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Tarski's theorem states that the first-order theory of the Field of Real Numbers is Decidable. However, the best-known Algorithm for eliminating Quantifiers is doubly exponential in the number of Quantifier blocks (Heintz et al. 1989).
References
Heintz, J.; Roy, R.-F.; and Solerno, P.  ``Complexité du principe de Tarski-Seidenberg.''
  C. R. Acad. Sci. Paris Sér. I Math. 309, 825-830, 1989.
 
Marker, D.  ``Model Theory and Exponentiation.''  Not. Amer. Math. Soc. 43, 753-759, 1996.
 
Tarski, A.  ``Sur les ensembles définissables de nombres réels.''  Fund. Math. 17, 210-239, 1931.
 
Tarski, A.  ``A Decision Method for Elementary Algebra and Geometry.''  RAND Corp. monograph, 1948.