## 98.3 The Hom functor

In this section we study the functor of homomorphisms defined below.

Situation 98.3.1. Let $S$ be a scheme. Let $f : X \to B$ be a morphism of algebraic spaces over $S$. Let $\mathcal{F}$, $\mathcal{G}$ be quasi-coherent $\mathcal{O}_ X$-modules. For any scheme $T$ over $B$ we will denote $\mathcal{F}_ T$ and $\mathcal{G}_ T$ the base changes of $\mathcal{F}$ and $\mathcal{G}$ to $T$, in other words, the pullbacks via the projection morphism $X_ T = X \times _ B T \to X$. We consider the functor

98.3.1.1
\begin{equation} \label{quot-equation-hom} \mathit{Hom}(\mathcal{F}, \mathcal{G}) : (\mathit{Sch}/B)^{opp} \longrightarrow \textit{Sets},\quad T \longrightarrow \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X_ T}}(\mathcal{F}_ T, \mathcal{G}_ T) \end{equation}

In Situation 98.3.1 we sometimes think of $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ as a functor $(\mathit{Sch}/S)^{opp} \to \textit{Sets}$ endowed with a morphism $\mathit{Hom}(\mathcal{F}, \mathcal{G}) \to B$. Namely, if $T$ is a scheme over $S$, then an element of $\mathit{Hom}(\mathcal{F}, \mathcal{G})(T)$ consists of a pair $(h, u)$, where $h$ is a morphism $h : T \to B$ and $u : \mathcal{F}_ T \to \mathcal{G}_ T$ is an $\mathcal{O}_{X_ T}$-module map where $X_ T = T \times _{h, B} X$ and $\mathcal{F}_ T$ and $\mathcal{G}_ T$ are the pullbacks to $X_ T$. In particular, when we say that $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is an algebraic space, we mean that the corresponding functor $(\mathit{Sch}/S)^{opp} \to \textit{Sets}$ is an algebraic space.

Lemma 98.3.2. In Situation 98.3.1 the functor $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ satisfies the sheaf property for the fpqc topology.

Proof. Let $\{ T_ i \to T\} _{i \in I}$ be an fpqc covering of schemes over $B$. Set $X_ i = X_{T_ i} = X \times _ S T_ i$ and $\mathcal{F}_ i = u_{T_ i}$ and $\mathcal{G}_ i = \mathcal{G}_{T_ i}$. Note that $\{ X_ i \to X_ T\} _{i \in I}$ is an fpqc covering of $X_ T$, see Topologies on Spaces, Lemma 72.9.3. Thus a family of maps $u_ i : \mathcal{F}_ i \to \mathcal{G}_ i$ such that $u_ i$ and $u_ j$ restrict to the same map on $X_{T_ i \times _ T T_ j}$ comes from a unique map $u : \mathcal{F}_ T \to \mathcal{G}_ T$ by descent (Descent on Spaces, Proposition 73.4.1). $\square$

Sanity check: $\mathit{Hom}$ sheaf plays the same role among algebraic spaces over $S$.

Lemma 98.3.3. In Situation 98.3.1. Let $T$ be an algebraic space over $S$. We have

$\mathop{\mathrm{Mor}}\nolimits _{\mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/S)_{fppf})}(T, \mathit{Hom}(\mathcal{F}, \mathcal{G})) = \{ (h, u) \mid h : T \to B, u : \mathcal{F}_ T \to \mathcal{G}_ T\}$

where $\mathcal{F}_ T, \mathcal{G}_ T$ denote the pullbacks of $\mathcal{F}$ and $\mathcal{G}$ to the algebraic space $X \times _{B, h} T$.

Proof. Choose a scheme $U$ and a surjective étale morphism $p : U \to T$. Let $R = U \times _ T U$ with projections $t, s : R \to U$.

Let $v : T \to \mathit{Hom}(\mathcal{F}, \mathcal{G})$ be a natural transformation. Then $v(p)$ corresponds to a pair $(h_ U, u_ U)$ over $U$. As $v$ is a transformation of functors we see that the pullbacks of $(h_ U, u_ U)$ by $s$ and $t$ agree. Since $T = U/R$ (Spaces, Lemma 64.9.1), we obtain a morphism $h : T \to B$ such that $h_ U = h \circ p$. Then $\mathcal{F}_ U$ is the pullback of $\mathcal{F}_ T$ to $X_ U$ and similarly for $\mathcal{G}_ U$. Hence $u_ U$ descends to a $\mathcal{O}_{X_ T}$-module map $u : \mathcal{F}_ T \to \mathcal{G}_ T$ by Descent on Spaces, Proposition 73.4.1.

