## 50.15 Log poles along a divisor

Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. If $X$ étale locally along $Y$ looks like $Y \times \mathbf{A}^1$, then there is a canonical short exact sequence of complexes

$0 \to \Omega ^\bullet _{X/S} \to \Omega ^\bullet _{X/S}(\log Y) \to \Omega ^\bullet _{Y/S}[-1] \to 0$

having many good properties we will discuss in this section. There is a variant of this construction where one starts with a normal crossings divisor (Étale Morphisms, Definition 41.21.1) which we will discuss elsewhere (insert future reference here).

Definition 50.15.1. Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. We say the de Rham complex of log poles is defined for $Y \subset X$ over $S$ if for all $y \in Y$ and local equation $f \in \mathcal{O}_{X, y}$ of $Y$ we have

1. $\mathcal{O}_{X, y} \to \Omega _{X/S, y}$, $g \mapsto g \text{d}f$ is a split injection, and

2. $\Omega ^ p_{X/S, y}$ is $f$-torsion free for all $p$.

An easy local calculation shows that it suffices for every $y \in Y$ to find one local equation $f$ for which conditions (1) and (2) hold.

Lemma 50.15.2. Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$. There is a canonical short exact sequence of complexes

$0 \to \Omega ^\bullet _{X/S} \to \Omega ^\bullet _{X/S}(\log Y) \to \Omega ^\bullet _{Y/S}[-1] \to 0$

Proof. Our assumption is that for every $y \in Y$ and local equation $f \in \mathcal{O}_{X, y}$ of $Y$ we have

$\Omega _{X/S, y} = \mathcal{O}_{X, y}\text{d}f \oplus M \quad \text{and}\quad \Omega ^ p_{X/S, y} = \wedge ^{p - 1}(M)\text{d}f \oplus \wedge ^ p(M)$

for some module $M$ with $f$-torsion free exterior powers $\wedge ^ p(M)$. It follows that

$\Omega ^ p_{Y/S, y} = \wedge ^ p(M/fM) = \wedge ^ p(M)/f\wedge ^ p(M)$

Below we will tacitly use these facts. In particular the sheaves $\Omega ^ p_{X/S}$ have no nonzero local sections supported on $Y$ and we have a canonical inclusion

$\Omega ^ p_{X/S} \subset \Omega ^ p_{X/S}(Y)$

see More on Flatness, Section 38.42. Let $U = \mathop{\mathrm{Spec}}(A)$ be an affine open subscheme such that $Y \cap U = V(f)$ for some nonzerodivisor $f \in A$. Let us consider the $\mathcal{O}_ U$-submodule of $\Omega ^ p_{X/S}(Y)|_ U$ generated by $\Omega ^ p_{X/S}|_ U$ and $\text{d}\log (f) \wedge \Omega ^{p - 1}_{X/S}$ where $\text{d}\log (f) = f^{-1}\text{d}(f)$. This is independent of the choice of $f$ as another generator of the ideal of $Y$ on $U$ is equal to $uf$ for a unit $u \in A$ and we get

$\text{d}\log (uf) - \text{d}\log (f) = \text{d}\log (u) = u^{-1}\text{d}u$

which is a section of $\Omega _{X/S}$ over $U$. These local sheaves glue to give a quasi-coherent submodule

$\Omega ^ p_{X/S} \subset \Omega ^ p_{X/S}(\log Y) \subset \Omega ^ p_{X/S}(Y)$

Let us agree to think of $\Omega ^ p_{Y/S}$ as a quasi-coherent $\mathcal{O}_ X$-module. There is a unique surjective $\mathcal{O}_ X$-linear map

$\text{Res} : \Omega ^ p_{X/S}(\log Y) \to \Omega ^{p - 1}_{Y/S}$

defined by the rule

$\text{Res}(\eta ' + \text{d}\log (f) \wedge \eta ) = \eta |_{Y \cap U}$

for all opens $U$ as above and all $\eta ' \in \Omega ^ p_{X/S}(U)$ and $\eta \in \Omega ^{p - 1}_{X/S}(U)$. If a form $\eta$ over $U$ restricts to zero on $Y \cap U$, then $\eta = \text{d}f \wedge \eta ' + f\eta ''$ for some forms $\eta '$ and $\eta ''$ over $U$. We conclude that we have a short exact sequence

