diff --git a/flat.tex b/flat.tex index 9086eff27..48e85a1b8 100644 --- a/flat.tex +++ b/flat.tex @@ -3628,93 +3628,285 @@ \section{Flat finite type modules, Part II} \label{section-finite-type-flat-II} \noindent -The following lemma will be superseded by the stronger +We will need the following lemma. + +\begin{lemma} +\label{lemma-weak-bourbaki-pre-pre} +Let $R \to S$ be a ring map of finite presentation. Let $N$ be a +finitely presented $S$-module. Let $\mathfrak q \subset S$ be a prime ideal +lying over $\mathfrak p \subset R$. Set +$\overline{S} = S \otimes_R \kappa(\mathfrak p)$, +$\overline{\mathfrak q} = \mathfrak q \overline{S}$, and +$\overline{N} = N \otimes_R \kappa(\mathfrak p)$. Then +we can find a $g \in S$ with +$g \not \in \mathfrak q$ such that +$\overline{g} \in \mathfrak r$ for all +$\mathfrak r \in \text{Ass}_{\overline{S}}(\overline{N})$ +such that $\mathfrak r \not \subset \overline{\mathfrak q}$. +\end{lemma} + +\begin{proof} +Namely, if $\text{Ass}_{\overline{S}}(\overline{N}) = +\{\mathfrak r_1, \ldots, \mathfrak r_n\}$ +(finiteness by Algebra, Lemma \ref{algebra-lemma-finite-ass}), +then after renumbering we may assume that +$$ +\mathfrak r_1 \subset \overline{\mathfrak q}, +\ldots, +\mathfrak r_r \subset \overline{\mathfrak q}, \quad +\mathfrak r_{r + 1} \not \subset \overline{\mathfrak q}, +\ldots, +\mathfrak r_n \not \subset \overline{\mathfrak q} +$$ +Since $\overline{\mathfrak q}$ is a prime ideal we see that the product +$\mathfrak r_{r + 1} \ldots \mathfrak r_n$ is not contained in +$\overline{\mathfrak q}$ and hence we can pick an element +$a$ of $\overline{S}$ contained in +$\mathfrak r_{r + 1}, \ldots, \mathfrak r_n$ but not in +$\overline{\mathfrak q}$. +If there exists $g \in S$ mapping to $a$, then $g$ +works. In general we can find a nonzero element +$\lambda \in \kappa(\mathfrak p)$ +such that $\lambda a$ is the image of a $g \in S$. +\end{proof} + +\noindent +The following lemma has a sligthly stronger variant Lemma \ref{lemma-weak-bourbaki} below. \begin{lemma} \label{lemma-weak-bourbaki-pre} -Let $(R, \mathfrak m)$ be a local ring. -Let $R \to S$ be of finite presentation. -Let $N$ be a finitely presented $S$-module which is free as an $R$-module. -Let $M$ be an $R$-module. -Let $\mathfrak q$ be a prime of $S$ lying over $\mathfrak m$. +Let $R \to S$ be a ring map of finite presentation. +Let $N$ be a finitely presented $S$-module +which is flat as an $R$-module. Let $M$ be an $R$-module. +Let $\mathfrak q$ be a prime of $S$ lying over $\mathfrak p \subset R$. Then -\begin{enumerate} -\item if $\mathfrak q \in \text{WeakAss}_S(M \otimes_R N)$ -then $\mathfrak m \in \text{WeakAss}_R(M)$ and -$\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$, -\item if $\mathfrak m \in \text{WeakAss}_R(M)$ and -$\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$ -is a maximal element then $\mathfrak q \in \text{WeakAss}_S(M \otimes_R N)$. -\end{enumerate} -Here $\overline{S} = S/\mathfrak m S$, +$$ +\mathfrak q \in \text{WeakAss}_S(M \otimes_R N) +\Leftrightarrow +\Big( +\mathfrak p \in \text{WeakAss}_R(M) +\text{ and } +\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N}) +\Big) +$$ +Here $\overline{S} = S \otimes_R \kappa(\mathfrak p)$, $\overline{\mathfrak q} = \mathfrak q \overline{S}$, and -$\overline{N} = N/\mathfrak m N$. +$\overline{N} = N \otimes_R \kappa(\mathfrak p)$. \end{lemma} \begin{proof} +Pick $g \in S$ as in Lemma \ref{lemma-weak-bourbaki-pre-pre}. +Apply Proposition \ref{proposition-finite-presentation-flat-at-point} +to the morphism of schemes $\Spec(S_g) \to \Spec(R)$, the quasi-coherent +module associated to $N_g$, and the points +corresponding to the primes $\mathfrak qS_g$ and $\mathfrak p$. Translating +into algebra we obtain a commutative diagram of rings +$$ +\xymatrix{ +S \ar[r] & S_g \ar[r] & S' \\ +& R \ar[lu] \ar[u] \ar[r] & R' \ar[u] +} +\quad\quad +\xymatrix{ +\mathfrak q \ar@{-}[r] \ar@{-}[rd] & +\mathfrak qS_g \ar@{-}[d] \ar@{-}[r] & \mathfrak q' \ar@{-}[d] \\ +& \mathfrak p \ar@{-}[r] & \mathfrak p' +} +$$ +endowed with primes as shown, the horizontal arrows are \'etale, +and $N \otimes_S S'$ is projective as an $R'$-module. Set +$N' = N \otimes_S S'$, $M' = M \otimes_R R'$, +$\overline{S}' = S' \otimes_{R'} \kappa(\mathfrak q')$, +$\overline{\mathfrak q}' = \mathfrak q' \overline{S}'$, +and +$$ +\overline{N}' = N' \otimes_{R'} \kappa(\mathfrak p') = +\overline{N} \otimes_{\overline{S}} \overline{S}' +$$ +By Lemma \ref{lemma-etale-weak-assassin-up-down} we have +\begin{align*} +\text{WeakAss}_{S'}(M' \otimes_{R'} N') & = +(\Spec(S') \to \Spec(S))^{-1}\text{WeakAss}_S(M \otimes_R N) \\ +\text{WeakAss}_{R'}(M') & = +(\Spec(R') \to \Spec(R))^{-1}\text{WeakAss}_R(M) \\ +\text{Ass}_{\overline{S}'}(\overline{N}') & = +(\Spec(\overline{S}') \to \Spec(\overline{S}))^{-1} +\text{Ass}_{\overline{S}}(\overline{N}) +\end{align*} +Use Algebra, Lemma \ref{algebra-lemma-ass-weakly-ass} +for $\overline{N}$ and $\overline{N}'$. In particular we have +\begin{align*} +\mathfrak q \in \text{WeakAss}_S(M \otimes_R N) +& \Leftrightarrow +\mathfrak q' \in \text{WeakAss}_{S'}(M' \otimes_{R'} N') \\ +\mathfrak p \in \text{WeakAss}_R(M) +& \Leftrightarrow +\mathfrak p' \in \text{WeakAss}_{R'}(M') \\ +\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N}) +& \Leftrightarrow +\overline{\mathfrak q}' \in \text{WeakAss}_{\overline{S}'}(\overline{N}') +\end{align*} +Our careful choice of $g$ and the formula for +$\text{Ass}_{\overline{S}'}(\overline{N}')$ above shows that +\begin{equation} +\label{equation-key-observation} +\text{if }\mathfrak r' \in \text{Ass}_{\overline{S}'}(\overline{N}') +\text{ lies over }\mathfrak r \subset \overline{S}\text{ then } +\mathfrak r \subset \overline{\mathfrak q} +\end{equation} +This will be a key observation later in the proof. We will use +the characterization of weakly associated primes given in +Algebra, Lemma \ref{algebra-lemma-weakly-ass-local} without further mention. + +\medskip\noindent Suppose that $\overline{\mathfrak q} \not \in \text{Ass}_{\overline{S}}(\overline{N})$. +Then +$\overline{\mathfrak q}' \not \in \text{Ass}_{\overline{S}'}(\overline{N}')$. By Algebra, Lemmas \ref{algebra-lemma-ass-zero-divisors}, \ref{algebra-lemma-finite-ass}, and \ref{algebra-lemma-silly} -there exists an element $\overline{g} \in \overline{\mathfrak q}$ -which is not a zerodivisor on $\overline{N}$. Let $g \in \mathfrak q$ -be an element which maps to $\overline{g}$ in $\overline{\mathfrak q}$. By +there exists an element $\overline{a}' \in \overline{\mathfrak q}'$ +which is not a zerodivisor on $\overline{N}'$. +After replacing $\overline{a}'$ by $\lambda \overline{a}'$ for some nonzero +$\lambda \in \kappa(\mathfrak p)$ we can find +$a' \in \mathfrak q'$ mapping to $\overline{a}'$. By Lemma \ref{lemma-invert-universally-injective} -the map $g : N \to N$ is $R$-universally injective. In particular -we see that $g : M \otimes_R N \to M \otimes_R N$ is injective. -Clearly this implies that -$\mathfrak q \not \in \text{WeakAss}_S(M \otimes_R N)$. +the map $a' : N'_{\mathfrak p'} \to N'_{\mathfrak p'}$ is +$R'_{\mathfrak p'}$-universally injective. In particular +we see that $a' : M' \otimes_{R'} N' \to M' \otimes_{R'} N'$ is +injective after localizating at $\mathfrak p'$ and hence after +localizing at $\mathfrak q'$. Clearly this implies that +$\mathfrak q' \not \in \text{WeakAss}_{S'}(M' \otimes_{R'} N')$. We conclude that $\mathfrak q \in \text{WeakAss}_S(M \otimes_R N)$ implies $\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$. \medskip\noindent -Assume $\mathfrak q \in \text{WeakAss}_S(M \otimes_R N)$. -Let $z \in M \otimes_R N$ be an element whose annihilator in $S$ -has radical $\mathfrak q$. As $N$ is a free $R$-module, we can find -a finite free direct summand $F \subset N$ such that -$z \in M \otimes_R F$. The radical of the annihilator of -$z \in M \otimes_R F$ in $R$ is $\mathfrak m$ (by our assumption on $z$ -and because $\mathfrak q$ lies over $\mathfrak m$). Hence we see that -$\mathfrak m \in \text{WeakAss}(M \otimes_R F)$ which implies -that $\mathfrak m \in \text{WeakAss}(M)$ by -Algebra, Lemma \ref{algebra-lemma-weakly-ass}. -This finishes the proof of (1). +Assume $\mathfrak q \in \text{WeakAss}_S(M \otimes_R N)$. We want +to show $\mathfrak p \in \text{WeakAss}_S(M)$. +Let $z \in M \otimes_R N$ be an element such that $\mathfrak q$ +is minimal over $J = \text{Ann}_S(z)$. +Let $f_i \in \mathfrak p$, $i \in I$ be a set of generators of the +ideal $\mathfrak p$. Since $\mathfrak q$ lies over $\mathfrak p$, for every $i$ +we can choose an $n_i \geq 1$ and $g_i \in S$, $g_i \not \in \mathfrak q$ +with $g_i f_i^{n_i} \in J$, i.e., $g_i f_i^{n_i} z = 0$. +Let $z' \in (M' \otimes_{R'} N')_{\mathfrak p'}$ be the image of $z$. +Observe that $z'$ is nonzero because $z$ has nonzero image in +$(M \otimes_R N)_\mathfrak q$ and because $S_\mathfrak q \to S'_{\mathfrak q'}$ +is faithfully flat. We claim that $f_i^{n_i} z' = 0$. + +\medskip\noindent +Proof of the claim: Let $g'_i \in S'$ be the image of $g_i$. +By