On Models of a Nondeterministic Computation

In this paper we consider a noudeterministic computation performed by deterministic multi-head 2-way antomata with a read-only access to an auxiliary memory. The memory contains additional data (a guess) and the computation is successful iff it is successful for some memory content. Also we consider the case of restricted guesses in which a guess should satisfy some constraint. We show that the standard complexity classes such as L; NL, P, NP, PSPACE can be characterized in terms of these models of nondeterministic computation. These characterizations differ from the well-known ones by absence of alternation.