Conversely, let $(h, u)$ be a pair over $T$. Then we get a natural transformation $v : T \to \mathit{Hom}(\mathcal{F}, \mathcal{G})$ by sending a morphism $a : T' \to T$ where $T'$ is a scheme to $(h \circ a, a^*u)$. We omit the verification that the construction of this and the previous paragraph are mutually inverse. $\square$

Remark 98.3.4. In Situation 98.3.1 let $B' \to B$ be a morphism of algebraic spaces over $S$. Set $X' = X \times _ B B'$ and denote $\mathcal{F}'$, $\mathcal{G}'$ the pullback of $\mathcal{F}$, $\mathcal{G}$ to $X'$. Then we obtain a functor $\mathit{Hom}(\mathcal{F}', \mathcal{G}') : (\mathit{Sch}/B')^{opp} \to \textit{Sets}$ associated to the base change $f' : X' \to B'$. For a scheme $T$ over $B'$ it is clear that we have

$\mathit{Hom}(\mathcal{F}', \mathcal{G}')(T) = \mathit{Hom}(\mathcal{F}, \mathcal{G})(T)$

where on the right hand side we think of $T$ as a scheme over $B$ via the composition $T \to B' \to B$. This trivial remark will occasionally be useful to change the base algebraic space.

Lemma 98.3.5. In Situation 98.3.1 let $\{ X_ i \to X\} _{i \in I}$ be an fppf covering and for each $i, j \in I$ let $\{ X_{ijk} \to X_ i \times _ X X_ j\}$ be an fppf covering. Denote $\mathcal{F}_ i$, resp. $\mathcal{F}_{ijk}$ the pullback of $\mathcal{F}$ to $X_ i$, resp. $X_{ijk}$. Similarly define $\mathcal{G}_ i$ and $\mathcal{G}_{ijk}$. For every scheme $T$ over $B$ the diagram

$\xymatrix{ \mathit{Hom}(\mathcal{F}, \mathcal{G})(T) \ar[r] & \prod \nolimits _ i \mathit{Hom}(\mathcal{F}_ i, \mathcal{G}_ i)(T) \ar@<1ex>[r]^-{\text{pr}_0^*} \ar@<-1ex>[r]_-{\text{pr}_1^*} & \prod \nolimits _{i, j, k} \mathit{Hom}(\mathcal{F}_{ijk}, \mathcal{G}_{ijk})(T) }$

presents the first arrow as the equalizer of the other two.

Proof. Let $u_ i : \mathcal{F}_{i, T} \to \mathcal{G}_{i, T}$ be an element in the equalizer of $\text{pr}_0^*$ and $\text{pr}_1^*$. Since the base change of an fppf covering is an fppf covering (Topologies on Spaces, Lemma 72.7.3) we see that $\{ X_{i, T} \to X_ T\} _{i \in I}$ and $\{ X_{ijk, T} \to X_{i, T} \times _{X_ T} X_{j, T}\}$ are fppf coverings. Applying Descent on Spaces, Proposition 73.4.1 we first conclude that $u_ i$ and $u_ j$ restrict to the same morphism over $X_{i, T} \times _{X_ T} X_{j, T}$, whereupon a second application shows that there is a unique morphism $u : \mathcal{F}_ T \to \mathcal{G}_ T$ restricting to $u_ i$ for each $i$. This finishes the proof. $\square$

Lemma 98.3.6. In Situation 98.3.1. If $\mathcal{F}$ is of finite presentation and $f$ is quasi-compact and quasi-separated, then $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is limit preserving.

Proof. Let $T = \mathop{\mathrm{lim}}\nolimits _{i \in I} T_ i$ be a directed limit of affine $B$-schemes. We have to show that

$\mathit{Hom}(\mathcal{F}, \mathcal{G})(T) = \mathop{\mathrm{colim}}\nolimits \mathit{Hom}(\mathcal{F}, \mathcal{G})(T_ i)$

Pick $0 \in I$. We may replace $B$ by $T_0$, $X$ by $X_{T_0}$, $\mathcal{F}$ by $\mathcal{F}_{T_0}$, $\mathcal{G}$ by $\mathcal{G}_{T_0}$, and $I$ by $\{ i \in I \mid i \geq 0\}$. See Remark 98.3.4. Thus we may assume $B = \mathop{\mathrm{Spec}}(R)$ is affine.

