## 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.

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