the key observation (\ref{equation-key-observation}) +we find that the image $\overline{g}'_i \in \overline{S}'$ +is not contained in $\mathfrak r'$ for any +$\mathfrak r' \in \text{Ass}_{\overline{S}'}(\overline{N})$. +Hence by Lemma \ref{lemma-invert-universally-injective} +we see that $g'_i : N'_{\mathfrak p'} \to N'_{\mathfrak p'}$ is +$R'_{\mathfrak p'}$-universally injective. In particular +we see that $g'_i : M' \otimes_{R'} N' \to M' \otimes_{R'} N'$ is +injective after localizating at $\mathfrak p'$. The claim +follows because $g_i f_i^{n_i} z' = 0$. + +\medskip\noindent +Our claim shows that the annihilator of $z'$ in $R'_{\mathfrak p'}$ +contains the elements $f_i^{n_i}$. As $R \to R'$ is \'etale we have +$\mathfrak p'R'_{\mathfrak p'} = \mathfrak pR'_{\mathfrak p'}$ +by Algebra, Lemma \ref{algebra-lemma-etale-at-prime}. +Hence the annihilator of $z'$ in $R'_{\mathfrak p'}$ has radical equal to +$\mathfrak p' R_{\mathfrak p'}$ (here we use $z'$ is not zero). +On the other hand +$$ +z' \in (M' \otimes_{R'} N')_{\mathfrak p'} = +M'_{\mathfrak p'} \otimes_{R'_{\mathfrak p'}} N'_{\mathfrak p'} +$$ +The module $N'_{\mathfrak p'}$ is projective over the local ring +$R'_{\mathfrak p'}$ and hence free +(Algebra, Theorem \ref{algebra-theorem-projective-free-over-local-ring}). +Thus we can find a finite free direct summand $F' \subset N'_{\mathfrak p'}$ +such that $z' \in M'_{\mathfrak p'} \otimes_{R'_{\mathfrak p'}} F'$. +If $F'$ has rank $n$, then we deduce that +$\mathfrak p' R'_{\mathfrak p'} \in +\text{WeakAss}_{R'_{\mathfrak p'}}({M'_{\mathfrak p'}}^{\oplus n})$. +This implies +$\mathfrak p'R'_{\mathfrak p'} \in \text{WeakAss}(M'_{\mathfrak p'})$ +for example by Algebra, Lemma \ref{algebra-lemma-weakly-ass}. +Then $\mathfrak p' \in \text{WeakAss}_{R'}(M')$ +which in turn gives $\mathfrak p \in \text{WeakAss}_R(M)$. +This finishes the proof of the implication +``$\Rightarrow$'' of the equivalence of the lemma. + +\medskip\noindent +Assume that $\mathfrak p \in \text{WeakAss}_R(M)$ and +$\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$. +We want to show that $\mathfrak q$ is weakly associated to $M \otimes_R N$. +Note that $\overline{\mathfrak q}'$ is a maximal element of +$\text{Ass}_{\overline{S}'}(\overline{N}')$. +This is a consequence of (\ref{equation-key-observation}) +and the fact that there are no inclusions among the primes +of $\overline{S}'$ lying over $\overline{\mathfrak q}$ +(as fibres of \'etale morphisms are discrete +Morphisms, Lemma \ref{morphisms-lemma-etale-over-field}). +Thus, after replacing $R, S, \mathfrak p, \mathfrak q, M, N$ by +$R', S', \mathfrak p', \mathfrak q', M', N'$ +we may assume, in addition to the assumptions of the lemma, that +\begin{enumerate} +\item $\mathfrak p \in \text{WeakAss}_R(M)$, +\item $\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$, +\item $N$ is projective as an $R$-module, and +\item $\overline{\mathfrak q}$ is maximal in +$\text{Ass}_{\overline{S}}(\overline{N})$. +\end{enumerate} +There is