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: "the separation problem for regular languages by piecewise testable languages"
  Abstract: Separation is a classical problem in mathematics and computer science. It asks whether, given two sets belonging to some class, it is possible to separate them by another set of a smaller class. We present and discuss the separation problem for regular languages. We then give a direct polynomial time algorithm to check whether two given regular languages are separable by a piecewise testable language, that is, whether a $B{\Sigma}1(<)$ sentence can witness that the languages are indeed disjoint. The proof is a reformulation and a refinement of an algebraic argument already given by Almeida and the second author.

Connected through hyperedge HE-1 (induced by source paper "the word problem for omega terms over the trotter weil hierarchy"):
  with members:
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "ehrenfeucht fraisse games on omega terms"
    - "the join levels of the trotter weil hierarchy are decidable"
    - "on logical hierarchies withinfoâ definable languages"
    - "the fo2 alternation hierarchy is decidable"
    - "representations of stream processors using nested fixed points"
    - "structure theorem and strict alternation hierarchy for fo2 on words"

Connected through hyperedge HE-2 (induced by source paper "a note on decidable separability by piecewise testable languages"):
  with members:
    - "on the state complexity of closures and interiors of regular languages with subwords and superwords"
    - "demystifying reachability in vector addition systems"
    - "an approach to computing downward closures"
    - "computing downward closures for stacked counter automata"
    - "the power of well structured systems"
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "piecewise testable tree languages"
    - "model checking vector addition systems with one zero test"

Connected through hyperedge HE-3 (induced by source paper "separating regular languages by piecewise testable and unambiguous languages"):
  with members:
    - "deciding definability in fo2 h v on trees"
    - "efficient separability of regular languages by subsequences and suffixes"

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.FL(Formal Languages and Automata Theory)