A field is a set paired with two operations on the set, which are designated as addition () and multiplication (). As a group can be conceptualized as an ordered pair of a set and an operation, , a field can be conceptualized as an ordered triple .
A set with addition and multiplication, , is a field if and only if it satisfies the following properties:
Commutativity of both addition and multiplication — For all, and ;
Associativity of both addition and multiplication — For all , and ;
Additive Identity — There exists a "zero" element, , called an additive identity, such that , for all ;
Additive Inverses — For each , there exists a , called an additive inverse of , such that ;
Multiplicative Identity — There exists a "one" element, , different from 0, called a multiplicative identity, such that , for all ;
Multiplicative Inverses — For each , except for 0, there exists a , called a multiplicative inverse of , such that ;
Closure of addition and multiplication — For all , and .
We will often abbreviate the multiplication of two elements, , by juxtaposition of the elements, . Also, when combining multiplication and addition in an expression, multiplication takes precedence over addition unless the addition is enclosed in parenthesis. That is, .
We can also denote and as additive and multiplicative inverses of any . Furthermore, we can define two more operations, called subtraction and division by , and provided that , .
Important Results
From the given criterion for a field, it can be shown that:
and are abelian groups (where is the set excluding 0), and therefore, group-theoretic properties can be applied to fields, such as uniqueness of identities and inverses, and cancellation properties;
, for all ;
, for all ;
, for all ;
, for all ;
, for all ;
Multiplication distributes over subtraction.
Optional Properties
A field is:
A subfield of a field if (see subset), where addition and multiplication on is a domain restriction on the addition and multiplication on . More commonly, we say that is an extension field of ;
An ordered field if there exists a total order on such that for all , if , then , (translation invariance), and if and , then .