$0 \to \Omega ^ p_{X/S} \to \Omega ^ p_{X/S}(\log Y) \to \Omega ^{p - 1}_{Y/S} \to 0$

for all $p$. We still have to define the differentials $\Omega ^ p_{X/S}(\log Y) \to \Omega ^{p + 1}_{X/S}(\log Y)$. On the subsheaf $\Omega ^ p_{X/S}$ we use the differential of the de Rham complex of $X$ over $S$. Finally, we define $\text{d}(\text{d}\log (f) \wedge \eta ) = -\text{d}\log (f) \wedge \text{d}\eta$. The sign is forced on us by the Leibniz rule (on $\Omega ^\bullet _{X/S}$) and it is compatible with the differential on $\Omega ^\bullet _{Y/S}[-1]$ which is after all $-\text{d}_{Y/S}$ by our sign convention in Homology, Definition 12.14.7. In this way we obtain a short exact sequence of complexes as stated in the lemma. $\square$

Definition 50.15.3. Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$. Then the complex

$\Omega ^\bullet _{X/S}(\log Y)$

constructed in Lemma 50.15.2 is the de Rham complex of log poles for $Y \subset X$ over $S$.

This complex has many good properties.

Lemma 50.15.4. Let $p : X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$.

1. The maps $\wedge : \Omega ^ p_{X/S} \times \Omega ^ q_{X/S} \to \Omega ^{p + q}_{X/S}$ extend uniquely to $\mathcal{O}_ X$-bilinear maps

$\wedge : \Omega ^ p_{X/S}(\log Y) \times \Omega ^ q_{X/S}(\log Y) \to \Omega ^{p + q}_{X/S}(\log Y)$

satisfying the Leibniz rule $\text{d}(\omega \wedge \eta ) = \text{d}(\omega ) \wedge \eta + (-1)^{\deg (\omega )} \omega \wedge \text{d}(\eta )$,

2. with multiplication as in (1) the map $\Omega ^\bullet _{X/S} \to \Omega ^\bullet _{X/S}(\log (Y)$ is a homomorphism of differential graded $\mathcal{O}_ S$-algebras,

3. via the maps in (1) we have $\Omega ^ p_{X/S}(\log Y) = \wedge ^ p(\Omega ^1_{X/S}(\log Y))$, and

4. the map $\text{Res} : \Omega ^\bullet _{X/S}(\log Y) \to \Omega ^\bullet _{Y/S}[-1]$ satisfies

$\text{Res}(\omega \wedge \eta ) = \text{Res}(\omega ) \wedge \eta |_ Y$

for $\omega$ a local section of $\Omega ^ p_{X/S}(\log Y)$ and $\eta$ a local section of $\Omega ^ q_{X/S}$.

Proof. This follows by direct calcuation from the local construction of the complex in the proof of Lemma 50.15.2. Details omitted. $\square$

Consider a commutative diagram

$\xymatrix{ X' \ar[r]_ f \ar[d] & X \ar[d] \\ S' \ar[r] & S }$

of schemes. Let $Y \subset X$ be an effective Cartier divisor whose pullback $Y' = f^*Y$ is defined (Divisors, Definition 31.13.12). Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$ and the de Rham complex of log poles is defined for $Y' \subset X'$ over $S'$. In this case we obtain a map of short exact sequences of complexes

$\xymatrix{ 0 \ar[r] & f^{-1}\Omega ^\bullet _{X/S} \ar[r] \ar[d] & f^{-1}\Omega ^\bullet _{X/S}(\log Y) \ar[r] \ar[d] & f^{-1}\Omega ^\bullet _{Y/S}[-1] \ar[r] \ar[d] & 0 \\ 0 \ar[r] & \Omega ^\bullet _{X'/S'} \ar[r] & \Omega ^\bullet _{X'/S'}(\log Y') \ar[r] & \Omega ^\bullet _{Y'/S'}[-1] \ar[r] & 0 }$

Linearizing, for every $p$ we obtain a linear map $f^*\Omega ^ p_{X/S}(\log Y) \to \Omega ^ p_{X'/S'}(\log Y')$.

Lemma 50.15.5. Let $f : X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$. Denote

$\delta : \Omega ^\bullet _{Y/S} \to \Omega ^\bullet _{X/S}[2]$

in $D(X, f^{-1}\mathcal{O}_ S)$ the “boundary” map coming from the short exact sequence in Lemma 50.15.2. Denote

$\xi ' : \Omega ^\bullet _{X/S} \to \Omega ^\bullet _{X/S}[2]$

in $D(X, f^{-1}\mathcal{O}_ S)$ the map of Remark 50.4.3 corresponding to $\xi = c_1^{dR}(\mathcal{O}_ X(-Y))$. Denote

$\zeta ' : \Omega ^\bullet _{Y/S} \to \Omega ^\bullet _{Y/S}[2]$

in $D(Y, f|_ Y^{-1}\mathcal{O}_ S)$ the map of Remark 50.4.3 corresponding to $\zeta = c_1^{dR}(\mathcal{O}_ X(-Y)|_ Y)$. Then the diagram

$\xymatrix{ \Omega ^\bullet _{X/S} \ar[d]_{\xi '} \ar[r] & \Omega ^\bullet _{Y/S} \ar[d]^{\zeta '} \ar[ld]_\delta \\ \Omega ^\bullet _{X/S}[2] \ar[r] & \Omega ^\bullet _{Y/S}[2] }$

is commutative in $D(X, f^{-1}\mathcal{O}_ S)$.

