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: "an optimal bifactor approximation algorithm for the metric uncapacitated facility location problem"
  Abstract: We obtain a 1.5-approximation algorithm for the metric uncapacitated facility location problem (UFL), which improves on the previously best known 1.52-approximation algorithm by Mahdian, Ye and Zhang. Note, that the approximability lower bound by Guha and Khuller is 1.463. #R##N#An algorithm is a {\em ($\lambda_f$,$\lambda_c$)-approximation algorithm} if the solution it produces has total cost at most $\lambda_f \cdot F^* + \lambda_c \cdot C^*$, where $F^*$ and $C^*$ are the facility and the connection cost of an optimal solution. Our new algorithm, which is a modification of the $(1+2/e)$-approximation algorithm of Chudak and Shmoys, is a (1.6774,1.3738)-approximation algorithm for the UFL problem and is the first one that touches the approximability limit curve $(\gamma_f, 1+2e^{-\gamma_f})$ established by Jain, Mahdian and Saberi. As a consequence, we obtain the first optimal approximation algorithm for instances dominated by connection costs. When combined with a (1.11,1.7764)-approximation algorithm proposed by Jain et al., and later analyzed by Mahdian et al., we obtain the overall approximation guarantee of 1.5 for the metric UFL problem. We also describe how to use our algorithm to improve the approximation ratio for the 3-level version of UFL.

Connected through hyperedge HE-1 (induced by source paper "constant factor approximation algorithms for parity constrained facility location problems"):
  with members:
    - "a tree structure for dynamic facility location"
    - "on facility location with general lower bounds"
    - "online facility location with deletions"
    - "optimal parameterized algorithms for planar facility location problems using voronoi diagrams"
    - "facility location in evolving metrics"
    - "lp based algorithms for capacitated facility location"
    - "eight fifth approximation for the path tsp"
    - "local search based approximation algorithms for mobile facility location problems"

Connected through hyperedge HE-2 (induced by source paper "concave connection cost facility location and the star inventory routing problem"):
  with members:
    - "inventory routing problem with facility location"
    - "on the factor revealing lp approach for facility location with penalties"
    - "approximation algorithms for inventory problems with submodular or routing costs"
    - "greedy facility location algorithms analyzed using dual fitting with factor revealing lp"

Connected through hyperedge HE-3 (induced by source paper "efficient black box reductions for separable cost sharing"):
  with members:
    - "a characterization of undirected graphs admitting optimal cost shares"
    - "designing networks with good equilibria under uncertainty"
    - "greedy algorithms for steiner forest"
    - "resource buying games"
    - "strategic cooperation in cost sharing games"
    - "an o lognloglogn upper bound on the price of stability for undirected shapley network design games"
    - "approximation schemes for steiner forest on planar graphs and graphs of bounded treewidth"

Connected through hyperedge HE-4 (induced by source paper "on facility location with general lower bounds"):
  with members:
    - "improved approximation guarantees for lower bounded facility location"
    - "greedy facility location algorithms analyzed using dual fitting with factor revealing lp"

Connected through hyperedge HE-5 (induced by source paper "constant approximation for k median and k means with outliers via iterative rounding"):
  with members:
    - "approximation schemes for clustering with outliers"
    - "the power of local search for clustering"
    - "better guarantees for k means and euclidean k median by primal dual algorithms"
    - "local search yields a ptas for k means in doubling metrics"
    - "improved and simplified inapproximability for k means"
    - "an improved approximation for k median and positive correlation in budgeted optimization"
    - "improved approximation algorithms for matroid and knapsack median problems and applications"
    - "approximating k median via pseudo approximation"

Connected through hyperedge HE-6 (induced by source paper "approximation algorithms for stochastic and risk averse optimization"):
  with members:
    - "greedy facility location algorithms analyzed using dual fitting with factor revealing lp"

Connected through hyperedge HE-7 (induced by source paper "better guarantees for k means and euclidean k median by primal dual algorithms"):
  with members:
    - "the power of local search for clustering"
    - "local search yields a ptas for k means in doubling metrics"
    - "improved and simplified inapproximability for k means"
    - "the hardness of approximation of euclidean k means"
    - "an improved approximation for k median and positive correlation in budgeted optimization"
    - "approximating k median via pseudo approximation"
    - "simpler analyses of local search algorithms for facility location"
    - "k means requires exponentially many iterations even in the plane"

Connected through hyperedge HE-8 (induced by source paper "on the factor revealing lp approach for facility location with penalties"):
  with members:
    - "a systematic approach to bound factor revealing lps and its application to the metric and squared metric facility location problems"
    - "greedy facility location algorithms analyzed using dual fitting with factor revealing lp"

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)