When $B$ is affine, then $X$ is quasi-compact and quasi-separated. Choose a surjective étale morphism $U \to X$ where $U$ is an affine scheme (Properties of Spaces, Lemma 65.6.3). Since $X$ is quasi-separated, the scheme $U \times _ X U$ is quasi-compact and we may choose a surjective étale morphism $V \to U \times _ X U$ where $V$ is an affine scheme. Applying Lemma 98.3.5 we see that $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is the equalizer of two maps between

$\mathit{Hom}(\mathcal{F}|_ U, \mathcal{G}|_ U) \quad \text{and}\quad \mathit{Hom}(\mathcal{F}|_ V, \mathcal{G}|_ V)$

This reduces us to the case that $X$ is affine.

In the affine case the statement of the lemma reduces to the following problem: Given a ring map $R \to A$, two $A$-modules $M$, $N$ and a directed system of $R$-algebras $C = \mathop{\mathrm{colim}}\nolimits C_ i$. When is it true that the map

$\mathop{\mathrm{colim}}\nolimits \mathop{\mathrm{Hom}}\nolimits _{A \otimes _ R C_ i}(M \otimes _ R C_ i, N \otimes _ R C_ i) \longrightarrow \mathop{\mathrm{Hom}}\nolimits _{A \otimes _ R C}(M \otimes _ R C, N \otimes _ R C)$

is bijective? By Algebra, Lemma 10.127.5 this holds if $M \otimes _ R C$ is of finite presentation over $A \otimes _ R C$, i.e., when $M$ is of finite presentation over $A$. $\square$

Lemma 98.3.7. Let $S$ be a scheme. Let $B$ be an algebraic space over $S$. Let $i : X' \to X$ be a closed immersion of algebraic spaces over $B$. Let $\mathcal{F}$ be a quasi-coherent $\mathcal{O}_ X$-module and let $\mathcal{G}'$ be a quasi-coherent $\mathcal{O}_{X'}$-module. Then

$\mathit{Hom}(\mathcal{F}, i_*\mathcal{G}') = \mathit{Hom}(i^*\mathcal{F}, \mathcal{G}')$

as functors on $(\mathit{Sch}/B)$.

Proof. Let $g : T \to B$ be a morphism where $T$ is a scheme. Denote $i_ T : X'_ T \to X_ T$ the base change of $i$. Denote $h : X_ T \to X$ and $h' : X'_ T \to X'$ the projections. Observe that $(h')^*i^*\mathcal{F} = i_ T^*h^*\mathcal{F}$. As a closed immersion is affine (Morphisms of Spaces, Lemma 66.20.6) we have $h^*i_*\mathcal{G} = i_{T, *}(h')^*\mathcal{G}$ by Cohomology of Spaces, Lemma 68.11.1. Thus we have

\begin{align*} \mathit{Hom}(\mathcal{F}, i_*\mathcal{G}')(T) & = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X_ T}}(h^*\mathcal{F}, h^*i_*\mathcal{G}') \\ & = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X_ T}}(h^*\mathcal{F}, i_{T, *}(h')^*\mathcal{G}) \\ & = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X'_ T}}(i_ T^*h^*\mathcal{F}, (h')^*\mathcal{G}) \\ & = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X'_ T}}((h')^*i^*\mathcal{F}, (h')^*\mathcal{G}) \\ & = \mathit{Hom}(i^*\mathcal{F}, \mathcal{G}')(T) \end{align*}

as desired. The middle equality follows from the adjointness of the functors $i_{T, *}$ and $i_ T^*$. $\square$

Lemma 98.3.8. Let $S$ be a scheme. Let $B$ be an algebraic space over $S$. Let $K$ be a pseudo-coherent object of $D(\mathcal{O}_ B)$.

1. If for all $g : T \to B$ in $(\mathit{Sch}/B)$ the cohomology sheaf $H^{-1}(Lg^*K)$ is zero, then the functor

$(\mathit{Sch}/B)^{opp} \longrightarrow \textit{Sets},\quad (g : T \to B) \longmapsto H^0(T, H^0(Lg^*K))$

is an algebraic space affine and of finite presentation over $B$.

2. If for all $g : T \to B$ in $(\mathit{Sch}/B)$ the cohomology sheaves $H^ i(Lg^*K)$ are zero for $i < 0$, then $K$ is perfect, $K$ locally has tor amplitude in $[0, b]$, and the functor

$(\mathit{Sch}/B)^{opp} \longrightarrow \textit{Sets},\quad (g : T \to B) \longmapsto H^0(T, Lg^*K)$

is an algebraic space affine and of finite presentation over $B$.

