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A symmetric Turing machine is one whose "yields" relation between configurations is symmetric. The space complexity classes for such machines are found to be intermediate between the corresponding deterministic and nondeterministic space complexity classes. Certain natural problems are shown to be complete for symmetric space complexity classes, and the relationship of symmetry to determinism...
We analyze the computational complexity of determining whether F is satisfiable when F is a formula of the classical predicate calculus which obeys certain syntactic restrictions. For example, for the monadic predicate calculus and the Gödel or ∃ ... ∃∀∀∃ ... ∃ prefix class we obtain lower and upper nondeterministic time bounds of the form cn/log n. The main tool in in these proofs is a finite version...
A number of decision problems that are unsolvable in general are solvable when restricted to systems with sufficiently simple "loop structure". Examples of such problems are the equivalence problems for flowchart schemata with nonintersecting loops and for the LOOP(l) programs of Meyer and Ritchie. We here present a theorem that gives a unifying view of the solvability of both of these problems,...
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