BPA bisimilarity is EXPTIME-hard

Given a basic process algebra (BPA) and two stack symbols, the BPA bisimilarity problem asks whether the two stack symbols are bisimilar. We show that this problem is EXPTIME-hard.

► It is shown that BPA bisimilarity is EXPTIME-hard.
► This improves on PSPACE-hardness, which was shown by Srba in 2002.
► The proof is based on EXPTIME-hardness of a reachability game involving succinct counters.