In mathematics, a positive polynomial (respectively non-negative polynomial) on a particular set is a polynomial whose values are positive (respectively non-negative) on that set. Precisely, Let ${\displaystyle p}$ be a polynomial in ${\displaystyle n}$ variables with real coefficients and let ${\displaystyle S}$ be a subset of the ${\displaystyle n}$-dimensional Euclidean space ${\displaystyle \mathbb {R} ^{n}}$. We say that:

For certain sets ${\displaystyle S}$, there exist algebraic descriptions of all polynomials that are positive (resp. non-negative) on ${\displaystyle S}$. Such a description is a positivstellensatz (resp. nichtnegativstellensatz). The importance of Positivstellensatz theorems in computation arises from its ability to transform problems of polynomial optimization into semidefinite programming problems, which can be efficiently solved using convex optimization techniques.

A real univariate polynomial is non-negative on ${\displaystyle \mathbb {R} }$ if and only if it is a sum of two squares of real univariate polynomials. This equivalence does not generalize to multivariate polynomials, which was originally shown by Hilbert. An explicit example of such a polynomial was not known until Theodore Motzkin showed in 1967 that ${\displaystyle X^{4}Y^{2}+X^{2}Y^{4}-3X^{2}Y^{2}+1}$ is not a sum of squares of polynomials but is non-negative on ${\displaystyle \mathbb {R} ^{2}}$, which follows from the AM-GM inequality.

In higher dimensions, a real polynomial in ${\displaystyle n}$ variables is non-negative on ${\displaystyle \mathbb {R} ^{n}}$ if and only if it is a sum of squares of real rational functions in ${\displaystyle n}$ variables. This was originally posed as Hilbert's seventeenth problem in 1900, and later solved by Emil Artin in 1927.

For homogeneous polynomials, more information can be determined about the denominator. Suppose that ${\displaystyle p\in \mathbb {R} [X_{1},\dots ,X_{n}]}$ is homogeneous of degree 2k. If it is positive on ${\displaystyle \mathbb {R} ^{n}\setminus \{0\}}$, then there exists an integer ${\displaystyle m}$ such that ${\displaystyle (X_{1}^{2}+\cdots +X_{n}^{2})^{m}p}$ is a sum of squares of homogeneous polynomials of degree ${\displaystyle m+2k}$.

For polynomials of degree${\displaystyle {}\leq 1}$ we have the following variant of Farkas lemma: If ${\displaystyle f,g_{1},\dots ,g_{k}}$ have degree${\displaystyle {}\leq 1}$ and ${\displaystyle f(x)\geq 0}$ for every ${\displaystyle x\in \mathbb {R} ^{n}}$ satisfying ${\displaystyle g_{1}(x)\geq 0,\dots ,g_{k}(x)\geq 0}$, then there exist non-negative real numbers ${\displaystyle c_{0},c_{1},\dots ,c_{k}}$ such that ${\displaystyle f=c_{0}+c_{1}g_{1}+\cdots +c_{k}g_{k}}$.

For higher degree polynomials on the simplex, Pólya showed that if ${\displaystyle p\in \mathbb {R} [X_{1},\dots ,X_{n}]}$ is homogeneous and positive on the set ${\displaystyle \{x\in \mathbb {R} ^{n}\mid x_{1}\geq 0,\dots ,x_{n}\geq 0,x_{1}+\cdots +x_{n}\neq 0\}}$, then there exists an integer ${\displaystyle m}$ such that ${\displaystyle (x_{1}+\cdots +x_{n})^{m}p}$ has non-negative coefficients.

For higher degree polynomials on general compact polytopes, we have Handelman's theorem: If ${\displaystyle K}$ is a compact polytope in Euclidean ${\displaystyle d}$-space, defined by linear inequalities ${\displaystyle g_{i}\geq 0}$, and if ${\displaystyle f}$ is a polynomial in ${\displaystyle d}$ variables that is positive on ${\displaystyle K}$, then ${\displaystyle f}$ can be expressed as a linear combination with non-negative coefficients of products of members of ${\displaystyle \{g_{i}\}}$.