The Stacks project

9.1 Introduction

In this chapter, we shall discuss the theory of fields. Recall that a field is a ring in which all nonzero elements are invertible. Equivalently, the only two ideals of a field are $(0)$ and $(1)$ since any nonzero element is a unit. Consequently fields will be the simplest cases of much of the theory developed later.

The theory of field extensions has a different feel from standard commutative algebra since, for instance, any morphism of fields is injective. Nonetheless, it turns out that questions involving rings can often be reduced to questions about fields. For instance, any domain can be embedded in a field (its quotient field), and any local ring (that is, a ring with a unique maximal ideal; we have not defined this term yet) has associated to it its residue field (that is, its quotient by the maximal ideal). A knowledge of field extensions will thus be useful.

Comments (2)

Comment #9535 by Joe Lamond on

The definition of a field appears to have a minor error in it. A field is a (commutative) ring in which and all nonzero elements are invertible. The definition given here allows for . (One can avoid this by defining a field to be a ring whose nonzero elements form a group under multiplication, or by defining a field as a ring with exactly two ideals.)

Comment #9536 by Joe Lamond on

Ah, I see that in the definition of "field" given in the next section does explicitly rule out . Apologies for not spotting this.

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