one more reduction, namely, we may replace +$R, S, M, N$ by their localizations at $\mathfrak p$. +This leads to one more condition, namely, +\begin{enumerate} +\item[(5)] $R$ is a local ring with maximal ideal $\mathfrak p$. +\end{enumerate} +We will finish by showing that (1) -- (5) imply +$\mathfrak q \in \text{WeakAss}(M \otimes_R N)$. \medskip\noindent -Assume that $\mathfrak m \in \text{WeakAss}_R(M)$ and -$\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$ -is a maximal element. -Let $y \in M$ be an element whose annihilator $I = \text{Ann}_R(y)$ -has radical $\mathfrak m$. Then $R/I \subset M$ and by flatness of $N$ -over $R$ we get $N/IN = R/I \otimes_R N \subset M \otimes_R N$. Hence -it is enough to show that $\mathfrak q \in \text{WeakAss}(N/IN)$. +Since $R$ is local and $\mathfrak p \in \text{WeakAss}_R(M)$ +we can pick a $y \in M$ whose annihilator $I$ has radical +equal to $\mathfrak p$. Write $\overline{\mathfrak q} = (\overline{g}_1, \ldots, \overline{g}_n)$ -for some $\overline{g}_i \in \overline{S}$. Choose lifts -$g_i \in \mathfrak q$. Consider the map +for some $\overline{g}_i \in \overline{S}$. Choose $g_i \in S$ +mapping to $\overline{g}_i$. +Then $\mathfrak q = \mathfrak pS + g_1S + \ldots + g_nS$. +Consider the map $$ -\Psi : N/IN \longrightarrow N/IN^{\oplus n}, \quad +\Psi : N/IN \longrightarrow (N/IN)^{\oplus n}, \quad z \longmapsto (g_1z, \ldots, g_nz). $$ -We may think of this as a map of free $R/I$-modules. As the ring -$R/I$ is auto-associated (since $\mathfrak m/I$ is locally nilpotent) -and since $\Psi \otimes R/\mathfrak m$ isn't injective (since -$\overline{\mathfrak q} \in \text{Ass}(\overline{N})$) we see by -More on Algebra, Lemma \ref{more-algebra-lemma-P-fPD-zero} -that $\Psi$ isn't injective. Pick $z \in N/IN$ nonzero in the kernel -of $\Psi$. The annihilator of $z$ contains $I$ and $g_i$, whence -its radical $J = \sqrt{\text{Ann}_S(z)}$ contains $\mathfrak q$. -Let $\mathfrak q' \supset J$ be a minimal prime over $J$. -Then $\mathfrak q' \in \text{WeakAss}(M \otimes_R N)$ (by definition) -and by (1) we see that -$\overline{\mathfrak q}' \in \text{Ass}(\overline{N})$. -Then since $\mathfrak q \subset \mathfrak q'$ by construction the -maximality of $\overline{\mathfrak q}$ implies $\mathfrak q = \mathfrak q'$ -whence $\mathfrak q \in \text{WeakAss}(M \otimes_R N)$. -This proves part (2) of the lemma. +This is a homomorphism of projective $R/I$-modules. +The local ring $R/I$ is auto-associated +(More on Algebra, Definition \ref{more-algebra-definition-auto-ass}) +as $\mathfrak p/I$ is locally nilpotent. +The map $\Psi \otimes \kappa(\mathfrak p)$ is not injective, because +$\overline{\mathfrak q} \in \text{Ass}_{\overline{S}}(\overline{N})$. +Hence More on Algebra, Lemma \ref{more-algebra-lemma-P-fPD-zero} +implies $\Psi$ is not injective. Pick $z \in N/IN$ +nonzero in the kernel of $\Psi$. The annihilator $J = \text{Ann}_S(z)$ +contains $IS$ and $g_i$ by construction. Thus +$\sqrt{J} \subset S$ contains $\mathfrak q$. +Let $\mathfrak s \subset S$ be a prime minimal over $J$. +Then $\mathfrak q \subset \mathfrak s$, +$\mathfrak s$ lies over $\mathfrak p$, and +$\mathfrak s \in \text{WeakAss}_S(N/IN)$. +The last fact by definition of weakly associated primes. +Apply the ``$\Rightarrow$'' part of the lemma (which we've already proven) +to the ring map $R \to S$ and the modules $R/I$ and $N$ +to conclude that +$\overline{\mathfrak s} \in \text{Ass}_{\overline{S}}(\overline{N})$. +Since $\overline{\mathfrak q} \subset \overline{\mathfrak s}$ +the maximality of $\overline{\mathfrak q}$, see condition (4) above, +implies that $\overline{\mathfrak q} = \overline{\mathfrak s}$. +This shows that $\mathfrak q = \mathfrak s$ and we conlude +what we want. \end{proof} \begin{lemma} @@ -3735,6 +3927,7 @@ \section{Flat finite type modules, Part II} \end{lemma} \begin{proof} +In this paragraph we reduce to $f$ being of finite presentation. The question is local on $X$ and $S$, hence we may assume $X$ and $S$ are affine. Write $X = \Spec(B)$, $S = \Spec(A)$ and write $B = A[x_1, \ldots, x_n]/I$. In other words we obtain a closed immersion @@ -3755,71 +3948,40 @@ \section{Flat finite type modules, Part II} if $t \in \text{Ass}_{\mathbf{A}^n_s}((i_*\mathcal{F})_s)$ (see Algebra, Lemma \ref{algebra-lemma-ass-quotient-ring}). Hence it suffices to prove the lemma in case $X = \mathbf{A}^n_S$. -In particular we may assume that $X \to S$ is of finite presentation. +Thus we may assume that $X \to S$ is of finite presentation. \medskip\noindent -Recall that $\text{Ass}_{X_s}(\mathcal{F}_s)$ is a locally finite subset -of the locally Noetherian scheme $X_s$, see -Divisors, Lemma \ref{divisors-lemma-finite-ass}. -After replacing $X$ by a suitable affine neighbourhood of $x$ we may -assume that -\begin{itemize} -\item[$(*)$] if $x' \in \text{Ass}_{X_s}(\mathcal{F}_s)$ and $x \leadsto x'$ -then $x = x'$. -\end{itemize} -(Proof omitted. Hint: using -Algebra, Lemma \ref{algebra-lemma-silly} -invert a function which does not vanish at $x$ but does vanish -in all the finitely many points of $\text{Ass}_{X_s}(\mathcal{F}_s)$ -which are specializations of $x$ but not equal to $x$.) -In words, no point of $\text{Ass}_{X_s}(\mathcal{F}_s)$ -is a proper specialization of $x$. +In this paragraph we reduce to $\mathcal{F}$ being of finite presentation +and flat over $S$. +Choose an elementary \'etale neighbourhood $e : (S', s') \to (S, s)$ +and an open $V \subset X \times_S \Spec(\mathcal{O}_{S', s'})$ +as in Proposition \ref{proposition-finite-type-flat-at-point}. +Let $x' \in X' = X \times_S S'$ be the unique point mapping to $x$ +and $s'$. Then it suffices to prove the statement for +$X' \to S'$, $x'$, $s'$, $(X' \to X)^*\mathcal{F}$, and $e^*\mathcal{G}$, see +Lemma \ref{lemma-etale-weak-assassin-up-down}. +Let $v \in V$ the unique point mapping to $x'$ +and let $s' \in \Spec(\mathcal{O}_{S', s'})$ be the closed point. +Then $\mathcal{O}_{V, v} = \mathcal{O}_{X', x'}$ +and $\mathcal{O}_{\Spec(\mathcal{O}_{S', s'}), s'} = +\mathcal{O}_{S', s'}$ and similarly for the stalks of pullbacks of +$\mathcal{F}$ and $\mathcal{G}$. +Also $V_{s'} \subset X'_{s'}$ is an open subscheme. +Since the condition of being a weakly associated point +depend only on the stalk of the sheaf, we may +replace +$X' \to S'$, $x'$, $s'$, $(X' \to X)^*\mathcal{F}$, and $e^*\mathcal{G}$ +by +$V \to \Spec(\mathcal{O}_{S', s'})$, $v$, $s'$, $(V \to X)^*\mathcal{F}$, +and $(\Spec(\mathcal{O}_{S', s'}) \to S)^*\mathcal{G}$. +Thus we may assume that $f$ is of finite presentation and +$\mathcal{F}$ of finite presentation and flat over $S$. \medskip\noindent -Suppose given a commutative diagram -$$ -\xymatrix{ -(X, x) \ar[d] & (X', x') \ar[l]^g \ar[d] \\ -(S, s) & (S', s') \ar[l]_e -} -$$ -of pointed schemes whose horizontal arrows are elementary \'etale -neighbourhoods. Then it suffices to prove the statement for -$x'$, $s'$, $g^*\mathcal{F}$ and $e^*\mathcal{G}$, see -Lemma \ref{lemma-etale-weak-assassin-up-down}. -Note that property $(*)$ is preserved by such an \'etale localization -by the same lemma (if there is a proper specialization -$x' \leadsto x''$ on $X'_{s'}$ then this maps to a proper -specialization on $X_s$ because the fibres of an \'etale morphism -are discrete). We may also replace $S$ by the spectrum of its local ring -as the condition of being an associated point of a quasi-coherent sheaf -depends only on the stalk of the sheaf. Again property $(*)$ is -preserved by this as well. Thus we may first apply -Proposition \ref{proposition-finite-type-flat-at-point} -to reduce to the case where $\mathcal{F}$ is of finite presentation -and flat over $S$, whereupon we may use -Proposition \ref{proposition-finite-presentation-flat-at-point} -to reduce to the case that $X \to S$ is a morphism of affines -and $\Gamma(X, \mathcal{F})$ is a finitely presented -$\Gamma(X, \mathcal{O}_X)$-module which is projective as a -$\Gamma(S, \mathcal{O}_S)$-module. Localizing $S$ once more we -may assume that $\Gamma(S, \mathcal{O}_S)$ is a local ring such that -$s$ corresponds to the maximal ideal. In this case -Algebra, Theorem \ref{algebra-theorem-projective-free-over-local-ring} -guarantees that $\Gamma(X, \mathcal{F})$ is free as an -$\Gamma(S, \mathcal{O}_S)$-module. The implication -$x \in \text{WeakAss}_X(\mathcal{F} \otimes_{\mathcal{O}_X} f^*\mathcal{G}) -\Rightarrow -s \in \text{WeakAss}_S(\mathcal{G}) -\text{ and } -x \in \text{Ass}_{X_s}(\mathcal{F}_s)$ follows from part (1) of -Lemma \ref{lemma-weak-bourbaki-pre}. -The converse implication follows from -part (2) of Lemma \ref{lemma-weak-bourbaki-pre} -as property $(*)$ insures that the prime corresponding -to $x$ gives rise to a maximal element of -$\text{Ass}_{\overline{S}}(\overline{N})$ exactly as in the statement -of part (2) of +Assume $f$ is of finite presentation and +$\mathcal{F}$ of finite presentation and flat over $S$. +After shrinking $X$ and $S$ to affine neighbourhoods +of $x$ and $s$, this case is handled by Lemma \ref{lemma-weak-bourbaki-pre}. \end{proof}