Monomial basis

In mathematics the monomial basis of a polynomial ring is its basis (as a vector space or free module over the field or ring of coefficients) that consists 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 univariate polynomials over a field K is a K-vector space, which has

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

as an (infinite) basis. More generally, if K is a ring then 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

$$\{1,x,x^{2},\ldots ,x^{d-1},x^{d}\}$$

as a basis.

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

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

or, using the shorter sigma notation:

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

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

$$1<x<x^{2}<\cdots ,$$

or by decreasing degrees

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

Several indeterminates

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

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

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

Similar to the case of univariate polynomials, the polynomials in $x_{1},\ldots ,x_{n}$ 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 $d$ form a subspace which has the monomials of degree $d=d_{1}+\cdots +d_{n}$ as a basis. The dimension of this subspace is the number of monomials of degree $d$, which is

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

where ${\textstyle {\binom {d+n-1}{d}}}$ is a binomial coefficient.

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

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

In contrast to the univariate case, there is no natural total order of the monomial basis in the multivariate case. For problems which require choosing a total order, such as Gröbner basis computations, one generally chooses an admissible monomial order – that is, a total order on the set of monomials such that

$$m<n\iff mq<nq$$

and

$$1\leq m$$

for every monomial $m,n,q.$

See also

• Horner's method
• Polynomial sequence
• Newton polynomial
• Lagrange polynomial
• Legendre polynomial
• Bernstein form
• Chebyshev form