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: "a polynomial multigrid smoother for the iterative solution of the heterogeneous helmholtz problem"
  Abstract: In this paper we develop a robust multigrid preconditioned Krylov subspace method for the solution of heterogeneous indefinite Helmholtz problems. The preconditioning operator is constructed by discretizing the original Helmholtz equation on a complex stretched grid. As this preconditioning operator has the same wavenumber as the original problem, and the only difference stems from the complex stretching of the discretization grid, its spectrum closely approximates the spectrum of the original Helmholtz operator, resulting in a fast converging Krylov subspace method. We have analyzed a multigrid cycle based on polynomial smoothers and have found a condition on the parameters used in the complex stretching of the discretization grid such that the existence of a stable third order polynomial smoother is guaranteed. In practice we use three iterations of GMRES as a smoother, and have found this to be a viable smoother for the multigrid preconditioning process. We apply the method to various test problems and report on the observed convergence rate.

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.NA(Numerical Analysis)