What is a good notation for the 'set' (or stack if you insist) of all principal G bundles over 'all' spaces for given G? BG is way over used. How about Bun(G)?
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$\begingroup$ Within the context of stacks in algebraic geometry, $BG$ (or $pt/G$) is absolutely the notation that would be used. I suppose you don't like this because it is also the notation for the classifying space of $G$? From my perspective, I don't particularly like the notation $Bun(G)$, only because $Bun_G(X)$ is typically used to denote the moduli of $G$-bundles on the space $X$. But maybe others disagree. I'm struggling to think of a good alternative though. $\endgroup$– Mike SkirvinMar 6, 2012 at 22:59
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1$\begingroup$ I certainly have seen $\operatorname{Bun}(G)$, but I agree with @Mike. Since from a stacky perspective $\operatorname{Bun}_G(X)$ is the space of maps $X \to $ this space that Jim is after, it's not bad to call the target space itself by $\operatorname{Bun}_G$. $\endgroup$– Theo Johnson-FreydMar 7, 2012 at 1:32
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$\begingroup$ I have made a small change to the formatting. The issue is that Markdown considers the backwards single quote ` to be a special character. $\endgroup$– Theo Johnson-FreydMar 7, 2012 at 1:33
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$\begingroup$ May I suggest that my learned colleagues' comments constitute genuine answers to the question asked, and might therefore be submitted as answers? $\endgroup$– Tom LeinsterMar 7, 2012 at 1:39
2 Answers
In my comment to the question, I may have misinterpreted Jim's intent. Upon a second reading, I think Jim would like to consider the collection of all pairs consisting of a space $X$ and a $G$-bundle over $X$. If this is the correct interpretation, then I think the correct notation for this category is $\text{Spaces}_{/\mathrm{B}G}$. The meaning of this notation is: $\mathrm{B}G = \operatorname{Bun}_G$ is the stack of $G$-bundles, defined so that a map $X \to \mathrm{B}G$ is a $G$-bundle over $X$. The category $\text{Spaces}$ is whatever category of spaces you want to consider (topological spaces, manifolds, schemes, ...). The category of pairs $(X,$ $G$-bundle over $X)$ is the category of pairs $(X,f: X \to \mathrm{B}G)$, which is precisely the "comma" category of "spaces over $\mathrm{B}G$", and that's what's notated by the subscript.
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$\begingroup$ I can live with Spaces/BG since it is not previously used and will encourage the reader to understand what it denotes. $\endgroup$ Mar 7, 2012 at 13:15
In algebraic geometry, $BG$ only ever refers to a stack, viewed as a fibered category over the category of schemes, or affine schemes, depending on your conventions. The objects are principal $G$-bundles $P \to X$ of schemes (or if $G$ is a group sheaf, it is a map of sheaves). The morphisms are $G$-equivariant fiber squares.
More to the point, there is no alternative object in the algebro-geometric universe that takes the name $BG$, so there is no source of confusion there.
The name $\operatorname{Bun}_G$ is usually reserved for the enriched Hom-stack construction $\underline{\operatorname{Hom}}(-, BG)$. In other words, $\operatorname{Bun}_G(X)(T) = BG(X \times T)$.
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$\begingroup$ Perhaps no confusion within alg-geom, but we are not all alg-geometers. So your BG is a category and alg-geom has no use for a classifying space for G-bundles? or it has some other symbol? $\endgroup$ Mar 7, 2012 at 13:12
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$\begingroup$ S. Carnahan's
$BG$
is the classifying space in the algebro-geometric context. You might (I do) find it more intuitive to turn the category fibred in groupoids into a sheaf of groupoids/homotopy 1-types, using the canonical cleavage' construction. Then we are talking about $BG$ as a sheaf of homotopy types on affine schemes. For a scheme$X$
,$\operatorname{Bun}_G(X)=\underline{\operatorname{Hom}}(X, BG)$
is another sheaf of homotopy types, and the global sections of$\pi_{0}\operatorname{Bun}_G(X)$
are the isomorphism classes of principal $G$-bundles over$X$
, just like in topology. $\endgroup$ Mar 7, 2012 at 14:58 -
$\begingroup$ That does clarify the relation, but notice how many words it required. $\endgroup$ Mar 8, 2012 at 14:02