# Definition:Degree of Polynomial

## One variable

Let $R$ be a commutative ring with unity.

Let $P \in R \sqbrk x$ be a nonzero polynomial over $R$ in one variable $x$.

The degree of $P$ is the largest natural number $k \in \N$ such that the coefficient of $x^k$ in $P$ is nonzero.

### General Ring

Let $\struct {R, +, \circ}$ be a ring.

Let $\struct {S, +, \circ}$ be a subring of $R$.

Let $x \in R$.

Let $\ds P = \sum_{j \mathop = 0}^n \paren {r_j \circ x^j} = r_0 + r_1 \circ x + \cdots + r_n \circ x^n$ be a polynomial in the element $x$ over $S$ such that $r_n \ne 0$.

Then the degree of $P$ is $n$.

The degree of $P$ can be denoted $\map \deg P$ or $\partial P$.

### Integral Domain

Let $\struct {R, +, \circ}$ be a commutative ring with unity whose zero is $0_R$.

Let $\struct {D, +, \circ}$ be an integral subdomain of $R$.

Let $X \in R$ be transcendental over $D$.

Let $\ds f = \sum_{j \mathop = 0}^n \paren {r_j \circ X^j} = r_0 + r_1 X + \cdots + r_n X^n$ be a polynomial over $D$ in $X$ such that $r_n \ne 0$.

Then the degree of $f$ is $n$.

The degree of $f$ is denoted on $\mathsf{Pr} \infty \mathsf{fWiki}$ by $\map \deg f$.

### Field

Let $\struct {F, +, \times}$ be a field whose zero is $0_F$.

Let $\struct {K, +, \times}$ be a subfield of $F$.

Let $x \in F$.

Let $\ds f = \sum_{j \mathop = 0}^n \paren {a_j x^j} = a_0 + a_1 x + \cdots + a_n x^n$ be a polynomial over $K$ in $x$ such that $a_n \ne 0$.

Then the degree of $f$ is $n$.

The degree of $f$ can be denoted $\map \deg f$ or $\partial f$.

## Sequence

### Ring

Let $f = \sequence {a_k} = \tuple {a_0, a_1, a_2, \ldots}$ be a polynomial over a ring $R$.

The degree of $f$ is defined as the largest $n \in \Z$ such that $a_n \ne 0$.

### Field

Let $f = \sequence {a_k} = \tuple {a_0, a_1, a_2, \ldots}$ be a polynomial over a field $F$.

The degree of $f$ is defined as the largest $n \in \Z$ such that $a_n \ne 0$.

## Polynomial Form

Let $f = a_1 \mathbf X^{k_1} + \cdots + a_r \mathbf X^{k_r}$ be a polynomial in the indeterminates $\family {X_j: j \in J}$ for some multiindices $k_1, \ldots, k_r$.

Let $f$ not be the null polynomial.

Let $k = \family {k_j}_{j \mathop \in J}$ be a multiindex.

Let $\ds \size k = \sum_{j \mathop \in J} k_j \ge 0$ be the degree of the monomial $\mathbf X^k$.

The degree of $f$ is the supremum:

$\ds \map \deg f = \max \set {\size {k_r}: i = 1, \ldots, r}$

## Degree Zero

A polynomial $f \in S \sqbrk x$ in $x$ over $S$ is of degree zero if and only if $x$ is a non-zero element of $S$, that is, a constant polynomial.

## Null Polynomial

The null polynomial $0_R \in S \left[{X}\right]$ does not have a degree.

## Also known as

The degree of a polynomial $f$ is also sometimes called the order of $f$.

Some sources denote $\map \deg f$ by $\partial f$.