For a decent algebraic space we can define the residue field and the henselian local ring at a point. For example, the following lemma tells us the residue field of a point on a decent space is defined.
Lemma 68.11.1. Let $S$ be a scheme. Let $X$ be an algebraic space over $S$. Consider the map This map is always injective. If $X$ is decent then this map is a bijection.
\[ \{ \mathop{\mathrm{Spec}}(k) \to X \text{ monomorphism where }k\text{ is a field}\} \longrightarrow |X| \]
Proof. We have seen in Properties of Spaces, Lemma 66.4.12 that the map is an injection in general. By Lemma 68.5.1 it is surjective when $X$ is decent (actually one can say this is part of the definition of being decent). $\square$
Let $S$ be a scheme. Let $X$ be an algebraic space over $S$. If a point $x \in |X|$ can be represented by a monomorphism $\mathop{\mathrm{Spec}}(k) \to X$, then the field $k$ is unique up to unique isomorphism. For a decent algebraic space such a monomorphism exists for every point by Lemma 68.11.1 and hence the following definition makes sense.
Definition 68.11.2. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $x \in |X|$. The residue field of $X$ at $x$ is the unique field $\kappa (x)$ which comes equipped with a monomorphism $\mathop{\mathrm{Spec}}(\kappa (x)) \to X$ representing $x$.
Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of decent algebraic spaces over $S$. Let $x \in |X|$ be a point. Set $y = f(x) \in |Y|$. Then the composition $\mathop{\mathrm{Spec}}(\kappa (x)) \to Y$ is in the equivalence class defining $y$ and hence factors through $\mathop{\mathrm{Spec}}(\kappa (y)) \to Y$. In other words we get a commutative diagram
The left vertical morphism corresponds to a homomorphism $\kappa (y) \to \kappa (x)$ of fields. We will often simply call this the homomorphism induced by $f$.
Lemma 68.11.3. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of decent algebraic spaces over $S$. Let $x \in |X|$ be a point with image $y = f(x) \in |Y|$. The following are equivalent
$f$ induces an isomorphism $\kappa (y) \to \kappa (x)$, and
the induced morphism $\mathop{\mathrm{Spec}}(\kappa (x)) \to Y$ is a monomorphism.
Proof. Immediate from the discussion above. $\square$
The following lemma tells us that the henselian local ring of a point on a decent algebraic space is defined.
Lemma 68.11.4. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. For every point $x \in |X|$ there exists an étale morphism where $U$ is an affine scheme, $u$ is the only point of $U$ lying over $x$, and the induced homomorphism $\kappa (x) \to \kappa (u)$ is an isomorphism.
\[ (U, u) \longrightarrow (X, x) \]
Proof. We may assume that $X$ is quasi-compact by replacing $X$ with a quasi-compact open containing $x$. Recall that $x$ can be represented by a quasi-compact (mono)morphism from the spectrum a field (by definition of decent spaces). Thus the lemma follows from Lemma 68.8.3. $\square$
Definition 68.11.5. Let $S$ be a scheme. Let $X$ be an algebraic space over $S$. Let $x \in X$ be a point. An elementary étale neighbourhood is an étale morphism $(U, u) \to (X, x)$ where $U$ is a scheme, $u \in U$ is a point mapping to $x$, and the morphism $u = \mathop{\mathrm{Spec}}(\kappa (u)) \to X$ is a monomorphism. A morphism of elementary étale neighbourhoods $(U, u) \to (U', u')$ is defined as a morphism $U \to U'$ over $X$ mapping $u$ to $u'$.
If $X$ is not decent then the category of elementary étale neighbourhoods may be empty.
Lemma 68.11.6. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $x$ be a point of $X$. The category of elementary étale neighborhoods of $(X, x)$ is cofiltered (see Categories, Definition 4.20.1).
Proof. The category is nonempty by Lemma 68.11.4. Suppose that we have two elementary étale neighbourhoods $(U_ i, u_ i) \to (X, x)$. Then consider $U = U_1 \times _ X U_2$. Since $\mathop{\mathrm{Spec}}(\kappa (u_ i)) \to X$, $i = 1, 2$ are both monomorphisms in the class of $x$ (Lemma 68.11.3) , we see that
is the spectrum of a field $\kappa (u)$ such that the induced maps $\kappa (u_ i) \to \kappa (u)$ are isomorphisms. Then $u \to U$ is a point of $U$ and we see that $(U, u) \to (X, x)$ is an elementary étale neighbourhood dominating $(U_ i, u_ i)$. If $a, b : (U_1, u_1) \to (U_2, u_2)$ are two morphisms between our elementary étale neighbourhoods, then we consider the scheme
Using Properties of Spaces, Lemma 66.16.6 we see that $U \to X$ is étale. Moreover, in exactly the same manner as before we see that $U$ has a point $u$ such that $(U, u) \to (X, x)$ is an elementary étale neighbourhood. Finally, $U \to U_1$ equalizes $a$ and $b$ and the proof is finished. $\square$
Definition 68.11.7. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $x \in |X|$. The henselian local ring of $X$ at $x$, is where the colimit is over the elementary étale neighbourhoods $(U, u) \to (X, x)$.
\[ \mathcal{O}_{X, x}^ h = \mathop{\mathrm{colim}}\nolimits \Gamma (U, \mathcal{O}_ U) \]
Here is the analogue of Properties of Spaces, Lemma 66.22.1.
