Properties that characterize LOGCFL

Abstract

Two properties, called semi-unboundedness, and polynomial proof-size, are identified as key properties shared by the definitions of LOGCFL on several models of computations. The semi-unboundedness property leads to the definition of new models of computation based on unbounded fan-in circuits. These are circuits obtained from unbounded fan-in circuits by restricting the fan-in of gates of one type. A new characterization of LOGCFL is obtained on such a model in which the fan-in of the AND gates are bounded by a constant. This property also suggests new characterizations of LOGCFL on the following models: alternating Turing machines [CKS81], nondeterministic auxiliary pushdown automata [Co71], and bounded fan-in Boolean circuits [Co85].