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What is known about the complexity of deciding whether a finite set of polynomial inequalities in $n$ real variables with integer coefficients is satisfiable? Decidability is guaranteed by Tarski's theorem (see the answers to this related question), but his proof is nonconstructive and gives no complexity bound.

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    $\begingroup$ This computational problem is known as the existential theory of the reals. It sits somewhere between NP and PSPACE, but not much more is known; it’s important enough that it’s been given a complexity class of its own ($\exists\mathbb R$). (The official definition as in the Wikipedia article allows the arbitrary existential sentences, but this easily reduces to the case of satisfiability of a single polynomial equation by introduction of a few extension variables.) $\endgroup$ Commented Jan 13, 2021 at 10:37

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