# Adjunction formula

In mathematics, especially in algebraic geometry and the theory of complex manifolds, the adjunction formula relates the canonical bundle of a variety and a hypersurface inside that variety. It is often used to deduce facts about varieties embedded in well-behaved spaces such as projective space or to prove theorems by induction.

## Adjunction for smooth varieties
### Formula for a smooth subvariety

Let X be a smooth algebraic variety or smooth complex manifold and Y be a smooth subvariety of X. Denote the inclusion map Y → X by i and the ideal sheaf of Y in X by ${\mathcal {I}}$. The conormal exact sequence for i is
    $0\to {\mathcal {I}}/{\mathcal {I}}^{2}\to i^{*}\Omega _{X}\to \Omega _{Y}\to 0,$
where Ω denotes a cotangent bundle. The determinant of this exact sequence is a natural isomorphism
    $\omega _{Y}=i^{*}\omega _{X}\otimes \operatorname {det} ({\mathcal {I}}/{\mathcal {I}}^{2})^{\vee },$
where $\vee$ denotes the dual of a line bundle.

### The particular case of a smooth divisor

Suppose that D is a smooth divisor on X. Its normal bundle extends to a line bundle ${\mathcal {O}}(D)$ on X, and the ideal sheaf of D corresponds to its dual ${\mathcal {O}}(-D)$. The conormal bundle ${\mathcal {I}}/{\mathcal {I}}^{2}$ is $i^{*}{\mathcal {O}}(-D)$, which, combined with the formula above, gives
    $\omega _{D}=i^{*}(\omega _{X}\otimes {\mathcal {O}}(D)).$
In terms of canonical classes, this says that
    $K_{D}=(K_{X}+D)|_{D}.$
Both of these two formulas are called the adjunction formula.

## Examples
### Degree d hypersurfaces

Given a smooth degree $d$ hypersurface $i:X\hookrightarrow \mathbb {P} _{S}^{n}$ we can compute its canonical and anti-canonical bundles using the adjunction formula. This reads as
   $\omega _{X}\cong i^{*}\omega _{\mathbb {P} ^{n}}\otimes {\mathcal {O}}_{X}(d)$
which is isomorphic to ${\mathcal {O}}_{X}(-n{-}1{+}d)$.

### Complete intersections

For a smooth complete intersection $i:X\hookrightarrow \mathbb {P} _{S}^{n}$ of degrees $(d_{1},d_{2})$, the conormal bundle ${\mathcal {I}}/{\mathcal {I}}^{2}$ is isomorphic to ${\mathcal {O}}(-d_{1})\oplus {\mathcal {O}}(-d_{2})$, so the determinant bundle is ${\mathcal {O}}(-d_{1}{-}d_{2})$ and its dual is ${\mathcal {O}}(d_{1}{+}d_{2})$, showing
   $\omega _{X}\,\cong \,{\mathcal {O}}_{X}(-n{-}1)\otimes {\mathcal {O}}_{X}(d_{1}{+}d_{2})\,\cong \,{\mathcal {O}}_{X}(-n{-}1{+}d_{1}{+}d_{2}).$
This generalizes in the same fashion for all complete intersections.

### Curves in a quadric surface

$\mathbb {P} ^{1}\times \mathbb {P} ^{1}$ embeds into $\mathbb {P} ^{3}$ as a quadric surface given by the vanishing locus of a quadratic polynomial coming from a non-singular symmetric matrix. We can then restrict our attention to curves on $Y=\mathbb {P} ^{1}\times \mathbb {P} ^{1}$. We can compute the cotangent bundle of $Y$ using the direct sum of the cotangent bundles on each $\mathbb {P} ^{1}$, so it is ${\mathcal {O}}(-2,0)\oplus {\mathcal {O}}(0,-2)$. Then, the canonical sheaf is given by ${\mathcal {O}}(-2,-2)$, which can be found using the decomposition of wedges of direct sums of vector bundles. Then, using the adjunction formula, a curve defined by the vanishing locus of a section $f\in \Gamma ({\mathcal {O}}(a,b))$, can be computed as
    $\omega _{C}\,\cong \,{\mathcal {O}}(-2,-2)\otimes {\mathcal {O}}_{C}(a,b)\,\cong \,{\mathcal {O}}_{C}(a{-}2,b{-}2).$

