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# 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 is this analogy that 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 = { f1, f2, ..., fm } and G = { g1, g2, ..., gr } finite sets of polynomials over R in n variables. Let W be the semialgebraic set

${\displaystyle W=\{x\in R^{n}\mid \forall f\in F,\,f(x)\geq 0;\,\forall g\in G,\,g(x)=0\},}$

and define the preordering associated with W as the set

${\displaystyle P(F,G)=\left\{\sum _{\alpha \in \{0,1\}^{m}}\sigma _{\alpha }f_{1}^{\alpha _{1}}\cdots f_{m}^{\alpha _{m}}+\sum _{\ell =1}^{r}\varphi _{\ell }g_{\ell }:\sigma _{\alpha }\in \Sigma ^{2}[X_{1},\ldots ,X_{n}];\ \varphi _{\ell }\in R[X_{1},\ldots ,X_{n}]\right\}}$

where Σ2[X1,…,Xn] 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[X1,…,Xn] generated by F and arbitrary squares) and I is the ideal generated by G.

Let p ∈ R[X1,…,Xn] be a polynomial. Krivine–Stengle Positivstellensatz states that

(i) ${\displaystyle \forall x\in W\;p(x)\geq 0}$ if and only if ${\displaystyle \exists q_{1},q_{2}\in P(F,G)}$ and ${\displaystyle s\in \mathbb {Z} }$ such that ${\displaystyle q_{1}p=p^{2s}+q_{2}}$.
(ii) ${\displaystyle \forall x\in W\;p(x)>0}$ if and only if ${\displaystyle \exists q_{1},q_{2}\in P(F,G)}$ such that ${\displaystyle q_{1}p=1+q_{2}}$.

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[X1,…,Xn]. Let C be the cone generated by F, and I the ideal generated by G. Then

${\displaystyle \{x\in R^{n}\mid \forall f\in F\,f(x)\geq 0\land \forall g\in G\,g(x)=0\land \forall h\in H\,h(x)\neq 0\}=\emptyset }$

if and only if

${\displaystyle \exists f\in C,g\in I,n\in \mathbb {N} \;f+g+\left(\prod H\right)^{2n}=0.}$

(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 ${\displaystyle R=\mathbb {R} }$. If the semialgebraic set ${\displaystyle W=\{x\in \mathbb {R} ^{n}\mid \forall f\in F,\,f(x)\geq 0\}}$ is compact, then each polynomial ${\displaystyle p\in \mathbb {R} [X_{1},\dots ,X_{n}]}$ that is strictly positive on ${\displaystyle W}$ can be written as a polynomial in the defining functions of ${\displaystyle W}$ with sums-of-squares coefficients, i.e. ${\displaystyle p\in P(F,\emptyset )}$. Here P is said to be strictly positive on ${\displaystyle W}$ if ${\displaystyle p(x)>0}$ for all ${\displaystyle x\in W}$. [1] Note that Schmüdgen's Positivstellensatz is stated for ${\displaystyle R=\mathbb {R} }$ and does not hold for arbitrary real closed fields.[2]

### Putinar's Positivstellensatz

Define the quadratic module associated with W as the set

${\displaystyle Q(F,G)=\left\{\sigma _{0}+\sum _{j=1}^{m}\sigma _{j}f_{j}+\sum _{\ell =1}^{r}\varphi _{\ell }g_{\ell }:\sigma _{j}\in \Sigma ^{2}[X_{1},\ldots ,X_{n}];\ \varphi _{\ell }\in \mathbb {R} [X_{1},\ldots ,X_{n}]\right\}}$

Assume there exists L > 0 such that the polynomial ${\displaystyle L-\sum _{i=1}^{n}x_{i}^{2}\in Q(F,G).}$ If ${\displaystyle \forall x\in W\;p(x)>0}$, then pQ(F,G).[3]