# The Stacks Project

## Tag 0B8B

Lemma 67.4.2. Let $S$ be a scheme. Let $f : X \to Y$ be a quasi-compact monomorphism of algebraic spaces $f : X \to Y$ such that for every $T \to X$ the map $$\mathcal{O}_T \to f_{T,*}\mathcal{O}_{X \times_Y T}$$ is injective. Then $f$ is an isomorphism (and hence representable by schemes).

Proof. The question is étale local on $Y$, hence we may assume $Y = \mathop{\rm Spec}(A)$ is affine. Then $X$ is quasi-compact and we may choose an affine scheme $U = \mathop{\rm Spec}(B)$ and a surjective étale morphism $U \to X$ (Properties of Spaces, Lemma 57.6.3). Note that $U \times_X U = \mathop{\rm Spec}(B \otimes_A B)$. Hence the category of quasi-coherent $\mathcal{O}_X$-modules is equivalent to the category $DD_{B/A}$ of descent data on modules for $A \to B$. See Properties of Spaces, Proposition 57.31.1, Descent, Definition 34.3.1, and Descent, Subsection 34.4.14. On the other hand, $$A \to B$$ is a universally injective ring map. Namely, given an $A$-module $M$ we see that $A \oplus M \to B \otimes_A (A \oplus M)$ is injective by the assumption of the lemma. Hence $DD_{B/A}$ is equivalent to the category of $A$-modules by Descent, Theorem 34.4.22. Thus pullback along $f : X \to \mathop{\rm Spec}(A)$ determines an equivalence of categories of quasi-coherent modules. In particular $f^*$ is exact on quasi-coherent modules and we see that $f$ is flat (small detail omitted). Moreover, it is clear that $f$ is surjective (for example because $\mathop{\rm Spec}(B) \to \mathop{\rm Spec}(A)$ is surjective). Hence we see that $\{X \to \mathop{\rm Spec}(A)\}$ is an fpqc cover. Then $X \to \mathop{\rm Spec}(A)$ is a morphism which becomes an isomorphism after base change by $X \to \mathop{\rm Spec}(A)$. Hence it is an isomorphism by fpqc descent, see Descent on Spaces, Lemma 65.10.15. $\square$

The code snippet corresponding to this tag is a part of the file spaces-more-morphisms.tex and is located in lines 318–326 (see updates for more information).

\begin{lemma}
\label{lemma-ui-case}
Let $S$ be a scheme. Let $f : X \to Y$ be a quasi-compact monomorphism
of algebraic spaces $f : X \to Y$ such that for every $T \to X$ the map
$$\mathcal{O}_T \to f_{T,*}\mathcal{O}_{X \times_Y T}$$
is injective. Then $f$ is an isomorphism (and hence representable by schemes).
\end{lemma}

\begin{proof}
The question is \'etale local on $Y$, hence we may assume $Y = \Spec(A)$
is affine. Then $X$ is quasi-compact and we may choose an affine scheme
$U = \Spec(B)$ and a surjective \'etale morphism $U \to X$
(Properties of Spaces, Lemma
\ref{spaces-properties-lemma-quasi-compact-affine-cover}).
Note that $U \times_X U = \Spec(B \otimes_A B)$. Hence the category of
quasi-coherent $\mathcal{O}_X$-modules is equivalent to the
category $DD_{B/A}$ of descent data on modules for $A \to B$.
See Properties of Spaces, Proposition
\ref{spaces-properties-proposition-quasi-coherent},
Descent, Definition \ref{descent-definition-descent-datum-modules}, and
Descent, Subsection \ref{descent-subsection-descent-modules-morphisms}.
On the other hand,
$$A \to B$$
is a universally injective ring map. Namely, given an
$A$-module $M$ we see that $A \oplus M \to B \otimes_A (A \oplus M)$
is injective by the assumption of the lemma. Hence
$DD_{B/A}$ is equivalent to the category of $A$-modules by
Descent, Theorem \ref{descent-theorem-descent}. Thus pullback along
$f : X \to \Spec(A)$ determines an equivalence of categories of
quasi-coherent modules. In particular $f^*$ is exact on
quasi-coherent modules and we see that $f$ is flat
(small detail omitted). Moreover, it is clear that $f$ is surjective
(for example because $\Spec(B) \to \Spec(A)$ is surjective).
Hence we see that $\{X \to \Spec(A)\}$ is an fpqc cover.
Then $X \to \Spec(A)$ is a morphism which becomes an isomorphism
after base change by $X \to \Spec(A)$. Hence it is an isomorphism by
fpqc descent, see Descent on Spaces, Lemma
\ref{spaces-descent-lemma-descending-property-isomorphism}.
\end{proof}

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