## Poincaré residue

The restriction map $\omega _{X}\otimes {\mathcal {O}}(D)\to \omega _{D}$ is called the Poincaré residue. Suppose that X is a complex manifold. Then on sections, the Poincaré residue can be expressed as follows. Fix an open set U on which D is given by the vanishing of a function f. Any section over U of ${\mathcal {O}}(D)$ can be written as s/f, where s is a holomorphic function on U. Let η be a section over U of ω$_X$. The Poincaré residue is the map
    $\eta \otimes {\frac {s}{f}}\mapsto s{\frac {\partial \eta }{\partial f}}{\bigg |}_{f=0},$
that is, it is formed by applying the vector field ∂/∂f to the volume form η, then multiplying by the holomorphic function s. If U admits local coordinates z$_1$, ..., z$_n$ such that for some i, ∂f/∂z$_i$ ≠ 0, then this can also be expressed as
    ${\frac {g(z)\,dz_{1}\wedge \dotsb \wedge dz_{n}}{f(z)}}\mapsto (-1)^{i-1}{\frac {g(z)\,dz_{1}\wedge \dotsb \wedge {\widehat {dz_{i}}}\wedge \dotsb \wedge dz_{n}}{\partial f/\partial z_{i}}}{\bigg |}_{f=0}.$
Another way of viewing Poincaré residue first reinterprets the adjunction formula as an isomorphism
    $\omega _{D}\otimes i^{*}{\mathcal {O}}(-D)=i^{*}\omega _{X}.$
On an open set U as before, a section of $i^{*}{\mathcal {O}}(-D)$ is the product of a holomorphic function s with the form df/f. The Poincaré residue is the map that takes the wedge product of a section of ω$_D$ and a section of $i^{*}{\mathcal {O}}(-D)$.

## Inversion of adjunction

The adjunction formula is false when the conormal exact sequence is not a short exact sequence. However, it is possible to use this failure to relate the singularities of X with the singularities of D. Theorems of this type are called inversion of adjunction. They are an important tool in modern birational geometry.

## The Canonical Divisor of a Plane Curve

Let $C\subset \mathbf {P} ^{2}$ be a smooth plane curve cut out by a degree $d$ homogeneous polynomial $F(X,Y,Z)$. We claim that the canonical divisor is $K=(d-3)[C\cap H]$ where $H$ is the hyperplane divisor.
First work in the affine chart $Z\neq 0$. The equation becomes $f(x,y)=F(x,y,1)=0$ where $x=X/Z$ and $y=Y/Z$. We will explicitly compute the divisor of the differential
    $\omega :={\frac {dx}{\partial f/\partial y}}={\frac {-dy}{\partial f/\partial x}}.$
At any point $(x_{0},y_{0})$ either $\partial f/\partial y\neq 0$ so $x-x_{0}$ is a local parameter or $\partial f/\partial x\neq 0$ so $y-y_{0}$ is a local parameter. In both cases the order of vanishing of $\omega$ at the point is zero. Thus all contributions to the divisor ${\text{div}}(\omega )$ are at the line at infinity, $Z=0$.
Now look on the line ${Z=0}$. Assume that $[1,0,0]\not \in C$ so it suffices to look in the chart $Y\neq 0$ with coordinates $u=1/y$ and $v=x/y$. The equation of the curve becomes
    $g(u,v)=F(v,1,u)=F(x/y,1,1/y)=y^{-d}F(x,y,1)=y^{-d}f(x,y).$
Hence
    $\partial f/\partial x=y^{d}{\frac {\partial g}{\partial v}}{\frac {\partial v}{\partial x}}=y^{d-1}{\frac {\partial g}{\partial v}}$
so
    $\omega ={\frac {-dy}{\partial f/\partial x}}={\frac {1}{u^{2}}}{\frac {du}{y^{d-1}\partial g/\partial v}}=u^{d-3}{\frac {du}{\partial g/\partial v}}$
with order of vanishing $\nu _{p}(\omega )=(d-3)\nu _{p}(u)$. Hence ${\text{div}}(\omega )=(d-3)[C\cap \{Z=0\}]$ which agrees with the adjunction formula.

