Segre class

In mathematics, the Segre class is a characteristic class used in the study of cones, a generalization of vector bundles. For vector bundles the total Segre class is inverse to the total Chern class, and thus provides equivalent information; the advantage of the Segre class is that it generalizes to more general cones, while the Chern class does not. The Segre class was introduced in the non-singular case by Beniamino Segre (1953).

Definition

Suppose $C$ is a cone over $X$ , $q$ is the projection from the projective completion $\mathbb {P} (C\oplus 1)$ of $C$ to $X$ , and ${\mathcal {O}}(1)$ is the anti-tautological line bundle on $\mathbb {P} (C\oplus 1)$ . The Chern class $c_{1}({\mathcal {O}}(1))$ is a group endomorphism of the Chow ring of $\mathbb {P} (C\oplus 1)$ . The $i$ th Segre class of $C$ is given by

s_{i}(C)=q_{*}(c_{1}({\mathcal {O}}(1))^{i}[\mathbb {P} (C\oplus 1)])

for non-negative integers $i$ . (Here, the power means composition.) The total Segre class is the sum of the Segre classes.

The reason for using $\mathbb {P} (C\oplus 1)$ rather than $\mathbb {P} (C)$ is that this makes the total Segre class stable under addition of the trivial bundle ${\mathcal {O}}$ .

If Z is a closed subscheme of an algebraic scheme X, then $s(Z,X)$ denote the Segre class of the normal cone to $Z \hookrightarrow X$ .

Relation to Chern classes for vector bundles

For a holomorphic vector bundle $E$ over a complex manifold $M$ a total Segre class $s(E)$ is the inverse to the total Chern class $c(E)$ , see e.g.^[1]

Explicitly, for a total Chern class

c(E)=1+c_{1}(E)+c_{2}(E)+\cdots \,

one gets the total Segre class

s(E)=1+s_{1}(E)+s_{2}(E)+\cdots \,

where

c_{1}(E)=-s_{1}(E),\quad c_{2}(E)=s_{1}(E)^{2}-s_{2}(E),\quad \dots ,\quad c_{n}(E)=-s_{1}(E)c_{{n-1}}(E)-s_{2}(E)c_{{n-2}}(E)-\cdots -s_{n}(E)

Let $x_{1},\dots ,x_{k}$ be Chern roots, i.e. formal eigenvalues of ${\frac {i\Omega }{2\pi }}$ where $\Omega$ is a curvature of a connection on $E$ .

While the Chern class c(E) is written as

c(E)=\prod _{{i=1}}^{{k}}(1+x_{i})=c_{0}+c_{1}+\cdots +c_{k}\,

where $c_{i}$ is an elementary symmetric polynomial of degree $i$ in variables $x_{1},\dots ,x_{k}$

the Segre for the dual bundle $E^{\vee }$ which has Chern roots $-x_{1},\dots ,-x_{k}$ is written as

s(E)=\prod _{{i=1}}^{{k}}{\frac {1}{1-x_{i}}}=s_{0}+s_{1}+\cdots

Expanding the above expression in powers of $x_{1},\dots x_{k}$ one can see that $s_{i}(E^{\vee })$ is represented by a complete homogeneous symmetric polynomial of $x_{1},\dots x_{k}$

Properties

A key property of a Segre class is birational invariance: this is contained in the following. Let $p:X\to Y$ be a proper morphism between algebraic schemes such that $Y$ is irreducible and each irreducible component of $X$ maps onto $Y$ . Then, for each closed subscheme $W\subset Y$ , $V=p^{-1}(W)$ and $p_{V}:V\to W$ the restriction of $p$ ,

{p_{V}}_{*}(s(V,X))=\operatorname {deg} (p)\,s(W,Y).

^[2]

Similarly, if $f:X\to Y$ is a flat morphism of constant relative dimension between pure-dimensional algebraic schemes, then, for each closed subscheme $W\subset Y$ , $V=f^{-1}(W)$ and $f_{V}:V\to W$ the restriction of $f$ ,

{f_{V}}^{*}(s(W,Y))=s(V,X).

^[3]

A basic example of binational invariance is provided by a blow-up. Let $\pi :{\widetilde {X}}\to X$ be a blow-up along some closed subscheme Z. Since the exceptional divisor $E:=\pi ^{-1}(Z)\hookrightarrow {\widetilde {X}}$ is an effective Cartier divisor and the normal cone (or normal bundle) to it is ${\mathcal {O}}_{E}(E):={\mathcal {O}}_{X}(E)|_{E}$ ,

{\begin{aligned}s(E,{\widetilde {X}})&=c({\mathcal {O}}_{E}(E))^{-1}[E]\\&=[E]-E\cdot [E]+E\cdot (E\cdot [E])+\cdots ,\end{aligned}}

where we used the notation $D\cdot \alpha =c_{1}({\mathcal {O}}(D))\alpha$ .^[4] Thus,

s(Z,X)=g_{*}\left(\sum _{k=1}^{\infty }(-1)^{k-1}E^{k}\right)

where $g:E=\pi ^{-1}(Z)\to Z$ is given by $\pi$ .

