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: "general and fractional hypertree decompositions hard and easy cases"
  Abstract: Hypertree decompositions, as well as the more powerful generalized hypertree decompositions (GHDs), and the yet more general fractional hypertree decompositions (FHD) are hypergraph decomposition methods successfully used for answering conjunctive queries and for the solution of constraint satisfaction problems. Every hypergraph H has a width relative to each of these decomposition methods: its hypertree width hw(H), its generalized hypertree width ghw(H), and its fractional hypertree width fhw(H), respectively. #R##N#It is known that hw(H) <= k can be checked in polynomial time for fixed k, while checking ghw(H) <= k is NP-complete for any k greater than or equal to 3. The complexity of checking fhw(H) <= k for a fixed k has been open for more than a decade. #R##N#We settle this open problem by showing that checking fhw(H) <= k is NP-complete, even for k=2. The same construction allows us to prove also the NP-completeness of checking ghw(H) <= k for k=2. After proving these hardness results, we identify meaningful restrictions, for which checking for bounded ghw or fhw becomes tractable.

Connected through hyperedge HE-1 (induced by source paper "complexity analysis of general and fractional hypertree decompositions"):
  with members:
    - "hyperbench a benchmark and tool for hypergraphs and empirical findings"
    - "tree projections and constraint optimization problems fixed parameter tractability and parallel algorithms"
    - "size bounds and query plans for relational joins"
    - "constraint solving via fractional edge covers"
    - "an analytical study of large sparql query logs"
    - "old techniques for new join algorithms a case study in rdf processing"
    - "myhill nerode methods for hypergraphs"
    - "faq questions asked frequently"

Connected through hyperedge HE-2 (induced by source paper "potential maximal cliques parameterized by edge clique cover"):
  with members:
    - "hyperbench a benchmark and tool for hypergraphs and empirical findings"
    - "constraint solving via fractional edge covers"
    - "on the tractability of optimization problems on h graphs"
    - "ranked enumeration of minimal triangulations"
    - "positive instance driven dynamic programming for treewidth"
    - "on distance d independent set and other problems in graphs with few minimal separators"
    - "on the number of minimal separators in graphs"
    - "algorithms parameterized by vertex cover and modular width through potential maximal cliques"

Connected through hyperedge HE-3 (induced by source paper "point width and max csps"):
  with members:
    - "understanding the complexity of sat using knowledge compilation"
    - "constraint solving via fractional edge covers"
    - "the complexity of general valued csps seen from the other side"
    - "a dichotomy theorem for nonuniform csps"
    - "the proof of csp dichotomy conjecture"
    - "the complexity of finite valued csps"
    - "understanding model counting for beta acyclic cnf formulas"
    - "tractable optimization problems through hypergraph based structural restrictions"

Connected through hyperedge HE-4 (induced by source paper "a join based hybrid parameter for constraint satisfaction"):
  with members:
    - "binary constraint satisfaction problems defined by excluded topological minors"
    - "size bounds and query plans for relational joins"
    - "constraint solving via fractional edge covers"
    - "what do shannon type inequalities submodular width and disjunctive datalog have to do with one another"
    - "faq questions asked frequently"
    - "tractable hypergraph properties for constraint satisfaction and conjunctive queries"
    - "hypertree decompositions and tractable queries"
    - "using pivot consistency to decompose and solve functional csps"

Connected through hyperedge HE-5 (induced by source paper "a more general theory of static approximations for conjunctive queries"):
  with members:
    - "constraint solving via fractional edge covers"
    - "semantic acyclicity under constraints"
    - "decidability results for the boundedness problem"
    - "querying the guarded fragment"
    - "beyond hypertree width decomposition methods without decompositions"
    - "hypertree decompositions and tractable queries"

Connected through hyperedge HE-6 (induced by source paper "solving linear programs on factorized databases"):
  with members:
    - "ranked enumeration of conjunctive query results"
    - "efficient inverse maintenance and faster algorithms for linear programming"
    - "aggregation in probabilistic databases via knowledge compilation"
    - "combining relational algebra sql constraint modelling and local search"
    - "hypertree decompositions and tractable queries"

Connected through hyperedge HE-7 (induced by source paper "hyperbench a benchmark and tool for hypergraphs and empirical findings"):
  with members:
    - "size bounds and query plans for relational joins"
    - "constraint solving via fractional edge covers"
    - "an analytical study of large sparql query logs"
    - "old techniques for new join algorithms a case study in rdf processing"
    - "xcsp3 an integrated format for benchmarking combinatorial constrained problems"
    - "faq questions asked frequently"
    - "joins via geometric resolutions worst case and beyond"
    - "aggregation and ordering in factorised databases"

Connected through hyperedge HE-8 (induced by source paper "hypertree decompositions revisited for pgms"):
  with members:
    - "size bounds and query plans for relational joins"
    - "faq questions asked frequently"
    - "skew strikes back new developments in the theory of join algorithms"
    - "join graph propagation algorithms"
    - "exploiting contextual independence in probabilistic inference"
    - "exploiting causal independence in bayesian network inference"

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)