function field
Let be a field.
Definition 1.
The rational function field over in one variable (), denoted by , is the field of all rational functions with polynomials and not identically zero.
Definition 2.
A function field (in one variable) over is a field , containing and at least one element , transcendental over , such that is a http://planetmath.org/node/FiniteExtensionfinite algebraic extension.
Let be a fixed algebraic closure of .
Definition 3.
Let be a function field over and let be a finite extension of . The extension of function fields is said to be geometric if .
Example 1.
The extension is geometric, but is not geometric.
Theorem 1 (Thm. I.6.9 of [1]).
Let be a function field over an algebraically closed field . There exists a nonsingular projective curve such that the function field of is isomorphic to .
Definition 4.
Let be a function field over a field . Let which is a function field over , a fixed algebraic closure of , and let be the curve given by the previous theorem. The genus of is, by definition, the genus of .
Definition 5.
Let be a function field over a field . A prime in is by definition a discrete valuation ring with maximal such that and the quotient field of is equal to . The prime is usually denoted after the maximal ideal of . The degree of , denoted by , is defined to be the dimension of over .
Example 2.
Let be the rational function field over and let . The prime ideals of are generated by monic irreducible polynomials in . Let be such a prime. Then , the localization of at the prime is a discrete valuation ring with and the quotient field of is . Thus is a prime of .
One can define an ‘extra’ prime in the following way. Let and let be the prime ideal of generated by . The localization ring is a prime of , called the prime at infinity.
References
- 1 R. Hartshorne, Algebraic Geometry, Springer-Verlag, New York.
- 2 M. Rosen, Number Theory in Function Fields, Springer-Verlag, New York.
Title | function field |
Canonical name | FunctionField |
Date of creation | 2013-03-22 15:34:35 |
Last modified on | 2013-03-22 15:34:35 |
Owner | alozano (2414) |
Last modified by | alozano (2414) |
Numerical id | 8 |
Author | alozano (2414) |
Entry type | Definition |
Classification | msc 11R58 |
Synonym | algebraic function field |
Related topic | SimpleTranscendentalFieldExtension |
Defines | rational function field |
Defines | geometric extension |
Defines | genus of a function field |
Defines | degree of a prime |