces with morphisms base-preserve continuous maps. The fundamental group gives a functor $FG: \text{ppTop}\to \text{...
Zhaoting Wei's user avatar
  • 9,019
1 vote
1 answer
277 views

What does the group of automorphisms corresponding to $\mathfrak{g}$

I am reading a book titled "Lectures on An Introduction to Grothendieck's Theory of the Fundamental Group" by J.P. Murre. I am in the chapter 4 titled "Fundamental groups". Here he fixes a base ...
Grobber's user avatar
  • 215
1 vote
1 answer
278 views

Trying to relate the fundamental groupoid to vector bundles

Fix a topological space $X$. Now consider a functor from the fundamental groupoid of $X$ to the category $Vect$. In other words, we assign a vector space to each point of $X$, we allow ourselves to ...
GraduateStudent's user avatar
36 votes
2 answers
5k views

Is the fundamental group functor a left-adjoint?

Theorem 1B.9 in Hatcher's Algebraic Topology says that for a (pointed) connected CW complex $X$ and group $G$, there is a bijection $\text{Hom}(\pi_1(X), G) \cong [X,K(G,1)]$, where $\pi_1(X)$ is the ...
ziggurism's user avatar
  • 1,446
36 votes
3 answers
3k views

Tannaka formalism and the étale fundamental group

For quite a while, I have been wondering if there is a general principle/theory that has both Tannaka fundamental groups and étale fundamental groups as a special case. To elaborate: The theory of ...
Lars's user avatar
  • 4,450