Morphism of varieties

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In algebraic geometry, a regular map between affine varieties is a mapping which is given by polynomials. To be explicit, suppose X and Y are subvarieties (or algebraic subsets) of An resp. Am. A regular map f from X to Y has the form f = (f_1, \dots, f_m) where the f_i are in k[x_1, \dots, x_n]/I, I the ideal defining X, so that the image f(X) lies in Y; i.e., satisfying the defining equations of Y. [1]

More generally, a map ƒ:XY between two varieties is regular at a point x if there is a neighbourhood U of x and a neighbourhood V of ƒ(x) such that the restricted function ƒ:UV is regular as a function on the coordinate patches of U and V. Then ƒ is called regular, if it is regular at all points of X.

A morphism between varieties is necessarily continuous.

In the particular case that Y equals A1 the map ƒ:XA1 is called a regular function, and correspond to scalar functions in differential geometry. In other words, a scalar function is regular at a point x if, in a neighborhood of x, it is a rational function (i.e., a fraction of polynomials) such that the denominator does not vanish at x. The ring of regular functions (that is the coordinate ring or more abstractly the ring of global sections of the structure sheaf) is a fundamental object in affine algebraic geometry. The only regular function on a connected projective variety is constant (this can be viewed as an algebraic analogue of Liouville's theorem in complex analysis); thus, in the projective case, one usually considers the global sections of a line bundle (or divisor) instead.

In fact taking the function field k(V) of an irreducible algebraic curve V, the functions F in the function field may all be realised as morphisms from V to the projective line over k. The image will either be a single point, or the whole projective line (this is a consequence of the completeness of projective varieties). That is, unless F is actually constant, we have to attribute to F the value ∞ at some points of V. Now in some sense F is no worse behaved at those points than anywhere else: ∞ is just the chosen point at infinity on the projective line, and by using a Möbius transformation we can move it anywhere we wish. But it is in some way inadequate to the needs of geometry to use only the affine line as target for functions, since we shall end up only with constants.

Regular maps are, by definition, morphisms in the category of algebraic varieties. In particular, a regular map between affine varieties corresponds contravariantly in one-to-one to a ring homomorphism between the coordinate rings.

A regular map whose inverse is also regular is called biregular, and are isomorphisms in the category of algebraic varieties. A morphism between algebraic varieties that is a homeomorphism between the underlying topological spaces need not be an isomorphism (a counterexample is given by a Frobenius morphism t \mapsto t^p.) On the other hand, if f is bijective birational and the target space of f is a normal variety, then f is biregular. (cf. Zariski's main theorem.)

Because regular and biregular are very restrictive conditions – there are no non-constant regular functions on projective space – the weaker condition of a rational map and birational maps are frequently used as well.

If f is a morphism between varieties, then the image of f contains an open dense subset of its closure. (cf. constructible set.)

A regular map between complex algebraic varieties is a holomorphic map. (There is actually a slight technical difference: a regular map is a holomorphic map with removable singularities, but the distinction is usually ignored in practice.) In particular, a regular map into the complex numbers is just a usual holomorphic function (complex-analytic function).

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  1. ^ This is perhaps the simplest definition and agrees with the more traditional definitions; cf. Milne, Proposition 3.16