Section 83.9 (04AB): Good quotients

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83.9 Good quotients

Especially when taking quotients by group actions the following definition is useful.

Definition 83.9.1. Let $S$ be a scheme and $B$ an algebraic space over $S$ . Let $j : R \to U \times _ B U$ be a pre-relation. A morphism $\phi : U \to X$ of algebraic spaces over $B$ is called a good quotient if

$\phi$ is invariant,
$\phi$ is affine,
$\phi$ is surjective,
condition (3) holds universally, and
the functions on $X$ are the $R$ -invariant functions on $U$ .

In [seshadri_quotients] Seshadri gives almost the same definition, except that instead of (4) he simply requires the condition (3) to hold – he does not require it to hold universally.

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83.9 Good quotients

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