Examples

Example 1

Let Z be a smooth curve that is a complete intersection of effective Cartier divisors $D_{1},\dots ,D_{n}$ on a variety X. Assume the dimension of X is n + 1. Then the Segre class of the normal cone $C_{Z/X}$ to $Z \hookrightarrow X$ is:^[5]

s(C_{Z/X})=[Z]-\sum _{i=1}^{n}D_{i}\cdot [Z].

Indeed, for example, if Z is regularly embedded into X, then, since $C_{Z/X}=N_{Z/X}$ is the normal bundle and $N_{Z/X}=\bigoplus _{i=1}^{n}N_{D_{i}/X}|_{Z}$ (see Normal cone#Properties), we have:

s(C_{Z/X})=c(N_{Z/X})^{-1}[Z]=\prod _{i=1}^{d}(1-c_{1}({\mathcal {O}}_{X}(D_{i})))[Z]=[Z]-\sum _{i=1}^{n}D_{i}\cdot [Z].

Example 2

The following is Example 3.2.22. of (Fulton 1998). It recovers some classical results from Schubert's book on enumerative geometry.

Viewing the dual projective space ${\breve {\mathbb {P} ^{3}}}$ as the Grassmann bundle $p:{\breve {\mathbb {P} ^{3}}}\to *$ parametrizing the 2-planes in ${\mathbb {P}}^{3}$ , consider the tautological exact sequence

0\to S\to p^{*}\mathbb {C} ^{3}\to Q\to 0

where $S,Q$ are the tautological sub and quotient bundles. With $E=\operatorname {Sym} ^{2}(S^{*}\otimes Q^{*})$ , the projective bundle $q:X=\mathbb {P} (E)\to {\breve {\mathbb {P} ^{3}}}$ is the variety of conics in ${\mathbb {P}}^{3}$ . With $\beta =c_{1}(Q^{*})$ , we have $c(S^{*}\otimes Q^{*})=2\beta +2\beta ^{2}$ and so, using Chern class#Computation formulae,

c(E)=1+8\beta +30\beta ^{2}+60\beta ^{3}

and thus

s(E)=1+8h+34h^{2}+92h^{3}

where $h=-\beta =c_{1}(Q).$ The coefficients in $s(E)$ have the enumerative geometric meanings; for example, 92 is the number of conics meeting 8 general lines.

Example 3

Let X be a surface and $A,B,D$ effective Cartier divisors on it. Let $Z\subset X$ be the scheme-theoretic intersection of $A+D$ and $B+D$ (viewing those divisors as closed subschemes). For simplicity, suppose $A,B$ meet only at a single point P with the same multiplicity m and that P is a smooth point of X. Then^[6]

s(Z,X)=[D]+(m^{2}[P]-D\cdot [D]).

To see this, consider the blow-up $\pi :{\widetilde {X}}\to X$ of X along P and let $g:{\widetilde {Z}}=\pi ^{-1}Z\to Z$ , the strict trasform of Z. By the formula at #Properties,

s(Z,X)=g_{*}([{\widetilde {Z}}])-g_{*}({\widetilde {Z}}\cdot [{\widetilde {Z}}]).

Since ${\widetilde {Z}}=\pi ^{*}D+mE$ where $E=\pi ^{-1}P$ , the formula above results.

Multiplicity along a subvariety

Let $(A,{\mathfrak {m}})$ be the local ring of a variety X at a closed subvariety V codimension n (for example, V can be a closed point). Then $\operatorname {length} _{A}(A/{\mathfrak {m}}^{t})$ is a polynomial of degree n in t for large t; i.e., it can be written as ${e(A)^{n} \over n!}t^{n}+$ the lower-degree terms and the integer $e(A)$ is called the multiplicity of A.

The Segre class $s(V,X)$ of $V\subset X$ encodes this multiplicity: the coefficient of $[V]$ in $s(V,X)$ is $e(A)$ .^[7]

References

↑ Fulton W. (1998). Intersection theory, p.50. Springer, 1998.
↑ Fulton, Proposition 4.2. (a)
↑ Fulton, Proposition 4.2. (b)
↑ Fulton, § 2.5.
↑ Fulton, Example 9.1.1.
↑ Fulton, Example 4.2.2.
↑ Fulton, Example 4.3.1.

Segre, Beniamino (1953), "Nuovi metodi e resultati nella geometria sulle varietà algebriche", Ann. Mat. Pura Appl. (in Italian), 35 (4): 1–127, MR 0061420

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] Fulton W. (1998). Intersection theory, p.50. Springer, 1998.

[2] Fulton, Proposition 4.2. (a)

[3] Fulton, Proposition 4.2. (b)

[4] Fulton, § 2.5.

[5] Fulton, Example 9.1.1.

[6] Fulton, Example 4.2.2.

[7] Fulton, Example 4.3.1.