Proof. More precisely, we define $\delta$ as the boundary map corresponding to the shifted short exact sequence

$0 \to \Omega ^\bullet _{X/S}[1] \to \Omega ^\bullet _{X/S}(\log Y)[1] \to \Omega ^\bullet _{Y/S} \to 0$

It suffices to prove each triangle commutes. Set $\mathcal{L} = \mathcal{O}_ X(-Y)$. Denote $\pi : L \to X$ the line bundle with $\pi _*\mathcal{O}_ L = \bigoplus _{n \geq 0} \mathcal{L}^{\otimes n}$.

Commutativity of the upper left triangle. By Lemma 50.10.3 the map $\xi '$ is the boundary map of the triangle given in Lemma 50.10.2. By functoriality it suffices to prove there exists a morphism of short exact sequences

$\xymatrix{ 0 \ar[r] & \Omega ^\bullet _{X/S}[1] \ar[r] \ar[d] & \Omega ^\bullet _{L^\star /S, 0}[1] \ar[r] \ar[d] & \Omega ^\bullet _{X/S} \ar[r] \ar[d] & 0 \\ 0 \ar[r] & \Omega ^\bullet _{X/S}[1] \ar[r] & \Omega ^\bullet _{X/S}(\log Y)[1] \ar[r] & \Omega ^\bullet _{Y/S} \ar[r] & 0 }$

where the left and right vertical arrows are the obvious ones. We can define the middle vertical arrow by the rule

$\omega ' + \text{d}\log (s) \wedge \omega \longmapsto \omega ' + \text{d}\log (f) \wedge \omega$

where $\omega ', \omega$ are local sections of $\Omega ^\bullet _{X/S}$ and where $s$ is a local generator of $\mathcal{L}$ and $f \in \mathcal{O}_ X(-Y)$ is the corresponding section of the ideal sheaf of $Y$ in $X$. Since the constructions of the maps in Lemmas 50.10.2 and 50.15.2 match exactly, this works.

Commutativity of the lower right triangle. Denote $\overline{L}$ the restriction of $L$ to $Y$. By Lemma 50.10.3 the map $\zeta '$ is the boundary map of the triangle given in Lemma 50.10.2 using the line bundle $\overline{L}$ on $Y$. By functoriality it suffices to prove there exists a morphism of short exact sequences

$\xymatrix{ 0 \ar[r] & \Omega ^\bullet _{X/S}[1] \ar[r] \ar[d] & \Omega ^\bullet _{X/S}(\log Y)[1] \ar[r] \ar[d] & \Omega ^\bullet _{Y/S} \ar[r] \ar[d] & 0 \\ 0 \ar[r] & \Omega ^\bullet _{Y/S}[1] \ar[r] & \Omega ^\bullet _{\overline{L}^\star /S, 0}[1] \ar[r] & \Omega ^\bullet _{Y/S} \ar[r] & 0 \\ }$

where the left and right vertical arrows are the obvious ones. We can define the middle vertical arrow by the rule

$\omega ' + \text{d}\log (f) \wedge \omega \longmapsto \omega '|_ Y + \text{d}\log (\overline{s}) \wedge \omega |_ Y$

where $\omega ', \omega$ are local sections of $\Omega ^\bullet _{X/S}$ and where $f$ is a local generator of $\mathcal{O}_ X(-Y)$ viewed as a function on $X$ and where $\overline{s}$ is $f|_ Y$ viewed as a section of $\mathcal{L}|_ Y = \mathcal{O}_ X(-Y)|_ Y$. Since the constructions of the maps in Lemmas 50.10.2 and 50.15.2 match exactly, this works. $\square$

Lemma 50.15.6. Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$. Let $b \in H^ m_{dR}(X/S)$ be a de Rham cohomology class whose restriction to $Y$ is zero. Then $c_1^{dR}(\mathcal{O}_ X(Y)) \cup b = 0$ in $H^{m + 2}_{dR}(X/S)$.

