The Complexity of Some Problems on Subsequences and Supersequences
Journal of the ACM (JACM)
Representing and querying XML with incomplete information
ACM Transactions on Database Systems (TODS)
XML with incomplete information: models, properties, and query answering
Proceedings of the twenty-eighth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
Conjunctive query containment over trees
DBPL'07 Proceedings of the 11th international conference on Database programming languages
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We deal with the problem of deciding whether a given set of string patterns implies the presence of a fixed pattern. While checking whether a set of patterns occurs in a string is solvable in polynomial time, this implication problem is well known to be intractable. Here we consider a version of the problem when patterns in the set are required to be disjoint. We show that for such a version of the problem the situation is reversed: checking whether a set of patterns occurs in a string is NP-complete, but the implication problem is solvable in polynomial time.