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The Stacks project

Lemma 101.7.6. Let

\xymatrix{ \mathcal{X} \ar[rr]_ f \ar[rd]_ p & & \mathcal{Y} \ar[dl]^ q \\ & \mathcal{Z} }

be a 2-commutative diagram of morphisms of algebraic stacks. If f is surjective and p is quasi-compact, then q is quasi-compact.

Proof. Let \mathcal{T} be a quasi-compact algebraic stack, and let \mathcal{T} \to \mathcal{Z} be a morphism. By Properties of Stacks, Lemma 100.5.3 the morphism \mathcal{T} \times _\mathcal {Z} \mathcal{X} \to \mathcal{T} \times _\mathcal {Z} \mathcal{Y} is surjective and by assumption \mathcal{T} \times _\mathcal {Z} \mathcal{X} is quasi-compact. Hence \mathcal{T} \times _\mathcal {Z} \mathcal{Y} is quasi-compact by Properties of Stacks, Lemma 100.6.2. \square


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