Proof. Under the assumptions of (2) we have $H^0(T, Lg^*K) = H^0(T, H^0(Lg^*K))$. Let us prove that the rule $T \mapsto H^0(T, H^0(Lg^*K))$ satisfies the sheaf property for the fppf topology. To do this assume we have an fppf covering $\{ h_ i : T_ i \to T\}$ of a scheme $g : T \to B$ over $B$. Set $g_ i = g \circ h_ i$. Note that since $h_ i$ is flat, we have $Lh_ i^* = h_ i^*$ and $h_ i^*$ commutes with taking cohomology. Hence

$H^0(T_ i, H^0(Lg_ i^*K)) = H^0(T_ i, H^0(h_ i^*Lg^*K)) = H^0(T, h_ i^*H^0(Lg^*K))$

Similarly for the pullback to $T_ i \times _ T T_ j$. Since $Lg^*K$ is a pseudo-coherent complex on $T$ (Cohomology on Sites, Lemma 21.45.3) the cohomology sheaf $\mathcal{F} = H^0(Lg^*K)$ is quasi-coherent (Derived Categories of Spaces, Lemma 74.13.6). Hence by Descent on Spaces, Proposition 73.4.1 we see that

$H^0(T, \mathcal{F}) = \mathop{\mathrm{Ker}}( \prod H^0(T_ i, h_ i^*\mathcal{F}) \to \prod H^0(T_ i \times _ T T_ j, (T_ i \times _ T T_ j \to T)^*\mathcal{F}))$

In this way we see that the rules in (1) and (2) satisfy the sheaf property for fppf coverings. This means we may apply Bootstrap, Lemma 79.11.2 to see it suffices to prove the representability étale locally on $B$. Moreover, we may check whether the end result is affine and of finite presentation étale locally on $B$, see Morphisms of Spaces, Lemmas 66.20.3 and 66.28.4. Hence we may assume that $B$ is an affine scheme.

Assume $B = \mathop{\mathrm{Spec}}(A)$ is an affine scheme. By the results of Derived Categories of Spaces, Lemmas 74.13.6, 74.4.2, and 74.13.2 we deduce that in the rest of the proof we may think of $K$ as a perfect object of the derived category of complexes of modules on $B$ in the Zariski topology. By Derived Categories of Schemes, Lemmas 36.10.1, 36.3.5, and 36.10.2 we can find a pseudo-coherent complex $M^\bullet$ of $A$-modules such that $K$ is the corresponding object of $D(\mathcal{O}_ B)$. Our assumption on pullbacks implies that $M^\bullet \otimes ^\mathbf {L}_ A \kappa (\mathfrak p)$ has vanishing $H^{-1}$ for all primes $\mathfrak p \subset A$. By More on Algebra, Lemma 15.76.4 we can write

$M^\bullet = \tau _{\geq 0}M^\bullet \oplus \tau _{\leq - 1}M^\bullet$

with $\tau _{\geq 0}M^\bullet$ perfect with Tor amplitude in $[0, b]$ for some $b \geq 0$ (here we also have used More on Algebra, Lemmas 15.74.12 and 15.66.16). Note that in case (2) we also see that $\tau _{\leq - 1}M^\bullet = 0$ in $D(A)$ whence $M^\bullet$ and $K$ are perfect with tor amplitude in $[0, b]$. For any $B$-scheme $g : T \to B$ we have

$H^0(T, H^0(Lg^*K)) = H^0(T, H^0(Lg^*\tau _{\geq 0}K))$

(by the dual of Derived Categories, Lemma 13.16.1) hence we may replace $K$ by $\tau _{\geq 0}K$ and correspondingly $M^\bullet$ by $\tau _{\geq 0}M^\bullet$. In other words, we may assume $M^\bullet$ has tor amplitude in $[0, b]$.

