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 characterization for decidable separability by piecewise testable languages"
  Abstract: The separability problem for word languages of a class $\mathcal{C}$ by languages of a class $\mathcal{S}$ asks, for two given languages $I$ and $E$ from $\mathcal{C}$, whether there exists a language $S$ from $\mathcal{S}$ that includes $I$ and excludes $E$, that is, $I \subseteq S$ and $S\cap E = \emptyset$. In this work, we assume some mild closure properties for $\mathcal{C}$ and study for which such classes separability by a piecewise testable language (PTL) is decidable. We characterize these classes in terms of decidability of (two variants of) an unboundedness problem. From this, we deduce that separability by PTL is decidable for a number of language classes, such as the context-free languages and languages of labeled vector addition systems. Furthermore, it follows that separability by PTL is decidable if and only if one can compute for any language of the class its downward closure wrt. the scattered substring ordering (i.e., if the set of scattered substrings of any language of the class is effectively regular). #R##N#The obtained decidability results contrast some undecidability results. In fact, for all (non-regular) language classes that we present as examples with decidable separability, it is undecidable whether a given language is a PTL itself. #R##N#Our characterization involves a result of independent interest, which states that for any kind of languages $I$ and $E$, non-separability by PTL is equivalent to the existence of common patterns in $I$ and $E$.

Connected through hyperedge HE-1 (induced by source paper "regular separability and intersection emptiness are independent problems"):
  with members:
    - "separation for dot depth two"
    - "the covering problem"
    - "regular separability of one counter automata"
    - "separating regular languages with first order logic"
    - "regular separability of parikh automata"
    - "separability of reachability sets of vector addition systems"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "an approach to computing downward closures"

Connected through hyperedge HE-2 (induced by source paper "ptl separability and closures for wqos on words"):
  with members:
    - "the diagonal problem for higher order recursion schemes is decidable"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "demystifying reachability in vector addition systems"
    - "computing downward closures for stacked counter automata"

Connected through hyperedge HE-3 (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"
    - "alternating towers and piecewise testable separators"
    - "separating regular languages by piecewise testable and unambiguous languages"
    - "efficient separability of regular languages by subsequences and suffixes"

Connected through hyperedge HE-4 (induced by source paper "on the upward downward closures of petri nets"):
  with members:
    - "complexity of regular abstractions of one counter languages"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "an approach to computing downward closures"
    - "finite automata for the sub and superword closure of cfls descriptional and computational complexity"
    - "computing downward closures for stacked counter automata"
    - "on selective unboundedness of vass"

Connected through hyperedge HE-5 (induced by source paper "the diagonal problem for higher order recursion schemes is decidable"):
  with members:
    - "on word and frontier languages of unsafe higher order grammars"
    - "unboundedness and downward closures of higher order pushdown automata"
    - "ordered tree pushdown systems"
    - "an approach to computing downward closures"
    - "using models to model check recursive schemes"
    - "finite automata for the sub and superword closure of cfls descriptional and computational complexity"
    - "computing downward closures for stacked counter automata"
    - "complexity of model checking recursion schemes for fragments of the modal mu calculus"

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)