Degree Bounds for Positivstellensätze of general semialgebraic sets
关于一般半代数集的正定定理的次数界
Olga Heijmans-Kuryatnikova, Juan C. Vera, Luis F. Zuluaga
AI总结 本文改进了Putinar和Schmüdgen的SOS正定定理的次数界,并为Krivine-Stengle和扩展Handelman的R_+正定定理提供了新的次数界,通过引入新的变量和Lojasiewicz不等式,统一了多种正定定理基于层次方法的次数界计算。
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令p_min表示多项式p在一般紧致半代数集S⊆R^n上的最小值。一种标准的近似方法是通过基于正定定理的层次结构来逼近p_min,这些层次结构通过求和平方或其他全局非负多项式来证明多项式在S上的非负性。随着证书的次数增加,这些层次结构生成的值渐近收敛到p_min。自然的问题是确定证书次数的明确界,以获得给定的ε近似值,或者等价地证明f:p - p_min + ε在S上的正性。我们改进了Putinar和Schmüdgen的SOS正定定理在S上的当前最佳次数界。同时,我们为Krivine-Stengle和最近引入的扩展Handelman的R_+正定定理在S上获得了次数界;为一般紧致半代数集上的线性优化基于层次方法提供了首次明确的次数界。我们的方法基于一种提升和投影构造,在其中我们添加新的变量以构造一个使用Lojasiewicz不等式的距离到集合S的代数表示。这使得将f在复集S上的正性证明问题转化为在更高维超立方体上证明相关多项式F的正性问题。通过投影出新增的变量,F在超立方体上的非负性证书成为f在S上的非负性证书。我们的方法为一般紧致集上的多种正定定理基于层次方法的次数界计算提供了一种统一的方法,缩小了超立方体(或其他简单集)与更一般半代数集之间的结果差距。
Let $p_{\min}$ denote the minimum of a polynomial $p$ over a (general) compact semialgebraic set $S \subseteq \mathbb{R}^n$. A standard way to approximate $p_{\min}$ is via hierarchies built from Positivstellensätze, which certify nonnegativity of polynomials on $S$ using sums of squares or other classes of globally nonnegative polynomials. As the degree of the certificate grows, the values generated by these hierarchies converge asymptotically to $p_{\min}$. A natural question is, then, to determine explicit bounds on the certificate's degree needed to obtain a prescribed $\varepsilon$-approximation to $p_{\min}$, or equivalently certify the positivity of $f:=p - p_{\min} + \varepsilon$ on $S$. We improve the current best degree bounds for Putinar's and Schmüdgen's SOS-Positivstellensatz over $S$. Also, we obtain degree bounds for Krivine--Stengle's and the recently introduced extended-Handelman's $\mathbb{R}_+$-Positivstellensätze over $S$; providing the first explicit degree bounds for linear optimization-based hierarchies over general compact semialgebraic sets. Our approach is based on a lift-and-project construction in which we add new variables to construct an algebraic representation of the distance to the set $S$ using Łojasiewicz's inequality. This lets us lift the problem of certifying the positivity of $f$ on the (complex) set $S$ to the problem of certifying the positivity of a related polynomial $F$ on a higher-dimensional hypercube. By projecting out the added variables, non-negativity certificates for $F$ on the hypercube become non-negativity certificates for $f$ on $S$. Our approach offers a unified methodology to obtain degree bounds for several Positivstellensatz-based hierarchies over general compact sets, narrowing the gap between results for the hypercube (or other simple sets) and more general semialgebraic sets.