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: "embedding ac power flow with voltage control in the complex plane the case of analytic continuation via pade approximants"
  Abstract: This paper proposes a method to embed the AC power flow problem with voltage magnitude constraints in the complex plane. Modeling the action of network controllers that regulate the magnitude of voltage phasors is a challenging task in the complex plane as it has to preserve the framework of holo- morphicity for obtention of these complex variables with fix ed magnitude. Hence this paper presents a significant step in th e development of the idea of Holomorphic Embedding Load Flow Method (HELM) (1), introduced in 2012, that exploits the theory of analytic continuation, especially the monodromy theorem (2) for resolving issues that have plagued conventional numerical methods for decades. This paper also illustrates the indispensable role of Padapproximants for analytic continuation of complex functions, expressed as power series, beyond the boundary of convergence of the series. Later the paper demonstrates the superiority of the proposed method over the well-established Newton-Raphson as well as the recently developed semidefini te and moment relaxation of power flow problems.

Connected through hyperedge HE-1 (induced by source paper "online voltage stability assessment for load areas based on the holomorphic embedding method"):
  with members:
    - "multi stage holomorphic embedding method for calculating the power voltage curve"
    - "embedding ac power flow in the complex plane part i modelling and mathematical foundation"
    - "embedding ac power flow in the complex plane part ii a reliable framework for voltage collapse analysis"
    - "fundamentals of the holomorphic embedding load flow method"

Connected through hyperedge HE-2 (induced by source paper "approximate analytical solutions of power flow equations based on multi dimensional holomorphic embedding method"):
  with members:
    - "online voltage stability assessment for load areas based on the holomorphic embedding method"
    - "embedding ac power flow in the complex plane part i modelling and mathematical foundation"
    - "embedding ac power flow in the complex plane part ii a reliable framework for voltage collapse analysis"

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.SY(Systems and Control)