human: Given a node-centered hypergraph: <hypergraph>, where nodes represent papers and hyperedges represent sets of papers co-cited by a source paper.

Details of the hypergraph:
Center node: "conjunctive queries over trees"
  Abstract: We study the complexity and expressive power of conjunctive queries over unranked labeled trees represented using a variety of structure relations such as ``child'', ``descendant'', and ``following'' as well as unary relations for node labels. We establish a framework for characterizing structures representing trees for which conjunctive queries can be evaluated efficiently. Then we completely chart the tractability frontier of the problem and establish a dichotomy theorem for our axis relations, i.e., we find all subset-maximal sets of axes for which query evaluation is in polynomial time and show that for all other cases, query evaluation is NP-complete. All polynomial-time results are obtained immediately using the proof techniques from our framework. Finally, we study the expressiveness of conjunctive queries over trees and show that for each conjunctive query, there is an equivalent acyclic positive query (i.e., a set of acyclic conjunctive queries), but that in general this query is not of polynomial size.

Connected through hyperedge HE-1 (induced by source paper "conjunctive queries on probabilistic graphs combined complexity"):
  with members:
    - "tractable lineages on treelike instances limits and extensions"
    - "combined tractability of query evaluation via tree automata and cycluits extended version"
    - "provenance circuits for trees and treelike instances extended version"
    - "complexity of counting subgraphs only the boundedness of the vertex cover number counts"
    - "understanding model counting for beta acyclic cnf formulas"
    - "symmetric weighted first order model counting"

Connected through hyperedge HE-2 (induced by source paper "json data model query languages and schema specification"):
  with members:
    - "a formal presentation of mongodb extended version"
    - "deterministic automata for unordered trees"

Connected through hyperedge HE-3 (induced by source paper "monadic datalog containment on trees"):
  with members:
    - "a note on monadic datalog on unranked trees"
    - "monadic datalog and the expressive power of languages for web information extraction"

Connected through hyperedge HE-4 (induced by source paper "logics for unranked trees an overview"):
  with members:
    - "on the complexity of nonrecursive xquery and functional query languages on complex values"
    - "tight lower bounds for query processing on streaming and external memory data"
    - "monadic datalog and the expressive power of languages for web information extraction"

Question: Please tell me which class the center node belongs to. The 40 classes are: cs.NA(Numerical Analysis), cs.MM(Multimedia), cs.LO(Logic in Computer Science), cs.CY(Computers and Society), cs.CR(Cryptography and Security), cs.DC(Distributed, Parallel, and Cluster Computing), cs.HC(Human-Computer Interaction), cs.CE(Computational Engineering, Finance, and Science), cs.NI(Networking and Internet Architecture), cs.CC(Computational Complexity), cs.AI(Artificial Intelligence), cs.MA(Multiagent Systems), cs.GL(General Literature), cs.NE(Neural and Evolutionary Computing), cs.SC(Symbolic Computation), cs.AR(Hardware Architecture), cs.CV(Computer Vision and Pattern Recognition), cs.GR(Graphics), cs.ET(Emerging Technologies), cs.SY(Systems and Control), cs.CG(Computational Geometry), cs.OH(Other Computer Science), cs.PL(Programming Languages), cs.SE(Software Engineering), cs.LG(Machine Learning), cs.SD(Sound), cs.SI(Social and Information Networks), cs.RO(Robotics), cs.IT(Information Theory), cs.PF(Performance), cs.CL(Computation and Language), cs.IR(Information Retrieval), cs.MS(Mathematical Software), cs.FL(Formal Languages and Automata Theory), cs.DS(Data Structures and Algorithms), cs.OS(Operating Systems), cs.GT(Computer Science and Game Theory), cs.DB(Databases), cs.DL(Digital Libraries), cs.DM(Discrete Mathematics). Directly output the class name.

gpt: cs.DB(Databases)