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The Stacks project

Lemma 101.5.4. Let f : \mathcal{X} \to \mathcal{Y} be a morphism of algebraic stacks. Let Z be an algebraic space and let x_ i : Z \to \mathcal{X}, i = 1, 2 be morphisms. Then

  1. \mathit{Isom}_{\mathcal{X}/\mathcal{Y}}(x_2, x_2) is a group algebraic space over Z,

  2. there is an exact sequence of groups

    0 \to \mathit{Isom}_{\mathcal{X}/\mathcal{Y}}(x_2, x_2) \to \mathit{Isom}_\mathcal {X}(x_2, x_2) \to \mathit{Isom}_\mathcal {Y}(f \circ x_2, f \circ x_2)
  3. there is a map of algebraic spaces \mathit{Isom}_\mathcal {X}(x_1, x_2) \to \mathit{Isom}_\mathcal {Y}(f \circ x_1, f \circ x_2) such that for any 2-morphism \alpha : f \circ x_1 \to f \circ x_2 we obtain a cartesian diagram

    \xymatrix{ \mathit{Isom}_{\mathcal{X}/\mathcal{Y}}^\alpha (x_1, x_2) \ar[d] \ar[r] & Z \ar[d]^\alpha \\ \mathit{Isom}_\mathcal {X}(x_1, x_2) \ar[r] & \mathit{Isom}_\mathcal {Y}(f \circ x_1, f \circ x_2) }
  4. for any 2-morphism \alpha : f \circ x_1 \to f \circ x_2 the algebraic space \mathit{Isom}_{\mathcal{X}/\mathcal{Y}}^\alpha (x_1, x_2) is a pseudo torsor for \mathit{Isom}_{\mathcal{X}/\mathcal{Y}}(x_2, x_2) over Z.

Proof. Part (1) follows from Definition 101.5.3. Part (2) comes from the exact sequence (101.5.2.1) étale locally on Z. Part (3) can be seen by unwinding the definitions. Locally on Z in the étale topology part (4) reduces to part (2) of Lemma 101.3.2. \square


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