Isogeny
In algebraic geometry, an isogeny between abelian varieties is a rational map which is also a group homomorphism, with finite kernel.
Elliptic curves
As 1-dimensional abelian varieties, elliptic curves provide a convenient introduction to the theory. If is a non-trivial rational map which maps the zero of E1 to the zero of E1, then it is necessarily a group homomorphism. The kernel of φ is a proper subvariety of E1 and hence a finite set of order d, the degree of φ. There is a dual isogeny defined by
the sum being taken on E1 of the d points on the fibre over Q. This is indeed an isogeny, and the composite is just multiplication by d.
The curves E1 and E2 are said to be isogenous: this is an equivalence relation on isomorphism classes of elliptic curves.
Example
Let E1 be an elliptic curve over a field K of characteristic not 2 or 3 in Weierstrass form.
Degree 2
A subgroup of order 2 on E1 must be of the form where P = (e,0) with e being a root of the cubic in X. Translating so that e=0 and the curve has equation , the map
is an isogeny from E1 to the isogenous curve E2 with equation .
Degree 3
A subgroup of order 3 must be of the form where x is in K but y need not be. We shall assume that (by taking a quadratic twist if necessary). Translating, we can put E in the form . The map
is an isogeny from E1 to the isogenous curve E2 with equation .
Elliptic curves over finite fields
Isogenous elliptic curves over a finite field have the same number of points (although not necessarily the same group structure). The converse is also true: this is the Honda-Tate theorem.