Exactly how is 'the diagonal is representable' used for algebraic stacks...

2011-03-03T22:53:17Z

...apart from stating properties of $(s,t):X_1 \to X_0\times X_0$ for ~~the~~ a presenting algebraic groupoid $X_1 \rightrightarrows X_0$?

Once we know that given a stack $\mathcal{X}$ we have a smooth representable $X_0 \to \mathcal{X}$ where $X_0$ is a scheme, then we can talk about the algebraic groupoid $X_1 :=X_0\times_\mathcal{X} X_0 \rightrightarrows X_0$, which has source and target smooth maps. We thus have the map $(s,t)$, and can talk about its properties, such as being separated or whatever. We can specify its properties (such as having 'property P') by demanding that the diagonal $\Delta:\mathcal{X} \to \mathcal{X} \times \mathcal{X}$ is representable and has 'property P'. But where else is representability and property $P$ of $\Delta$ used, in a way that couldn't be otherwise derived from property $P$ of $(s,t)$? 'Everybody knows' that algebraic stacks and algebraic groupoids form the objects of two equivalent bicategories (there is a 1996 article by Dorette Pronk that makes this precise, and recent work by myself - available on the arXiv if you care - expands hers to be applied in more general situations). Thus I wonder what properties of the diagonal $\Delta$ are used that couldn't be instead derived from $(s,t)$ of a presenting groupoid. (Edit: or can all (stable under pullback) properties of the diagonal be so described - and also used?)

Pointers (in comments) to any relevant questions where example situations are discussed in detail would be appreciated.

(NB This is a spin-off from comments at this question.)

Answer by David Carchedi for Exactly how is 'the diagonal is representable' used for algebraic stacks...

2011-03-04T14:38:27Z

If $X_1 \rightrightarrows X_0$ is a groupoid and $\mathcal{X}$ is the associated stack, consider the atlas $p:X_0 \to \mathcal{X}$ . If you form the weak $2$-pullback of $p \times p:X_0 \times X_0 \to \mathcal{X} \times \mathcal{X}$ against the diagonal $\Delta:\mathcal{X} \to \mathcal{X} \times \mathcal{X}$ you get by first projection $\left(s,t\right):X_1 \to X_0 \times X_0$ . But $p \times p$ is an atlas for $\mathcal{X} \times \mathcal{X}$ so it follows that $\Delta$ is representable, and it has "property $P$" if and only if $\Delta$ does.

Exactly how is 'the diagonal is representable' used for algebraic stacks... - MathOverflow

Exactly how is 'the diagonal is representable' used for algebraic stacks...

Answer by David Carchedi for Exactly how is 'the diagonal is representable' used for algebraic stacks...