# Monomial basis

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In mathematics the monomial basis of a polynomial ring is its basis (as vector space or free module over the field or ring of coefficients) that consists in the set of all monomials. The monomials form a basis because every polynomial may be uniquely written as a finite linear combination of monomials (this is an immediate consequence of the definition of a polynomial).

## One indeterminate

The polynomial ring K[x] of the univariate polynomial over a field K is a K-vector space, which has

${\displaystyle 1,x,x^{2},x^{3},\ldots }$

as an (infinite) basis. More generally, if K is a ring, K[x] is a free module, which has the same basis.

The polynomials of degree at most d form also a vector space (or a free module in the case of a ring of coefficients), which has

${\displaystyle 1,x,x^{2},\ldots }$

as a basis

The canonical form of a polynomial is its expression on this basis:

${\displaystyle a_{0}+a_{1}x+a_{2}x^{2}+\ldots +a_{d}x^{d},}$

or, using the shorter sigma notation:

${\displaystyle \sum _{i=0}^{d}a_{i}x^{i}.}$

The monomial basis is naturally totally ordered, either by increasing degrees

${\displaystyle 1

or by decreasing degrees

${\displaystyle 1>x>x^{2}>\cdots .}$

## Several indeterminates

In the case of several indeterminates ${\displaystyle x_{1},\ldots ,x_{n},}$ a monomial is a product

${\displaystyle x_{1}^{d_{1}}x_{2}^{d_{2}}\cdots x_{n}^{d_{n}},}$

where the ${\displaystyle d_{i}}$ are non-negative integers. Note that, as ${\displaystyle x_{i}^{0}=1,}$ an exponent equal to zero means that the corresponding indeterminate does not appear in the monomial; in particular ${\displaystyle 1=x_{1}^{0}x_{2}^{0}\cdots x_{n}^{0}}$ is a monomial.

Similar to the case of univariate polynomials, the polynomials in ${\displaystyle x_{1},\ldots ,x_{n}}$ form a vector space (if the coefficients belong to a field) or a free module (if the coefficients belong to a ring), which has the set of all monomials as a basis, called the monomial basis

The homogeneous polynomials of degree ${\displaystyle d}$ form a subspace which has the monomials of degree ${\displaystyle d=d_{1}+\cdots +d_{n}}$ as a basis. The dimension of this subspace is the number of monomials of degree ${\displaystyle d}$, which is

${\displaystyle {\binom {d+n-1}{d}}={\frac {n(n+1)\cdots (n+d-1)}{d!}},}$

where ${\displaystyle {\binom {d+n-1}{d}}}$ denotes a binomial coefficient.

The polynomials of degree at most ${\displaystyle d}$ form also a subspace, which has the monomials of degree at most ${\displaystyle d}$ as a basis. The number of these monomials is the dimension of this subspace, equal to

${\displaystyle {\binom {d+n}{d}}={\binom {d+n}{n}}={\frac {(d+1)\cdots (d+n)}{n!}}.}$

Despite the univariate case, there is no natural total order of the monomial basis. For problem which require to choose a total order, such Gröbner basis computation, one generally chooses an admissible monomial order that is a total order on the set of monomials such that

${\displaystyle m

and

${\displaystyle 1\leq m}$

for every monomials ${\displaystyle m,n,q.}$

A polynomial can always be converted into monomial form by calculating its Taylor expansion around 0. For example, a polynomial in ${\displaystyle \Pi _{4}}$:

${\displaystyle 1+x+3x^{4}}$