# Residual class field

**Residual class** fields play an important role in various areas of algebra and number theory . In their simplest form they are the mathematical abstraction of the remainder when dividing by a prime number , in algebraic geometry they appear when the local structure of a geometric object is described in a point.

## definition

Be a ring with a maximum ideal . Then is the quotient ring , which is a factor ring of a maximal ideal one body, the residual body of respect .

## Examples

### Residual class field modulo a prime number

Be the ring of whole numbers . Since there is a main ideal ring , maximum ideals of just the ideals generated by prime elements . So if a prime number , the remainder class ring is a field , more precisely a finite field with elements. It is called the remainder class field modulo and is usually denoted by. Note, however, that there are also finite fields , which have nothing to do with the respective residue class rings.

For more details on finite fields *see * finite field .

### Residual class field of local rings

Be a local ring , i.e. a ring in which there is only one maximum ideal . Then there is only one remainder class field, namely , and we speak of *the* remainder class field of .

#### Residual class field of discrete evaluation rings

Be the evaluation ring of a discretely evaluated body . Then there is a local main ideal ring such that the maximum ideal of is generated by an element . Such an element is called a uniformizing element and in this case it is also called a remainder class field of .

#### Residual class field of points on schemas

Be a scheme with a point . Then the remainder class field of the local ring is called the remainder class field of in and is usually denoted by.

If a schema is over a field , then all remainder class fields are of field extensions of . Is locally finite type and is a closed point, then is a finite extension of . This is essentially the statement of Hilbert's zero theorem .