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: "13 9 approximation for graphic tsp"
  Abstract: The Travelling Salesman Problem is one the most fundamental and most studied problems in approximation algorithms. For more than 30 years, the best algorithm known for general metrics has been Christofides's algorithm with approximation factor of 3/2, even though the so-called Held-Karp LP relaxation of the problem is conjectured to have the integrality gap of only 4/3. Very recently, significant progress has been made for the important special case of graphic metrics, first by Oveis Gharan et al., and then by Momke and Svensson. In this paper, we provide an improved analysis for the approach introduced by Momke and Svensson yielding a bound of 13/9 on the approximation factor, as well as a bound of 19/12+epsilon for any epsilon>0 for a more general Travelling Salesman Path Problem in graphic metrics.

Connected through hyperedge HE-1 (induced by source paper "reducing path tsp to tsp"):
  with members:
    - "beating the integrality ratio for s t tours in graphs"
    - "a 1 5 approximation for path tsp"
    - "better s t tours by gao trees"
    - "reassembling trees for the traveling salesman"
    - "eight fifth approximation for the path tsp"
    - "new inapproximability bounds for tsp"
    - "approximating minimum cost connected t joins"
    - "improving christofides algorithm for the s t path tsp"

Connected through hyperedge HE-2 (induced by source paper "characterizing the integrality gap of the subtour lp for the circulant traveling salesman problem"):
  with members:
    - "semidefinite programming relaxations of the traveling salesman problem and their integrality gaps"
    - "the unbounded integrality gap of a semidefinite relaxation of the traveling salesman problem"
    - "new inapproximability bounds for tsp"
    - "on approximation lower bounds for tsp with bounded metrics"

Connected through hyperedge HE-3 (induced by source paper "an improved approximation algorithm for tsp in the half integral case"):
  with members:
    - "towards improving christofides algorithm for half integer tsp"
    - "cover and conquer augmenting decompositions for connectivity problems"
    - "improved inapproximability for tsp"

Connected through hyperedge HE-4 (induced by source paper "5 4 approximation for symmetric tsp"):
  with members:
    - "low degree spanning trees of small weight"

Connected through hyperedge HE-5 (induced by source paper "a 1 5 approximation for path tsp"):
  with members:
    - "better s t tours by gao trees"
    - "reassembling trees for the traveling salesman"
    - "on the metric s t path traveling salesman problem"
    - "minimum cuts in near linear time"

Connected through hyperedge HE-6 (induced by source paper "a constant factor approximation algorithm for the asymmetric traveling salesman problem"):
  with members:
    - "the asymmetric traveling salesman path lp has constant integrality ratio"
    - "constant factor approximation for atsp with two edge weights"
    - "approximating atsp by relaxing connectivity"
    - "effective resistance reducing flows spectrally thin trees and asymmetric tsp"
    - "new inapproximability bounds for tsp"
    - "shorter tours by nicer ears 7 5 approximation for graphic tsp 3 2 for the path version and 4 3 for two edge connected subgraphs"
    - "improving christofides algorithm for the s t path tsp"
    - "the asymmetric traveling salesman problem on graphs with bounded genus"

Connected through hyperedge HE-7 (induced by source paper "shorter tours and longer detours uniform covers and a bit beyond"):
  with members:
    - "coloring down 3 2 approximation for special cases of the weighted tree augmentation problem"
    - "shorter tours by nicer ears 7 5 approximation for graphic tsp 3 2 for the path version and 4 3 for two edge connected subgraphs"
    - "the traveling salesman problem on cubic and subcubic graphs"

Connected through hyperedge HE-8 (induced by source paper "the unbounded integrality gap of a semidefinite relaxation of the traveling salesman problem"):
  with members:
    - "on the metric s t path traveling salesman problem"
    - "new inapproximability bounds for tsp"

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)