# Krivine–Stengle Positivstellensatz

In real algebraic geometry, Krivine–Stengle Positivstellensatz (German for "positive-locus-theorem") characterizes polynomials that are positive on a semialgebraic set, which is defined by systems of inequalities of polynomials with real coefficients, or more generally, coefficients from any real closed field.
It can be thought of as a real analogue of Hilbert's Nullstellensatz (which concern complex zeros of polynomial ideals), and this analogy is at the origin of its name. It was proved by French mathematician Jean-Louis Krivine [fr; de] and then rediscovered by the Canadian Gilbert Stengle [Wikidata].

## Statement

Let R be a real closed field, and F = {f$_1$, f$_2$, ..., f$_m$} and G = {g$_1$, g$_2$, ..., g$_r$} finite sets of polynomials over R in n variables. Let W be the semialgebraic set
    $W=\{x\in R^{n}\mid \forall f\in F,\,f(x)\geq 0;\,\forall g\in G,\,g(x)=0\},$
and define the preorder associated with W as the set
    $P(F,G)=\left\{\sum _{\alpha \in \{0,1\}^{m}}\sigma _{\alpha }f_{1}^{\alpha _{1}}\cdots f_{m}^{\alpha _{m}}+\sum _{\ell =1}^{r}\varphi _{\ell }g_{\ell }:\sigma _{\alpha }\in \Sigma ^{2}[X_{1},\ldots ,X_{n}];\ \varphi _{\ell }\in R[X_{1},\ldots ,X_{n}]\right\}$
where Σ$^2$[X$_1$,...,X$_n$] is the set of sum-of-squares polynomials. In other words, P(F, G) = C + I, where C is the cone generated by F (i.e., the subsemiring of R[X$_1$,...,X$_n$] generated by F and arbitrary squares) and I is the ideal generated by G.
Let p ∈ R[X$_1$,...,X$_n$] be a polynomial. Krivine–Stengle Positivstellensatz states that
    (i) $\forall x\in W\;p(x)\geq 0$ if and only if $\exists q_{1},q_{2}\in P(F,G)$ and $s\in \mathbb {Z}$ such that $q_{1}p=p^{2s}+q_{2}$.
    (ii) $\forall x\in W\;p(x)>0$ if and only if $\exists q_{1},q_{2}\in P(F,G)$ such that $q_{1}p=1+q_{2}$.
The weak Positivstellensatz is the following variant of the Positivstellensatz. Let R be a real closed field, and F, G, and H finite subsets of R[X$_1$,...,X$_n$]. Let C be the cone generated by F, and I the ideal generated by G. Then
    $\{x\in R^{n}\mid \forall f\in F\,f(x)\geq 0\land \forall g\in G\,g(x)=0\land \forall h\in H\,h(x)\neq 0\}=\emptyset$
if and only if
    $\exists f\in C,g\in I,n\in \mathbb {N} \;f+g+\left(\prod H\right)^{\!2n}=0.$
(Unlike Nullstellensatz, the "weak" form actually includes the "strong" form as a special case, so the terminology is a misnomer.)

## Variants

The Krivine–Stengle Positivstellensatz also has the following refinements under additional assumptions. It should be remarked that Schmüdgen's Positivstellensatz has a weaker assumption than Putinar's Positivstellensatz, but the conclusion is also weaker.

### Schmüdgen's Positivstellensatz

Suppose that $R=\mathbb {R}$. If the semialgebraic set $W=\{x\in \mathbb {R} ^{n}\mid \forall f\in F,\,f(x)\geq 0\}$ is compact, then each polynomial $p\in \mathbb {R} [X_{1},\dots ,X_{n}]$ that is strictly positive on $W$ can be written as a polynomial in the defining functions of $W$ with sums-of-squares coefficients, i.e. $p\in P(F,\emptyset )$. Here P is said to be strictly positive on $W$ if $p(x)>0$ for all $x\in W$. Note that Schmüdgen's Positivstellensatz is stated for $R=\mathbb {R}$ and does not hold for arbitrary real closed fields.

### Putinar's Positivstellensatz

Define the quadratic module associated with W as the set
    $Q(F,G)=\left\{\sigma _{0}+\sum _{j=1}^{m}\sigma _{j}f_{j}+\sum _{\ell =1}^{r}\varphi _{\ell }g_{\ell }:\sigma _{j}\in \Sigma ^{2}[X_{1},\ldots ,X_{n}];\ \varphi _{\ell }\in \mathbb {R} [X_{1},\ldots ,X_{n}]\right\}$
Assume there exists L > 0 such that the polynomial $L-\sum _{i=1}^{n}x_{i}^{2}\in Q(F,G).$ If $p(x)>0$ for all $x\in W$, then p ∈ Q(F,G).