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In mathematics, an elliptic curve is a plane algebraic curve defined by an equation of the form : that is non-singular; that is, its graph has no cusps or self-intersections. (When the characteristic of the coefficient field is equal to 2 or 3, the above equation is not quite general enough to comprise all non-singular cubic curves; see below for a more precise definition.) Formally, an elliptic curve is a smooth, projective algebraic curve of genus one, on which there is a specified point ''O''. An elliptic curve is in fact an abelian variety – that is, it has a multiplication defined algebraically, with respect to which it is a (necessarily commutative) group – and ''O'' serves as the identity element. Often the curve itself, without ''O'' specified, is called an elliptic curve. The point ''O'' is actually the "point at infinity" in the projective plane. If ''y''2 = ''P''(''x''), where ''P'' is any polynomial of degree three in ''x'' with no repeated roots, then we obtain a nonsingular plane curve of genus one, which is thus an elliptic curve. If ''P'' has degree four and is square-free this equation again describes a plane curve of genus one; however, it has no natural choice of identity element. More generally, any algebraic curve of genus one, for example from the intersection of two quadric surfaces embedded in three-dimensional projective space, is called an elliptic curve, provided that it has at least one rational point to act as the identity. Using the theory of elliptic functions, it can be shown that elliptic curves defined over the complex numbers correspond to embeddings of the torus into the complex projective plane. The torus is also an abelian group, and in fact this correspondence is also a group isomorphism. Elliptic curves are especially important in number theory, and constitute a major area of current research; for example, they were used in the proof, by Andrew Wiles (assisted by Richard Taylor), of Fermat's Last Theorem. They also find applications in elliptic curve cryptography (ECC) and integer factorization. An elliptic curve is ''not'' an ellipse: see elliptic integral for the origin of the term. Topologically, a complex elliptic curve is a torus. ==Elliptic curves over the real numbers== Although the formal definition of an elliptic curve is fairly technical and requires some background in algebraic geometry, it is possible to describe some features of elliptic curves over the real numbers using only high school algebra and geometry. In this context, an elliptic curve is a plane curve defined by an equation of the form : where ''a'' and ''b'' are real numbers. This type of equation is called a Weierstrass equation. The definition of elliptic curve also requires that the curve be non-singular. Geometrically, this means that the graph has no cusps, self-intersections, or isolated points. Algebraically, this involves calculating the discriminant : The curve is non-singular if and only if the discriminant is not equal to zero. (Although the factor −16 is irrelevant to whether or not the curve is non-singular, this definition of the discriminant is useful in a more advanced study of elliptic curves.) The (real) graph of a non-singular curve has ''two'' components if its discriminant is positive, and ''one'' component if it is negative. For example, in the graphs shown in figure to the right, the discriminant in the first case is 64, and in the second case is −368. 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「elliptic curve」の詳細全文を読む スポンサード リンク
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