|
In mathematics, an isogeny is a morphism of algebraic groups that is surjective and has a finite kernel. If the groups are abelian varieties, then any morphism ''f'' : ''A'' → ''B'' of the underlying algebraic varieties which is surjective with finite fibres is automatically an isogeny, provided that ''f''(1''A'') = 1''B''. Such an isogeny ''f'' then provides a group homomorphism between the groups of ''k''-valued points of ''A'' and ''B'', for any field ''k'' over which ''f'' is defined. The terms "isogeny" and "isogenous" come from the Greek word ισογενη-ς, meaning "equal in kind or nature". ==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 ''f'' : ''E''1 → ''E''2 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 ''E''1 → ''E''2. This is an equivalence relation, symmetry being 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Isogeny」の詳細全文を読む スポンサード リンク
|