# Definition:Ring of Polynomial Forms

## Definition

Let $R$ be a commutative ring with unity.

Let $I$ be a set

Let $\left\{{X_i: i \in I}\right\}$ be an indexed set.

Let $A = R \left[{\left\{{X_i: i \in I}\right\}}\right]$ be the set of all polynomial forms over $R$ in $\left\{{X_i: i \in I}\right\}$.

Let $+$ and $\circ$ denote the standard addition and multiplication of polynomial forms.

The **ring of polynomial forms** is the ordered triple $\left( A, +, \circ \right)$.

## Also known as

Because the **ring of sequences of polynomial forms** can be used to construct the polynomial ring over $R$, it may be referred to as a **polynomial ring**.

The elements of the set $\left\{{X_j: j \in J}\right\}$ are called **indeterminates**.

### Notation

Suppose we let $a_k \mathbf X^k$ denote the polynomial that has value $a_k$ on $\mathbf X^k$ and $0_R$ otherwise.

It follows from Unique Representation in Polynomial Forms that $f$ can then be uniquely written as a finite sum of non-zero mononomials:

- $f = a_1 \mathbf X^{k_1} + \cdots + a_r \mathbf X^{k_r}$

or non-uniquely by relaxing the condition that $\forall i = 1, \ldots, r: a_i \ne 0$.

This is the notation most frequently used when working with polynomials.

It is also sometimes helpful to include all the zero terms in this sum, in which case:

- $\displaystyle f = \sum_{k \in Z} a_k \mathbf X^k$

where $Z$ is the set of multiindices indexed by $J$.

## Also see

## Sources

- 2000: Pierre A. Grillet:
*Abstract Algebra*: $\S \text{III}.6$