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: "improved approximation for the directed spanner problem"
  Abstract: We prove that the size of the sparsest directed k-spanner of a graph can be approximated in polynomial time to within a factor of $\tilde{O}(\sqrt{n})$, for all k >= 3. This improves the $\tilde{O}(n^{2/3})$-approximation recently shown by Dinitz and Krauthgamer.

Connected through hyperedge HE-1 (induced by source paper "a fast network decomposition algorithm and its applications to constant time distributed computation"):
  with members:
    - "distributed symmetry breaking in hypergraphs"
    - "distributed approximation algorithms for weighted shortest paths"
    - "the locality of distributed symmetry breaking"
    - "super fast 3 ruling sets"
    - "on the locality of some np complete problems"
    - "deterministic distributed vertex coloring in polylogarithmic time"
    - "directed spanners via flow based linear programs"

Connected through hyperedge HE-2 (induced by source paper "fixed parameter and approximation algorithms a new look"):
  with members:
    - "completely inapproximable monotone and antimonotone parameterized problems"
    - "on problems as hard as cnf sat"
    - "fixed parameter tractability of multicut parameterized by the size of the cutset"
    - "known algorithms on graphs of bounded treewidth are probably optimal"
    - "almost 2 sat is fixed parameter tractable"

Connected through hyperedge HE-3 (induced by source paper "everywhere sparse spanners via dense subgraphs"):
  with members:
    - "fault tolerant spanners better and simpler"
    - "approximating sparsest cut in graphs of bounded treewidth"
    - "dial a ride from k forest"
    - "lower stretch spanning trees"
    - "nearly linear time algorithms for graph partitioning graph sparsification and solving linear systems"

Connected through hyperedge HE-4 (induced by source paper "label cover instances with large girth and the hardness of approximating basic k spanner"):
  with members:
    - "parameters of two prover one round game and the hardness of connectivity problems"
    - "fault tolerant spanners better and simpler"
    - "directed spanners via flow based linear programs"
    - "transitive closure spanners"

Connected through hyperedge HE-5 (induced by source paper "collective additive tree spanners of bounded tree breadth graphs with generalizations and consequences"):
  with members:
    - "label cover instances with large girth and the hardness of approximating basic k spanner"
    - "directed spanners via flow based linear programs"
    - "transitive closure spanners"
    - "lower stretch spanning trees"

Connected through hyperedge HE-6 (induced by source paper "ibgp and constrained connectivity"):
  with members:
    - "directed spanners via flow based linear programs"
    - "transitive closure spanners"

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.DS(Data Structures and Algorithms)