1 research outputs found
The Power of Centralized PC Systems of Pushdown Automata
Parallel communicating systems of pushdown automata (PCPA) were introduced in
(Csuhaj-Varj{\'u} et. al. 2000) and in their centralized variants shown to be
able to simulate nondeterministic one-way multi-head pushdown automata. A
claimed converse simulation for returning mode (Balan 2009) turned out to be
incomplete (Otto 2012) and a language was suggested for separating these PCPA
of degree two (number of pushdown automata) from nondeterministic one-way
two-head pushdown automata. We show that the suggested language can be accepted
by the latter computational model. We present a different example over a single
letter alphabet indeed ruling out the possibility of a simulation between the
models. The open question about the power of centralized PCPA working in
returning mode is then settled by showing them to be universal. Since the
construction is possible using systems of degree two, this also improves the
previous bound three for generating all recursively enumerable languages.
Finally PCPAs are restricted in such a way that a simulation by multi-head
automata is possible