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: "separating regular languages by piecewise testable and unambiguous languages"
  Abstract: Separation is a classical problem asking whether, given two sets belonging to some class, it is possible to separate them by a set from a smaller class. We discuss the separation problem for regular languages. We give a Ptime 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 disjoint. The proof refines an algebraic argument from Almeida and the third author. When separation is possible, we also express a separator by saturating one of the original languages by a suitable congruence. Following the same line, we show that one can as well decide whether two regular languages can be separated by an unambiguous language, albeit with a higher complexity.

Connected through hyperedge HE-1 (induced by source paper "partially ordered automata and piecewise testability"):
  with members:
    - "the covering problem"
    - "complexity of detectability opacity and a diagnosability for modular discrete event systems"
    - "complexity of universality and related problems for partially ordered nfas"
    - "separability by piecewise testable languages is ptime complete"
    - "complexity of deciding detectability in discrete event systems"
    - "piecewise testable languages and nondeterministic automata"
    - "the height of piecewise testable languages and the complexity of the logic of subwords"
    - "on separation by locally testable and locally threshold testable languages"

Connected through hyperedge HE-2 (induced by source paper "separation for dot depth two"):
  with members:
    - "the covering problem"
    - "generic results for concatenation hierarchies"
    - "separating regular languages with first order logic"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "efficient separability of regular languages by subsequences and suffixes"

Connected through hyperedge HE-3 (induced by source paper "covering and separation for logical fragments with modular predicates"):
  with members:
    - "separation for dot depth two"
    - "the covering problem"
    - "adding successor a transfer theorem for separation and covering"
    - "separating regular languages with first order logic"
    - "adding modular predicates to first order fragments"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "one quantifier alternation in first order logic with modular predicates"
    - "efficient separability of regular languages by subsequences and suffixes"

Connected through hyperedge HE-4 (induced by source paper "the word problem for omega terms over the trotter weil hierarchy"):
  with members:
    - "the separation problem for regular languages by piecewise testable 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-5 (induced by source paper "regular separability of one counter automata"):
  with members:
    - "regular separability of well structured transition systems"
    - "separating regular languages with first order logic"
    - "regular separability of parikh automata"
    - "separability of reachability sets of vector addition systems"
    - "invisible pushdown languages"
    - "reachability in two dimensional vector addition systems with states is pspace complete"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "reachability in two clock timed automata is pspace complete"

Connected through hyperedge HE-6 (induced by source paper "adding successor a transfer theorem for separation and covering"):
  with members:
    - "the covering problem"
    - "separating regular languages with two quantifier alternations"
    - "going higher in first order quantifier alternation hierarchies on words"
    - "separating regular languages with first order logic"
    - "quantifier alternation for infinite words"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "efficient separability of regular languages by subsequences and suffixes"
    - "around dot depth one"

Connected through hyperedge HE-7 (induced by source paper "on the height of towers of subsequences and prefixes"):
  with members:
    - "separability by piecewise testable languages is ptime complete"
    - "on the index of simon s congruence for piecewise testability"
    - "on upper and lower bounds on the length of alternating towers"
    - "a characterization for decidable separability by piecewise testable languages"
    - "alternating towers and piecewise testable separators"
    - "efficient separability of regular languages by subsequences and suffixes"

Connected through hyperedge HE-8 (induced by source paper "going higher in first order quantifier alternation hierarchies on words"):
  with members:
    - "separating regular languages with two quantifier alternations"
    - "separating regular languages with first order logic"
    - "on the index of simon s congruence for piecewise testability"
    - "going higher in the first order quantifier alternation hierarchy on words"
    - "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)