Lemma 68.11.8. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $x \in |X|$. Let $(U, u) \to (X, x)$ be an elementary étale neighbourhood. Then In words: the henselian local ring of $X$ at $x$ is equal to the henselization $\mathcal{O}_{U, u}^ h$ of the local ring $\mathcal{O}_{U, u}$ of $U$ at $u$.
\[ \mathcal{O}_{X, x}^ h = \mathcal{O}_{U, u}^ h \]
Proof. Since the category of elementary étale neighbourhood of $(X, x)$ is cofiltered (Lemma 68.11.6) we see that the category of elementary étale neighbourhoods of $(U, u)$ is initial in the category of elementary étale neighbourhood of $(X, x)$. Then the equality follows from More on Morphisms, Lemma 37.35.5 and Categories, Lemma 4.17.2 (initial is turned into cofinal because the colimit defining henselian local rings is over the opposite of the category of elementary étale neighbourhoods). $\square$
Lemma 68.11.9. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $\overline{x}$ be a geometric point of $X$ lying over $x \in |X|$. The étale local ring $\mathcal{O}_{X, \overline{x}}$ of $X$ at $\overline{x}$ (Properties of Spaces, Definition 66.22.2) is the strict henselization of the henselian local ring $\mathcal{O}_{X, x}^ h$ of $X$ at $x$.
Proof. Follows from Lemma 68.11.8, Properties of Spaces, Lemma 66.22.1 and the fact that $(R^ h)^{sh} = R^{sh}$ for a local ring $(R, \mathfrak m, \kappa )$ and a given separable algebraic closure $\kappa ^{sep}$ of $\kappa $. This equality follows from Algebra, Lemma 10.154.7. $\square$
Lemma 68.11.10. Let $S$ be a scheme. Let $X$ be a decent algebraic space over $S$. Let $x \in |X|$. The residue field of the henselian local ring of $X$ at $x$ (Definition 68.11.7) is the residue field of $X$ at $x$ (Definition 68.11.2).
Proof. Choose an elementary étale neighbourhood $(U, u) \to (X, x)$. Then $\kappa (u) = \kappa (x)$ and $\mathcal{O}_{X, x}^ h = \mathcal{O}_{U, u}^ h$ (Lemma 68.11.8). The residue field of $\mathcal{O}_{U, u}^ h$ is $\kappa (u)$ by Algebra, Lemma 10.155.1 (the output of this lemma is the construction/definition of the henselization of a local ring, see Algebra, Definition 10.155.3). $\square$
Remark 68.11.11. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of decent algebraic spaces over $S$. Let $x \in |X|$ with image $y \in |Y|$. Choose an elementary étale neighbourhood $(V, v) \to (Y, y)$ (possible by Lemma 68.11.4). Then $V \times _ Y X$ is an algebraic space étale over $X$ which has a unique point $x'$ mapping to $x$ in $X$ and to $v$ in $V$. (Details omitted; use that all points can be represented by monomorphisms from spectra of fields.) Choose an elementary étale neighbourhood $(U, u) \to (V \times _ Y X, x')$. Then we obtain the following commutative diagram This comes from the identifications $\mathcal{O}_{X, \overline{x}} = \mathcal{O}_{U, u}^{sh}$, $\mathcal{O}_{X, x}^ h = \mathcal{O}_{U, u}^ h$, $\mathcal{O}_{Y, \overline{y}} = \mathcal{O}_{V, v}^{sh}$, $\mathcal{O}_{Y, y}^ h = \mathcal{O}_{V, v}^ h$ see in Lemma 68.11.8 and Properties of Spaces, Lemma 66.22.1 and the functoriality of the (strict) henselization discussed in Algebra, Sections 10.154 and 10.155.
\[ \xymatrix{ \mathop{\mathrm{Spec}}(\mathcal{O}_{X, \overline{x}}) \ar[r] \ar[d] & \mathop{\mathrm{Spec}}(\mathcal{O}_{X, x}^ h) \ar[r] \ar[d] & \mathop{\mathrm{Spec}}(\mathcal{O}_{U, u}) \ar[r] \ar[d] & U \ar[r] \ar[d] & X \ar[d] \\ \mathop{\mathrm{Spec}}(\mathcal{O}_{Y, \overline{y}}) \ar[r] & \mathop{\mathrm{Spec}}(\mathcal{O}_{Y, y}^ h) \ar[r] & \mathop{\mathrm{Spec}}(\mathcal{O}_{V, v}) \ar[r] & V \ar[r] & Y } \]
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