Assume $M^\bullet$ has tor amplitude in $[0, b]$. We may assume $M^\bullet$ is a bounded above complex of finite free $A$-modules (by our definition of pseudo-coherent complexes, see More on Algebra, Definition 15.64.1 and the discussion following the definition). By More on Algebra, Lemma 15.66.2 we see that $M = \mathop{\mathrm{Coker}}(M^{- 1} \to M^0)$ is flat. By Algebra, Lemma 10.78.2 we see that $M$ is finite locally free. Hence $M^\bullet$ is quasi-isomorphic to

$M \to M^1 \to M^2 \to \ldots \to M^ d \to 0 \ldots$

Note that this is a K-flat complex (Cohomology, Lemma 20.26.9), hence derived pullback of $K$ via a morphism $T \to B$ is computed by the complex

$g^*\widetilde{M} \to g^*\widetilde{M^1} \to \ldots$

Thus it suffices to show that the functor

$(g : T \to B) \longmapsto \mathop{\mathrm{Ker}}( \Gamma (T,g^*\widetilde{M}) \to \Gamma (T, g^*(\widetilde{M^1}) )$

is representable by an affine scheme of finite presentation over $B$.

We may still replace $B$ by the members of an affine open covering in order to prove this last statement. Hence we may assume that $M$ is finite free (recall that $M^1$ is finite free to begin with). Write $M = A^{\oplus n}$ and $M^1 = A^{\oplus m}$. Let the map $M \to M^1$ be given by the $m \times n$ matrix $(a_{ij})$ with coefficients in $A$. Then $\widetilde{M} = \mathcal{O}_ B^{\oplus n}$ and $\widetilde{M^1} = \mathcal{O}_ B^{\oplus m}$. Thus the functor above is equal to the functor

$(g : T \to B) \longmapsto \{ (f_1, \ldots , f_ n) \in \Gamma (T, \mathcal{O}_ T) \mid \sum g^\sharp (a_{ij})f_ i = 0,\ j = 1, \ldots , m\}$

Clearly this is representable by the affine scheme

$\mathop{\mathrm{Spec}}\left(A[x_1, \ldots , x_ n]/(\sum a_{ij}x_ i; j = 1, \ldots , m)\right)$

and the lemma has been proved. $\square$

The functor $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is representable in a number of situations. All of our results will be based on the following basic case. The proof of this lemma as given below is in some sense the natural generalization to the proof of [III, Cor 7.7.8, EGA].

Lemma 98.3.9. In Situation 98.3.1 assume that

1. $B$ is a Noetherian algebraic space,

2. $f$ is locally of finite type and quasi-separated,

3. $\mathcal{F}$ is a finite type $\mathcal{O}_ X$-module, and

4. $\mathcal{G}$ is a finite type $\mathcal{O}_ X$-module, flat over $B$, with support proper over $B$.

Then the functor $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is an algebraic space affine and of finite presentation over $B$.

Proof. We may replace $X$ by a quasi-compact open neighbourhood of the support of $\mathcal{G}$, hence we may assume $X$ is Noetherian. In this case $X$ and $f$ are quasi-compact and quasi-separated. Choose an approximation $P \to \mathcal{F}$ by a perfect complex $P$ of the triple $(X, \mathcal{F}, -1)$, see Derived Categories of Spaces, Definition 74.14.1 and Theorem 74.14.7). Then the induced map

$\mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ X}(\mathcal{F}, \mathcal{G}) \longrightarrow \mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_ X)}(P, \mathcal{G})$

is an isomorphism because $P \to \mathcal{F}$ induces an isomorphism $H^0(P) \to \mathcal{F}$ and $H^ i(P) = 0$ for $i > 0$. Moreover, for any morphism $g : T \to B$ denote $h : X_ T = T \times _ B X \to X$ the projection and set $P_ T = Lh^*P$. Then it is equally true that

$\mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_{X_ T}}(\mathcal{F}_ T, \mathcal{G}_ T) \longrightarrow \mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_{X_ T})}(P_ T, \mathcal{G}_ T)$

is an isomorphism, as $P_ T = Lh^*P \to Lh^*\mathcal{F} \to \mathcal{F}_ T$ induces an isomorphism $H^0(P_ T) \to \mathcal{F}_ T$ (because $h^*$ is right exact and $H^ i(P) = 0$ for $i > 0$). Thus it suffices to prove the result for the functor

$T \longmapsto \mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_{X_ T})}(P_ T, \mathcal{G}_ T).$

By the Leray spectral sequence (see Cohomology on Sites, Remark 21.14.4) we have

$\mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_{X_ T})}(P_ T, \mathcal{G}_ T) = H^0(X_ T, R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P_ T, \mathcal{G}_ T)) = H^0(T, Rf_{T, *}R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P_ T, \mathcal{G}_ T))$

where $f_ T : X_ T \to T$ is the base change of $f$. By Derived Categories of Spaces, Lemma 74.21.5 we have

