Here is the characterization of quasi-compact formal algebraic spaces.

Lemma 87.17.1. Let $S$ be a scheme. Let $X$ be a formal algebraic space over $S$. The following are equivalent

the reduction of $X$ (Lemma 87.12.1) is a quasi-compact algebraic space,

we can find $\{ X_ i \to X\} _{i \in I}$ as in Definition 87.11.1 with $I$ finite,

there exists a morphism $Y \to X$ representable by algebraic spaces which is étale and surjective and where $Y$ is an affine formal algebraic space.

**Proof.**
Omitted.
$\square$

Definition 87.17.2. Let $S$ be a scheme. Let $X$ be a formal algebraic space over $S$. We say $X$ is *quasi-compact* if the equivalent conditions of Lemma 87.17.1 are satisfied.

Lemma 87.17.3. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of formal algebraic spaces over $S$. The following are equivalent

the induced map $f_{red} : X_{red} \to Y_{red}$ between reductions (Lemma 87.12.1) is a quasi-compact morphism of algebraic spaces,

for every quasi-compact scheme $T$ and morphism $T \to Y$ the fibre product $X \times _ Y T$ is a quasi-compact formal algebraic space,

for every affine scheme $T$ and morphism $T \to Y$ the fibre product $X \times _ Y T$ is a quasi-compact formal algebraic space, and

there exists a covering $\{ Y_ j \to Y\} $ as in Definition 87.11.1 such that each $X \times _ Y Y_ j$ is a quasi-compact formal algebraic space.

**Proof.**
Omitted.
$\square$

Definition 87.17.4. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of formal algebraic spaces over $S$. We say $f$ is *quasi-compact* if the equivalent conditions of Lemma 87.17.3 are satisfied.

This agrees with the already existing notion when the morphism is representable by algebraic spaces (and in particular when it is representable).

Lemma 87.17.5. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of formal algebraic spaces over $S$ which is representable by algebraic spaces. Then $f$ is quasi-compact in the sense of Definition 87.17.4 if and only if $f$ is quasi-compact in the sense of Bootstrap, Definition 80.4.1.

**Proof.**
This is immediate from the definitions and Lemma 87.17.3.
$\square$

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