## Applications to curves

The genus-degree formula for plane curves can be deduced from the adjunction formula. Let C ⊂ P$^2$ be a smooth plane curve of degree d and genus g. Let H be the class of a hyperplane in P$^2$, that is, the class of a line. The canonical class of P$^2$ is −3H. Consequently, the adjunction formula says that the restriction of (d − 3)H to C equals the canonical class of C. This restriction is the same as the intersection product (d − 3)H ⋅ dH restricted to C, and so the degree of the canonical class of C is d(d−3). By the Riemann–Roch theorem, g − 1 = (d−3)d − g + 1, which implies the formula
    $g={\tfrac {1}{2}}(d{-}1)(d{-}2).$
Similarly, if C is a smooth curve on the quadric surface P$^1$×P$^1$ with bidegree (d$_1$,d$_2$) (meaning d$_1$,d$_2$ are its intersection degrees with a fiber of each projection to P$^1$), since the canonical class of P$^1$×P$^1$ has bidegree (−2,−2), the adjunction formula shows that the canonical class of C is the intersection product of divisors of bidegrees (d$_1$,d$_2$) and (d$_1$−2,d$_2$−2). The intersection form on P$^1$×P$^1$ is $((d_{1},d_{2}),(e_{1},e_{2}))\mapsto d_{1}e_{2}+d_{2}e_{1}$ by definition of the bidegree and by bilinearity, so applying Riemann–Roch gives $2g-2=d_{1}(d_{2}{-}2)+d_{2}(d_{1}{-}2)$ or
    $g=(d_{1}{-}1)(d_{2}{-}1)\,=\,d_{1}d_{2}-d_{1}-d_{2}+1.$
The genus of a curve C which is the complete intersection of two surfaces D and E in P$^3$ can also be computed using the adjunction formula. Suppose that d and e are the degrees of D and E, respectively. Applying the adjunction formula to D shows that its canonical divisor is (d − 4)H|$_D$, which is the intersection product of (d − 4)H and D. Doing this again with E, which is possible because C is a complete intersection, shows that the canonical divisor C is the product (d + e − 4)H ⋅ dH ⋅ eH, that is, it has degree de(d + e − 4). By the Riemann–Roch theorem, this implies that the genus of C is
    $g=de(d+e-4)/2+1.$
More generally, if C is the complete intersection of n − 1 hypersurfaces D$_1$, ..., D$_{n−1}$ of degrees d$_1$, ..., d$_{n−1}$ in P$^n$, then an inductive computation shows that the canonical class of C is $(d_{1}+\cdots +d_{n-1}-n-1)d_{1}\cdots d_{n-1}H^{n-1}$. The Riemann–Roch theorem implies that the genus of this curve is
    $g=1+{\tfrac {1}{2}}(d_{1}+\cdots +d_{n-1}-n-1)d_{1}\cdots d_{n-1}.$

## In low dimensional topology

Let S be a complex surface (in particular a 4-dimensional manifold) and let $C\to S$ be a smooth (non-singular) connected complex curve. Then
$2g(C)-2=[C]^{2}-c_{1}(S)[C]$
where $g(C)$ is the genus of C, $[C]^{2}$ denotes the self-intersections and $c_{1}(S)[C]$ denotes the Kronecker pairing $<c_{1}(S),[C]>$.