Isogeny

In mathematics, particularly in algebraic geometry, an isogeny is a morphism of algebraic groups (also known as group varieties) that is surjective and has a finite kernel.

If the groups are abelian varieties, then any morphism of the underlying algebraic varieties which is surjective with finite fibres is automatically an isogeny, provided that . Such an isogeny then provides a group homomorphism between the groups of -valued points of and , for any field over which is defined.

The terms "isogeny" and "isogenous" come from the Greek word ισογενη-ς, meaning "equal in kind or nature". The term "isogeny" was introduced by Weil; before this, the term "isomorphism" was somewhat confusingly used for what is now called an isogeny.

Degree of isogeny

Let be isogeny between two algebraic groups.
This mapping induces a pullback mapping between their rational function fields. Since the mapping is nontrivial, it is a field embedding and $\operatorname f^*$ is a subfield of . The degree of the extension $K(A) / \operatorname f^*$ is called degree of isogeny:
: \deg f := (A): \operatorname f^*/math>

Properties of degree:
* If $f:X \rightarrow Y$, $g:Y \rightarrow Z$ are isogenies of algebraic groups, then: $\deg (g\circ f)=\deg g \cdot \deg f$
* If $char \; K \nmid \deg f$, then $\deg f = , \ker\; f,$

Case of abelian varieties

For abelian varieties, such as elliptic curves, this notion can also be formulated as follows:

Let ''E''₁ and ''E''₂ be abelian varieties of the same dimension over a field ''k''. An isogeny between ''E''₁ and ''E''₂ is a dense morphism of varieties that preserves basepoints (i.e. ''f'' maps the identity point on ''E''₁ to that on ''E''₂).

This is equivalent to the above notion, as every dense morphism between two abelian varieties of the same dimension is automatically surjective with finite fibres, and if it preserves identities then it is a homomorphism of groups.

Two abelian varieties ''E''₁ and ''E''₂ are called isogenous if there is an isogeny . This can be shown to be an equivalence relation; in the case of elliptic curves, symmetry is due to the existence of the dual isogeny. As above, every isogeny induces homomorphisms of the groups of the k-valued points of the abelian varieties.

See also

* Abelian varieties up to isogeny
* Selmer group

References

External links

*
* {{cite book , last=Mumford , first=David , author-link=David Mumford , year=1974 , title=Abelian Varieties , publisher=Oxford University Press , isbn=0-19-560528-4

Morphisms of schemes