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 internal model approach to optimal frequency regulation in power grids with time varying voltages"
  Abstract: This paper studies the problem of frequency regulation in power grids under unknown and possible time-varying load changes, while minimizing the generation costs. We formulate this problem as an output agreement problem for distribution networks and address it using incremental passivity and distributed internal-model-based controllers. Incremental passivity enables a systematic approach to study convergence to the steady state with zero frequency deviation and to design the controller in the presence of time-varying voltages, whereas the internal-model principle is applied to tackle the uncertain nature of the loads.

Connected through hyperedge HE-1 (induced by source paper "an energy based analysis of reduced order models of networked synchronous machines"):
  with members:
    - "a survey on modeling of microgrids from fundamental physics to phasors and voltage sources"

Connected through hyperedge HE-2 (induced by source paper "optimal regulation of flow networks with transient constraints"):
  with members:
    - "on resilient control of dynamical flow networks"
    - "modeling and control of hvdc transmission systems from theory to practice and back"
    - "a compartmental model for traffic networks and its dynamical behavior"
    - "zero forcing sets and controllability of dynamical systems defined on graphs"
    - "zero forcing number constrained matchings and strong structural controllability"
    - "dynamic coupling design for nonlinear output agreement and time varying flow control"

Connected through hyperedge HE-3 (induced by source paper "conditions on shifted passivity of port hamiltonian systems"):
  with members:
    - "permanent magnet synchronous motors are globally asymptotically stabilizable with pi current control"
    - "new results on disturbance rejection for energy shaping controlled port hamiltonian systems"
    - "on differential passivity of physical systems"
    - "a differential lyapunov framework for contraction analysis"

Connected through hyperedge HE-4 (induced by source paper "stability and frequency regulation of inverters with capacitive inertia"):
  with members:
    - "output impedance diffusion into lossy power lines"
    - "nonlinear analysis of an improved swing equation"

Connected through hyperedge HE-5 (induced by source paper "passivity based design of sliding modes for optimal load frequency control"):
  with members:
    - "multi level power imbalance allocation control for secondary frequency control in power systems"
    - "a novel reduced model for electrical networks with constant power loads"

Connected through hyperedge HE-6 (induced by source paper "distributed resource allocation over a class of nonlinear multi agent systems"):
  with members:
    - "reaching an optimal consensus dynamical systems that compute intersections of convex sets"

Connected through hyperedge HE-7 (induced by source paper "adaptation and disturbance rejection for output synchronization of incrementally output feedback passive systems"):
  with members:
    - "output synchronization of nonlinear systems under input disturbances"
    - "dynamic coupling design for nonlinear output agreement and time varying flow control"
    - "distributed consensus of linear multi agent systems with adaptive dynamic protocols"

Connected through hyperedge HE-8 (induced by source paper "distributed robust adaptive frequency control of power systems with dynamic loads"):
  with members:
    - "design and stability of load side primary frequency control in power systems"
    - "energy imbalance management using a robust pricing scheme"

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)