Upper and lower bounds on continuous-time computation Manuel Lameiras Campagnolo1 and Cristopher Moore2,3,4 1 D.M./I.S.A., Universidade T´ecnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
[email protected] 2 Computer Science Department, University of New Mexico, Albuquerque NM 87131
[email protected] 3 Physics and Astronomy Department, University of New Mexico, Albuquerque NM 87131 4 Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501 Abstract. We consider various extensions and modifications of Shannon’s General Purpose Analog Com- puter, which is a model of computation by differential equations in continuous time. We show that several classical computation classes have natural analog counterparts, including the primitive recursive functions, the elementary functions, the levels of the Grzegorczyk hierarchy, and the arithmetical and analytical hierarchies. Key words: Continuous-time computation, differential equations, recursion theory, dynamical systems, elementary func- tions, Grzegorczyk hierarchy, primitive recursive functions, computable functions, arithmetical and analytical hierarchies. 1 Introduction The theory of analog computation, where the internal states of a computer are continuous rather than discrete, has enjoyed a recent resurgence of interest. This stems partly from a wider program of exploring alternative approaches to computation, such as quantum and DNA computation; partly as an idealization of numerical algorithms where real numbers can be thought of as quantities in themselves, rather than as strings of digits; and partly from a desire to use the tools of computation theory to better classify the variety of continuous dynamical systems we see in the world (or at least in its classical idealization).