Real recursive functions and their hierarchy

In the last years, recursive functions over the reals (Theoret. Comput. Sci. 162 (1996) 23) have been 9 considered, first as a model of analog computation, and second to obtain analog characterizations of classical computational complexity classes (Unconventional Models of Computation, UMC 2002, 11 Lecture Notes in Computer Science, Vol. 2509, Springer, Berlin, pp. 1–14). However, one of the operators introduced in the seminal paper byMoore (1996), the minimalization operator, has not been 13 considered: (a) although differential recursion (the analog counterpart of classical recurrence) is, in some extent, directly implementable in the General Purpose Analog Computer of Claude Shannon, 15 analog minimalization is far from physical realizability, and (b) analog minimalization was borrowed from classical recursion theory and does not fit well the analytic realm of analog computation. In 17 this paper, we show that a most natural operator captured from analysis—the operator of taking a limit—can be used properly to enhance the theory of recursion over the reals, providing good solutions 19 to puzzling problems raised by the original model. © 2004 Published by Elsevier Inc. 21