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Summary: On the power of deep pushdown stacks
A. Arratia Quesada
Departament de Llenguatges i Sistemes Inform`atics
Universitat Polit`ecnica de Catalunya, Barcelona, Spain
I.A. Stewart
Department of Computer Science, Durham University,
Science Labs, South Road, Durham DH1 3LE, U.K.
Abstract
Inspired by recent work of Meduna on deep pushdown automata, we consider
the computational power of a class of basic program schemes, NPSDSs, based
around assignments, while-loops and non-deterministic guessing but with access
to a deep pushdown stack which, apart from having the usual push and pop
instructions, also has deep-push instructions which allow elements to be pushed
to stack locations deep within the stack. We syntactically define sub-classes of
NPSDSs by restricting the occurrences of pops, pushes and deep-pushes and
capture the complexity classes NP and PSPACE. Furthermore, we show that
all problems accepted by program schemes of NPSDSs are in EXPTIME.
1 Introduction
In automata theory, there is a variety of machine models, both restricting and en-
hancing pushdown automata, such as finite-turn pushdown automata [7, 23] and two-
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