Improving the previously known best bound, we show that any
recursively enumerable language can be generated with a non-returning
parallel communicating (PC) grammar system having six context-free
components. We also present a non-returning universal PC grammar
system generating unary languages, that is, a system where not only
the number of components, but also the number of productions and the
number of nonterminals are limited by certain constants, and
these size parameters do not depend on the generated language. |