Abel–Jacobi map

In mathematics, the Abel–Jacobi map is a construction of algebraic geometry which relates an algebraic curve to its Jacobian variety. In Riemannian geometry, it is a more general construction mapping a manifold to its Jacobi torus. The name derives from the theorem of Abel and Jacobi that two effective divisors are linearly equivalent if and only if they are indistinguishable under the Abel–Jacobi map.

Construction of the map

In complex algebraic geometry, the Jacobian of a curve C is constructed using path integration. Namely, suppose C has genus g, which means topologically that

H_{1}(C,\mathbb {Z} )\cong \mathbb {Z} ^{2g}.

Geometrically, this homology group consists of (homology classes of) cycles in C, or in other words, closed loops. Therefore, we can choose 2g loops $\gamma _{1},\dots ,\gamma _{2g}$ generating it. On the other hand, another more algebro-geometric way of saying that the genus of C is g is that

H^{0}(C,K)\cong \mathbb {C} ^{g},

where K is the canonical bundle on C.

By definition, this is the space of globally defined holomorphic differential forms on C, so we can choose g linearly independent forms $\omega _{1},\dots ,\omega _{g}$ . Given forms and closed loops we can integrate, and we define 2g vectors

\Omega _{j}=\left(\int _{\gamma _{j}}\omega _{1},\dots ,\int _{\gamma _{j}}\omega _{g}\right)\in \mathbb {C} ^{g}.

It follows from the Riemann bilinear relations that the $\Omega _{j}$ generate a nondegenerate lattice $\Lambda$ (that is, they are a real basis for $\mathbb {C} ^{g}\cong \mathbb {R} ^{2g}$ ), and the Jacobian is defined by

J(C)=\mathbb {C} ^{g}/\Lambda .

The Abel–Jacobi map is then defined as follows. We pick some base point $p_{0}\in C$ and, nearly mimicking the definition of $\Lambda$ , define the map

u\colon C\to J(C),u(p)=\left(\int _{p_{0}}^{p}\omega _{1},\dots ,\int _{p_{0}}^{p}\omega _{g}\right){\bmod {\Lambda }}.

Although this is seemingly dependent on a path from $p_{0}$ to $p,$ any two such paths define a closed loop in $C$ and, therefore, an element of $H_{1}(C,\mathbb {Z} ),$ so integration over it gives an element of $\Lambda .$ Thus the difference is erased in the passage to the quotient by $\Lambda$ . Changing base-point $p_{0}$ does change the map, but only by a translation of the torus.

The Abel–Jacobi map of a Riemannian manifold

Let $M$ be a smooth compact manifold. Let $\pi =\pi _{1}(M)$ be its fundamental group. Let $f:\pi \to \pi ^{ab}$ be its abelianisation map. Let $tor=tor(\pi ^{ab})$ be the torsion subgroup of $\pi ^{ab}$ . Let $g:\pi ^{ab}\to \pi ^{ab}/tor$ be the quotient by torsion. If $M$ is a surface, $\pi ^{ab}/tor$ is non-canonically isomorphic to $\mathbb {Z} ^{2g}$ , where $g$ is the genus; more generally, $\pi ^{ab}/tor$ is non-canonically isomorphic to $\mathbb {Z} ^{b}$ , where $b$ is the first Betti number. Let $\phi =g\circ f:\pi \to \mathbb {Z} ^{b}$ be the composite homomorphism.

Definition. The cover ${\bar {M}}$ of the manifold $M$ corresponding to the subgroup $\mathrm {Ker} (\phi )\subset \pi$ is called the universal (or maximal) free abelian cover.

Now assume M has a Riemannian metric. Let $E$ be the space of harmonic $1$ -forms on $M$ , with dual $E^{*}$ canonically identified with $H_{1}(M,\mathbb {R} )$ . By integrating an integral harmonic $1$ -form along paths from a basepoint $x_{0}\in M$ , we obtain a map to the circle $\mathbb {R} /\mathbb {Z} =S^{1}$ .