$Rf_{T, *}R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P_ T, \mathcal{G}_ T) = Lg^*Rf_*R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P, \mathcal{G}).$

By Derived Categories of Spaces, Lemma 74.22.3 the object $K = Rf_*R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P, \mathcal{G})$ of $D(\mathcal{O}_ B)$ is perfect. This means we can apply Lemma 98.3.8 as long as we can prove that the cohomology sheaf $H^ i(Lg^*K)$ is $0$ for all $i < 0$ and $g : T \to B$ as above. This is clear from the last displayed formula as the cohomology sheaves of $Rf_{T, *}R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P_ T, \mathcal{G}_ T)$ are zero in negative degrees due to the fact that $R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (P_ T, \mathcal{G}_ T)$ has vanishing cohomology sheaves in negative degrees as $P_ T$ is perfect with vanishing cohomology sheaves in positive degrees. $\square$

Here is a cheap consequence of Lemma 98.3.9.

Proposition 98.3.10. In Situation 98.3.1 assume that

1. $f$ is of finite presentation, and

2. $\mathcal{G}$ is a finitely presented $\mathcal{O}_ X$-module, flat over $B$, with support proper over $B$.

Then the functor $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is an algebraic space affine over $B$. If $\mathcal{F}$ is of finite presentation, then $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ is of finite presentation over $B$.

Proof. By Lemma 98.3.2 the functor $\mathit{Hom}(\mathcal{F}, \mathcal{G})$ satisfies the sheaf property for fppf coverings. This mean we may1 apply Bootstrap, Lemma 79.11.1 to check the representability étale locally on $B$. Moreover, we may check whether the end result is affine or of finite presentation étale locally on $B$, see Morphisms of Spaces, Lemmas 66.20.3 and 66.28.4. Hence we may assume that $B$ is an affine scheme.

Assume $B$ is an affine scheme. As $f$ is of finite presentation, it follows $X$ is quasi-compact and quasi-separated. Thus we can write $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}_ i$ as a filtered colimit of $\mathcal{O}_ X$-modules of finite presentation (Limits of Spaces, Lemma 69.9.1). It is clear that

$\mathit{Hom}(\mathcal{F}, \mathcal{G}) = \mathop{\mathrm{lim}}\nolimits \mathit{Hom}(\mathcal{F}_ i, \mathcal{G})$

Hence if we can show that each $\mathit{Hom}(\mathcal{F}_ i, \mathcal{G})$ is representable by an affine scheme, then we see that the same thing holds for $\mathit{Hom}(\mathcal{F}, \mathcal{G})$. Use the material in Limits, Section 32.2 and Limits of Spaces, Section 69.4. Thus we may assume that $\mathcal{F}$ is of finite presentation.

Say $B = \mathop{\mathrm{Spec}}(R)$. Write $R = \mathop{\mathrm{colim}}\nolimits R_ i$ with each $R_ i$ a finite type $\mathbf{Z}$-algebra. Set $B_ i = \mathop{\mathrm{Spec}}(R_ i)$. By the results of Limits of Spaces, Lemmas 69.7.1 and 69.7.2 we can find an $i$, a morphism of algebraic spaces $X_ i \to B_ i$, and finitely presented $\mathcal{O}_{X_ i}$-modules $\mathcal{F}_ i$ and $\mathcal{G}_ i$ such that the base change of $(X_ i, \mathcal{F}_ i, \mathcal{G}_ i)$ to $B$ recovers $(X, \mathcal{F}, \mathcal{G})$. By Limits of Spaces, Lemma 69.6.12 we may, after increasing $i$, assume that $\mathcal{G}_ i$ is flat over $B_ i$. By Limits of Spaces, Lemma 69.12.3 we may similarly assume the scheme theoretic support of $\mathcal{G}_ i$ is proper over $B_ i$. At this point we can apply Lemma 98.3.9 to see that $H_ i = \mathit{Hom}(\mathcal{F}_ i, \mathcal{G}_ i)$ is an algebraic space affine of finite presentation over $B_ i$. Pulling back to $B$ (using Remark 98.3.4) we see that $H_ i \times _{B_ i} B = \mathit{Hom}(\mathcal{F}, \mathcal{G})$ and we win. $\square$

 We omit the verification of the set theoretical condition (3) of the referenced lemma.

Comment #1616 by David Hansen on

In the second-to-last inset equation in the proof of Lemma 81.3.7, there is a small typo - the parenthesis after the g^\sharp has no closing partner.

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