We introduce the complexity class \(\exists \mathbb {R}\) based on the existential theory of the reals. We show that the definition of \(\exists \mathbb {R}\) is robust in the sense that even the fragment of the theory expressing solvability of systems of strict polynomial inequalities leads to the same complexity class. Several natural and well-known problems turn out to be complete for \(\exists \mathbb {R}\); here we show that the complexity of decision variants of fixed-point problems, including Nash equilibria, are complete for this class, complementing work by Etessami and Yannakakis [13].
['Marcus Schaefer', 'Daniel Štefankovič']
Fixed Points, Nash Equilibria, and the Existential Theory of the Reals