# Monomial

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In mathematics, in the context of polynomials, the word monomial can have one of two different meanings:

## Comparison of the two definitions

With either definition, the set of monomials is a subset of all polynomials that is closed under multiplication.

Both uses of this notion can be found, and in many cases the distinction is simply ignored, see for instance examples for the first[1] and second[2] meaning, and an unclear definition. In informal discussions the distinction is seldom important, and tendency is towards the broader second meaning. When studying the structure of polynomials however, one often definitely needs a notion with the first meaning. This is for instance the case when considering a monomial basis of a polynomial ring, or a monomial ordering of that basis. An argument in favor of the first meaning is also that no obvious other notion is available to designate these values (the term power product is in use, but it does not make the absence of constants clear either), while the notion term of a polynomial unambiguously coincides with the second meaning of monomial.

## As bases

The most obvious fact about monomials (first meaning) is that any polynomial is a linear combination of them, so they form a basis of the vector space of all polynomials - a fact of constant implicit use in mathematics.

## Number

The number of monomials of degree d in n variables is the number of multicombinations of d elements chosen among the n variables (a variable can be chosen more than once, but order does not matter), which is given by the multiset coefficient ${\displaystyle \textstyle {\left(\!\!{n \choose d}\!\!\right)}}$. This expression can also be given in the form of a binomial coefficient, as a polynomial expression in d, or using a rising factorial power of d + 1:

${\displaystyle \left(\!\!{n \choose d}\!\!\right)={\binom {n+d-1}{d}}={\binom {d+(n-1)}{n-1}}={\frac {(d+1)\times (d+2)\times \cdots \times (d+n-1)}{1\times 2\times \cdots \times (n-1)}}={\frac {1}{(n-1)!}}(d+1)^{\overline {n-1}}.}$

The latter forms are particularly useful when one fixes the number of variables and lets the degree vary. From these expressions one sees that for fixed n, the number of monomials of degree d is a polynomial expression in d of degree ${\displaystyle n-1}$ with leading coefficient ${\displaystyle {\tfrac {1}{(n-1)!}}}$.

For example, the number of monomials in three variables (${\displaystyle n=3}$) of degree d is ${\displaystyle \textstyle {\frac {1}{2}}(d+1)^{\overline {2}}=\textstyle {\frac {1}{2}}(d+1)(d+2)}$; these numbers form the sequence 1, 3, 6, 10, 15, ... of triangular numbers.

## Notation

Notation for monomials is constantly required in fields like partial differential equations. If the variables being used form an indexed family like ${\displaystyle x_{1}}$, ${\displaystyle x_{2}}$, ${\displaystyle x_{3}}$, ..., then multi-index notation is helpful: if we write

${\displaystyle \alpha =(a,b,c)}$

we can define

${\displaystyle x^{\alpha }=x_{1}^{a}\,x_{2}^{b}\,x_{3}^{c}}$

and save a great deal of space.

## Degree

The degree of a monomial is defined as the sum of all the exponents of the variables, including the implicit exponents of 1 for the variables which appear without exponent; e.g., in the example of the previous section, the degree is ${\displaystyle a+b+c}$. The degree of ${\displaystyle xyz^{2}}$ is 1+1+2=4.

The degree of a monomial is sometimes called order, mainly in the context of series. It is also called total degree when it is needed to distinguish it from the degree in one of the variables.

Monomial degree is fundamental to the theory of univariate and multivariate polynomials. Explicitly, it is used to define the degree of a polynomial and the notion of homogeneous polynomial, as well as for graded monomial orderings used in formulating and computing Gröbner bases. Implicitly, it is used in grouping the terms of a Taylor series in several variables.

## Geometry

In algebraic geometry the varieties defined by monomial equations ${\displaystyle x^{\alpha }=0}$ for some set of α have special properties of homogeneity. This can be phrased in the language of algebraic groups, in terms of the existence of a group action of an algebraic torus (equivalently by a multiplicative group of diagonal matrices). This area is studied under the name of torus embeddings.

## Notes

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