Monomial

In mathematics, in the context of polynomials, the word monomial means one of two different things:

• The first meaning is a product of powers of variables, or formally any value obtained from 1 by finitely many multiplications by a variable. If only a single variable $x$ is considered this means that any monomial is either 1 or a power $x^{n}$ of $x$, with $n$ a positive integer. If several variables are considered, say, $x$, $y$, $z$, then each can be given an exponent, so that any monomial is of the form $x^{a}y^{b}z^{c}$ with $a,b,c$ nonnegative integers (taking note that any exponent 0 makes the corresponding factor equal to 1).
• The second meaning of monomial includes monomials in the first sense, but also allows multiplication by any constant, so that $-7x^{5}$ and $(3-4i)x^{4}yz^{13}$ are also considered to be monomials (the second example assuming polynomials in $x$, $y$, $z$ over the complex numbers are considered).

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 and second 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. For an isolated polynomial consisting of a single term, one could if necessary use the uncontracted form mononomial, analogous to binomial and trinomial.

The remainder of this article assumes the first meaning of "monomial".

this is so confusing

As bases

The most obvious fact about monomials is that any polynomial is a linear combination of them, so they can serve as basis vectors in a vector space of polynomials - a fact of constant implicit use in mathematics.

Notation

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

$\alpha =(a,b,c)$

we can define

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

and save a great deal of space.

Geometry

In algebraic geometry the varieties defined by monomial equations $x^{\alpha }=0$ 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.

See also

• monomial order
• monomial representation
• monomial matrix
• homogeneous polynomial
• homogeneous function
• multilinear form