This paper describes the simulation of an S(n) space-bounded deterministic Turing machine by a reversible Turing machine operating in space S(n). It thus answers a question posed by Bennett in 1989 and refutes the conjecture, made by Li and Vitanyi in 1996, that any reversible simulation of an irreversible computation must obey Bennett's reversible pebble game rules.

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The concept of unambiguity of circuits is considered. Several classes of unambiguous circuitfamilies within the NC-hierarchy are introduced and related to unambiguous automata and to PRAM's with exclusive writeaccess. In particular, we show CREW-TIME(logn) = Unum bA C1.

This paper concerns some of the theoretical complexity aspects of the reconfigurable network model. The computational power of the model is investigated under several variants, depending on the type of switches (or switch operations) assumed by the network nodes. Computational power is evaluated by focusing on the set of problems computable in constant time in each variant. A hierarchy of such problem classes corresponding to different variants is shown to exist and is placed relative to traditional classes of complexity theory.