Proof. This follows immediately from Lemma 50.15.5. Namely, we have

$c_1^{dR}(\mathcal{O}_ X(Y)) \cup b = -c_1^{dR}(\mathcal{O}_ X(-Y)) \cup b = -\xi '(b) = -\delta (b|_ Y) = 0$

as desired. For the second equality, see Remark 50.4.3. $\square$

Lemma 50.15.7. Let $X \to T \to S$ be morphisms of schemes. Let $Y \subset X$ be an effective Cartier divisor. If both $X \to T$ and $Y \to T$ are smooth, then the de Rham complex of log poles is defined for $Y \subset X$ over $S$.

Proof. Let $y \in Y$ be a point. By More on Morphisms, Lemma 37.17.1 there exists an integer $0 \geq m$ and a commutative diagram

$\xymatrix{ Y \ar[d] & V \ar[l] \ar[d] \ar[r] & \mathbf{A}^ m_ T \ar[d]^{(a_1, \ldots , a_ m) \mapsto (a_1, \ldots , a_ m, 0)} \\ X & U \ar[l] \ar[r]^-\pi & \mathbf{A}^{m + 1}_ T }$

where $U \subset X$ is open, $V = Y \cap U$, $\pi$ is étale, $V = \pi ^{-1}(\mathbf{A}^ m_ T)$, and $y \in V$. Denote $z \in \mathbf{A}^ m_ T$ the image of $y$. Then we have

$\Omega ^ p_{X/S, y} = \Omega ^ p_{\mathbf{A}^{m + 1}_ T/S, z} \otimes _{\mathcal{O}_{\mathbf{A}^{m + 1}_ T, z}} \mathcal{O}_{X, x}$

by Lemma 50.2.2. Denote $x_1, \ldots , x_{m + 1}$ the coordinate functions on $\mathbf{A}^{m + 1}_ T$. Since the conditions (1) and (2) in Definition 50.15.1 do not depend on the choice of the local coordinate, it suffices to check the conditions (1) and (2) when $f$ is the image of $x_{m + 1}$ by the flat local ring homomorphism $\mathcal{O}_{\mathbf{A}^{m + 1}_ T, z} \to \mathcal{O}_{X, x}$. In this way we see that it suffices to check conditions (1) and (2) for $\mathbf{A}^ m_ T \subset \mathbf{A}^{m + 1}_ T$ and the point $z$. To prove this case we may assume $S = \mathop{\mathrm{Spec}}(A)$ and $T = \mathop{\mathrm{Spec}}(B)$ are affine. Let $A \to B$ be the ring map corresponding to the morphism $T \to S$ and set $P = B[x_1, \ldots , x_{m + 1}]$ so that $\mathbf{A}^{m + 1}_ T = \mathop{\mathrm{Spec}}(B)$. We have

$\Omega _{P/A} = \Omega _{B/A} \otimes _ B P \oplus \bigoplus \nolimits _{j = 1, \ldots , m} P \text{d}x_ j \oplus P \text{d}x_{m + 1}$

Hence the map $P \to \Omega _{P/A}$, $g \mapsto g \text{d}x_{m + 1}$ is a split injection and $x_{m + 1}$ is a nonzerodivisor on $\Omega ^ p_{P/A}$ for all $p \geq 0$. Localizing at the prime ideal corresponding to $z$ finishes the proof. $\square$

Remark 50.15.8. Let $S$ be a locally Noetherian scheme. Let $X$ be locally of finite type over $S$. Let $Y \subset X$ be an effective Cartier divisor. If the map

$\mathcal{O}_{X, y}^\wedge \longrightarrow \mathcal{O}_{Y, y}^\wedge$

has a section for all $y \in Y$, then the de Rham complex of log poles is defined for $Y \subset X$ over $S$. If we ever need this result we will formulate a precise statement and add a proof here.

Remark 50.15.9. Let $S$ be a locally Noetherian scheme. Let $X$ be locally of finite type over $S$. Let $Y \subset X$ be an effective Cartier divisor. If for every $y \in Y$ we can find a diagram of schemes over $S$

$X \xleftarrow {\varphi } U \xrightarrow {\psi } V$

with $\varphi$ étale and $\psi |_{\varphi ^{-1}(Y)} : \varphi ^{-1}(Y) \to V$ étale, then the de Rham complex of log poles is defined for $Y \subset X$ over $S$. A special case is when the pair $(X, Y)$ étale locally looks like $(V \times \mathbf{A}^1, V \times \{ 0\} )$. If we ever need this result we will formulate a precise statement and add a proof here.

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