Summary: Effective Lossy Queue Languages
Parosh Aziz Abdulla 1 , Luc Boasson 2 , and Ahmed Bouajjani 2
1 DoCS, Uppsala University, P.O. Box 325, S75105 Uppsala, Sweden.
Liafa, Univ. of Paris 7, Case 7014, 2 place Jussieu, F75251 Paris Cedex 05, France.
Abstract. Although the set of reachable states of a lossy channel system
(LCS) is regular, it is wellknown that this set cannot be constructed
effectively. In this paper, we characterize significant classes of LCS for
which the set of reachable states can be computed. Furthermore, we
show that, for slight generatlizations of these classes, computability can
no longer be achieved.
To carry out our study, we define rewriting systems which capture the be
haviour of LCS, in the sense that (i) they have a FIFOlike semantics and
(ii) their languages are downward closed with respect to the substring
relation. The main result of the paper shows that, for contextfree rewrit
ing systems, the corresponding language can be computed. An interesting
consequence of our results is that we get a characterization of classes of
metatransitions whose postimages can be effectively constructed. These