We summarize in a n intuitiue vein a few concepts froin recursion theory and fi-om the theory ofjormal systems and then state and comment our recent 1-esults on the incoinpleteness of elementary real analysis and its consequences. Their relation to forcing is also dealt with. 0 1995 John Wiiviley & Sons, Inc.
a MOTIVATIONhere are simple noncomputable expressions for functions within mathematics; those expressions creep up T even within languages close to arithmetic, and lead to very simple questions such as, "does the integer-valued function O(n) equal 0 or 12" which turn out to be undecidable in the general case. Analogous nai've looking but intractable expressions for functions can also be found within more elaborate languages, as classical elementary analysis. With their help we can generate infinitely many undecidable sentences with a trivial appearance from arithmetic on and all the way up to the whole of mathematics.Some of those intractable expressions represent the halting function e (m, n), that tells us whether the Turing machine M,,,(n) stops over its input n. Once we have an expression for the halting function, we can obtain explicit expressions for all complete arithmetic degrees and even beyond. Therefore, the associated undecidable predicates represent problems in all the corresponding degrees of unsolvability, both inside and outside the arithmetic hierarchy.We used those undecidable predicates and noncomputable functions to settle several open decision problems, mainly in dynamical systems theory. Two such problems deal with frequently handled questions:
0Is there a decision procedure for chaos? Chaos theory has been a fast growing research area since the early 1970s, a decade after the discovery by E. Lorenz of an apparent, but still unproved chaotic behavior in a deterministic nonlinear dynamical system (for references see [7]). Chaos scientists usually proceed in one of two ways: whenever they wish to know if a given physical process is chaotic the usual starting point is to write down the equations that describe the process; out of them one tries to check whether the process satisfies some of the established mathematical criteria for chaos and randomness.However, those equations are in most cases intractable nonlinear differential equations as they cannot in general be given explicit analytical solutions. Therefore, chaos theorists turn to computer simulations and for most non-