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We observe that many important computational problems in NC1 share a simple self-reducibility property. We then show that, for any problem A having this self-reducibility property, A has polynomial size TC0 circuits if and only if it has TC0 circuits of size n1+isin for every isin>0 (counting the number of wires in a circuit as the size of the circuit). As an example of what this observation yields, consider the Boolean formula evaluation problem (BFE), which is complete for NC1. It follows from a lower bound of Impagliazzo, Paturi, and Saks, that BFE requires depth d TC0 circuits of size n1+isin d. If one were able to improve this lower bound to show that there is some constant isin>0 such that every TC0 circuit family recognizing BFE has size n1+isin, then it would follow that TC0neNC1. We also show that problems with small uniform constant- depth circuits have algorithms that simultaneously have small space and time bounds. We then make use of known time-space tradeoff lower bounds to show that SAT requires uniform depth d TC0 and AC0  circuits of size n1+c for some constant c depending on d.
Date of Conference: 23-26 June 2008