Hardness Results for Coverability Problem of Well-Structured Pushdown Systems.

Well-Structured Pushdown Systems (WSPDSs) are push-down systems extended with states and stack alphabet to be vectors, for modeling (restricted) recursive concurrent programs. It has been considered to be "very close to the border of undecidability". In this paper, we prove some hardness results for the coverability problem of WSPDSs. We show that for WSPDS with three dimensional vectors as states and WSPDS with three dimensional vectors as stack alphabet, the coverability problem becomes Ackermann-hard.