Similarly, in order to define a map $M\to H_{1}(M,\mathbb {R} )/H_{1}(M,\mathbb {Z} )_{\mathbb {R} }$ without choosing a basis for cohomology, we argue as follows. Let $x$ be a point in the universal cover ${\tilde {M}}$ of $M$ . Thus $x$ is represented by a point of $M$ together with a path $c$ from $x_{0}$ to it. By integrating along the path $c$ , we obtain a linear form, $h\to \int _{c}h$ , on $E$ . We thus obtain a map ${\tilde {M}}\to E^{*}=H_{1}(M,\mathbb {R} )$ , which, furthermore, descends to a map

{\overline {A}}_{M}:{\overline {M}}\to E^{*},\;\;c\mapsto \left(h\mapsto \int _{c}h\right),

where ${\overline {M}}$ is the universal free abelian cover.

Definition. The Jacobi variety (Jacobi torus) of $M$ is the torus

J_{1}(M)=H_{1}(M,\mathbb {R} )/H_{1}(M,\mathbb {Z} )_{\mathbb {R} }.

Definition. The Abel–Jacobi map

A_{M}:M\to J_{1}(M),

is obtained from the map above by passing to quotients.

The Abel–Jacobi map is unique up to translations of the Jacobi torus. The map has applications in Systolic geometry. Interestingly, the Abel–Jacobi map of a Riemannian manifold shows up in the large time asymptotics of the heat kernel on a periodic manifold (Kotani & Sunada (2000) and Sunada (2012)).

In much the same way, one can define a graph-theoretic analogue of Abel–Jacobi map as a piecewise-linear map from a finite graph into a flat torus (or a Cayley graph associated with a finite abelian group), which is closely related to asymptotic behaviors of random walks on crystal lattices, and can be used for design of crystal structures.

Abel–Jacobi theorem

The following theorem was proved by Abel: Suppose that

D=\sum _{i}n_{i}p_{i}\

is a divisor (meaning a formal integer-linear combination of points of C). We can define

u(D)=\sum _{i}n_{i}u(p_{i})\

and therefore speak of the value of the Abel–Jacobi map on divisors. The theorem is then that if D and E are two effective divisors, meaning that the $n_{i}$ are all positive integers, then

u(D)=u(E)\

if and only if

D

is linearly equivalent to

E.

This implies that the Abel–Jacobi map induces an injective map (of abelian groups) from the space of divisor classes of degree zero to the Jacobian.

Jacobi proved that this map is also surjective, so the two groups are naturally isomorphic.

The Abel–Jacobi theorem implies that the Albanese variety of a compact complex curve (dual of holomorphic 1-forms modulo periods) is isomorphic to its Jacobian variety (divisors of degree 0 modulo equivalence). For higher-dimensional compact projective varieties the Albanese variety and the Picard variety are dual but need not be isomorphic.

References

E. Arbarello; M. Cornalba; P. Griffiths; J. Harris (1985). "1.3, Abel's Theorem". Geometry of Algebraic Curves, Vol. 1. Grundlehren der Mathematischen Wissenschaften. Springer-Verlag. ISBN 978-0-387-90997-4.

Kotani, Motoko; Sunada, Toshikazu (2000), "Albanese maps and an off diagonal long time asymptotic for the heat kernel", Comm. Math. Phys., 209: 633–670, Bibcode:2000CMaPh.209..633K, doi:10.1007/s002200050033
Sunada, Toshikazu (2012), "Lecture on topological crystallography", Japan. J. Math., 7: 1–39, doi:10.1007/s11537-012-1144-4

Topics in algebraic curves

Rational curves

Elliptic curves

Analytic theory	Elliptic function Elliptic integral Fundamental pair of periods Modular form
Arithmetic theory	Counting points on elliptic curves Division polynomials Hasse's theorem on elliptic curves Mazur's torsion theorem Modular elliptic curve Modularity theorem Mordell–Weil theorem Nagell–Lutz theorem Supersingular elliptic curve Schoof's algorithm Schoof–Elkies–Atkin algorithm
Applications	Elliptic curve cryptography Elliptic curve primality

Higher genus

Plane curves

Riemann surfaces

Constructions

Structure of curves

Divisors on curves	Abel–Jacobi map Brill–Noether theory Clifford's theorem on special divisors Gonality of an algebraic curve Jacobian variety Riemann–Roch theorem Weierstrass point Weil reciprocity law
Moduli	ELSV formula Gromov–Witten invariant Hodge bundle Moduli of algebraic curves Stable curve
Morphisms	Hasse–Witt matrix Riemann–Hurwitz formula Prym variety Weber's theorem
Singularities	Acnode Crunode Cusp Delta invariant Tacnode
Vector bundles	Birkhoff–Grothendieck theorem Stable vector bundle Vector bundles on algebraic curves

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