# Definition:Commutative and Unitary Ring

## Definition

A commutative and unitary ring $\struct {R, +, \circ}$ is a ring with unity which is also commutative.

That is, it is a ring such that the ring product $\struct {R, \circ}$ is commutative and has an identity element.

That is, such that the multiplicative semigroup $\struct {R, \circ}$ is a commutative monoid.

The identity element is usually denoted by $1_R$ or $1$ and called a unity.

### Commutative and Unitary Ring Axioms

A commutative and unitary ring is an algebraic structure $\struct {R, *, \circ}$, on which are defined two binary operations $\circ$ and $*$, which satisfy the following conditions:

 $(\text A 0)$ $:$ Closure under addition $\displaystyle \forall a, b \in R:$ $\displaystyle a * b \in R$ $(\text A 1)$ $:$ Associativity of addition $\displaystyle \forall a, b, c \in R:$ $\displaystyle \paren {a * b} * c = a * \paren {b * c}$ $(\text A 2)$ $:$ Commutativity of addition $\displaystyle \forall a, b \in R:$ $\displaystyle a * b = b * a$ $(\text A 3)$ $:$ Identity element for addition: the zero $\displaystyle \exists 0_R \in R: \forall a \in R:$ $\displaystyle a * 0_R = a = 0_R * a$ $(\text A 4)$ $:$ Inverse elements for addition: negative elements $\displaystyle \forall a \in R: \exists a' \in R:$ $\displaystyle a * a' = 0_R = a' * a$ $(\text M 0)$ $:$ Closure under product $\displaystyle \forall a, b \in R:$ $\displaystyle a \circ b \in R$ $(\text M 1)$ $:$ Associativity of product $\displaystyle \forall a, b, c \in R:$ $\displaystyle \paren {a \circ b} \circ c = a \circ \paren {b \circ c}$ $(\text M 2)$ $:$ Commutativity of product $\displaystyle \forall a, b \in R:$ $\displaystyle a \circ b = b \circ a$ $(\text M 3)$ $:$ Identity element for product: the unity $\displaystyle \exists 1_R \in R: \forall a \in R:$ $\displaystyle a \circ 1_R = a = 1_R \circ a$ $(\text D)$ $:$ Product is distributive over addition $\displaystyle \forall a, b, c \in R:$ $\displaystyle a \circ \paren {b * c} = \paren {a \circ b} * \paren {a \circ c}$ $\displaystyle \paren {a * b} \circ c = \paren {a \circ c} * \paren {b \circ c}$

These criteria are called the commutative and unitary ring axioms.

## Also known as

Other nomenclature includes:

• Commutative and unital ring
• Commutative ring with unity
• Commutative ring with identity

## Also see

• Results about commutative and unitary rings can be found here.