Delta-Decidability over the Reals
  Given any collection F of computable functions over the reals, we show that there exists an algorithm that, given any L_F-sentence \varphi containing only bounded quantifiers, and any positive rational number \delta, decides either "\varphi is true", or "a \delta-strengthening of \varphi is false". Under mild assumptions, for a C-computable signature F, the \delta-decision problem for bounded \Sigma_k-sentences in L_F resides in (\Sigma_k^P)^C. The results stand in sharp contrast to the well-known undecidability results, and serve as a theoretical basis for the use of numerical methods in decision procedures for nonlinear first-order theories over the reals. 
Sicun Gao, Jeremy Avigad, Edmund Clarke
cs.LO