# Monomial

In mathematics, a monomial is a particular kind of polynomial, having just one term. Given a natural number n and a variable x, the power function defined by the rule f(x)=xn is therefore a monomial. Given several unknown variables (say, x, y, z) and corresponding natural number exponents (say, a, b, c), the product of the resulting univariate monomials is also a monomial (e.g., the function determined by the rule f(x)=xaybzc).

If coefficients are allowed (this may not be consistent), then a constant multiple of a monomial is also counted as a monomial (e.g., 7xaybzc).

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. An interesting fact from functional analysis is that the full set of monomials tn is not required to span a linear subspace of C[0,1] that is dense for the uniform norm (sharpening the Stone-Weierstrass theorem). It is enough that the sum of the reciprocals n-1 diverge (the Müntz-Szasz theorem).

Notation for monomials is constantly required in fields like partial differential equations. Multi-index notation is helpful: if we write

α = (a, b, c)

we can define

xα = x1a x2b x3c

and save a great deal of space.

In algebraic geometry the varieties defined by monomial equations xα = 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.

In group representation theory, a monomial representation is a particular kind of induced representation.

In propositional logic, a monomial is a conjunction of literals. (See also Clause, Minterm.)

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