Definition 1: Ring

A ring R is a nonempty set with two binary operations: addition (a + b) and multiplication (ab).

It has the following properties:

1. Commutative Rule for Addition: a + b = b + a

2. Associative Rule for Addition: (a + b) + c = a + (b + c)

3. Additive Identity: there exists a value 0 ∈ R such that a + 0 = a for all a ∈ R

4. Additive Inverse: for all elements a ∈ R, there exists a value -a ∈ R such that a + -a = 0.

5. Associative Rule for Multiplication: a(bc) = (ab)c

6. Distributive Rule: a(b + c) = ab + ac and (b + c)a = ba + ca

Definition 2: Commutative Ring

A ring that has the following additional property:

7. Commutative Rule for Multiplication: ab = ba

Definition 3: Field

A field is a commutative ring R with unity in which every nonzero element is a unit.

It has all the properties of a commutative ring plus:

8. Multiplicative Identity (unity): there exists a value 1 ∈ R such that such that a * 1 = a for all a ∈ R

9. Multiplicative Inverse (every nonzero element is a unit): for all nonzero elements a ∈ R, there exists an inverse a

^{-1}∈ R such that a*a

^{-1}= 1.

Examples:

1. The set of integers Z is a commutative ring with unity 1. [See here for more information on the integers]

2. The set Z

_{n}is a commutative ring with unity 1. [See here for more information on modular arithmetic]

3. The set of 2x2 matrices with integer entries is a noncommutative ring with unity. [See here for more information on 2x2 matrices]

4. If p is prime, then the set Z

_{p}is a field. [See here for more information on modular arithmetic]

5. The set of rational numbers Q is a field.

6. The set of real numbers R is a field.

7. The set of complex numbers is a field. [See here for more information on complex numbers]

8. The set of Gaussian Integers is a ring. [See here for more information on Gaussian Integers]

9. The set of Eisenstein Integers is a ring. [See here for more information on Eisenstein Integers]

References

- Joseph A. Gallian, Contemporary Abstract Algebra
- Rings, Wikipedia