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: "bpa bisimilarity is exptime hard"
  Abstract: Given a basic process algebra (BPA) and two stack symbols, the BPA bisimilarity problem asks whether the two stack symbols are bisimilar. We show that this problem is EXPTIME-hard.

Connected through hyperedge HE-1 (induced by source paper "bisimulation equivalence of first order grammars is ackermann complete"):
  with members:
    - "equivalence of pushdown automata via first order grammars"
    - "complexity hierarchies beyond elementary"
    - "branching bisimilarity checking for prs"
    - "bisimulation equivalence of first order grammars"
    - "complexity bounds for ordinal based termination"
    - "beyond language equivalence on visibly pushdown automata"
    - "the bisimulation problem for equational graphs of finite out degree"

Connected through hyperedge HE-2 (induced by source paper "expspace hardness of behavioural equivalences of succinct one counter nets"):
  with members:
    - "simulation problems over one counter nets"
    - "reachability in succinct one counter games"
    - "beyond language equivalence on visibly pushdown automata"
    - "model checking probabilistic timed automata with one or two clocks"

Connected through hyperedge HE-3 (induced by source paper "branching bisimilarity of normed bpa processes as a rational monoid"):
  with members:
    - "two lower bounds for bpa"
    - "branching bisimilarity on normed bpa is exptime complete"
    - "a polynomial time algorithm for deciding branching bisimilarity on totally normed bpa"
    - "branching bisimilarity of normed bpa processes is in nexptime"
    - "branching bisimilarity checking for prs"
    - "bisimilarity on basic process algebra is in 2 exptime an explicit proof"

Connected through hyperedge HE-4 (induced by source paper "branching bisimilarity on normed bpa is exptime complete"):
  with members:
    - "dividing line between decidable pda s and undecidable ones"
    - "a polynomial time algorithm for deciding branching bisimilarity on totally normed bpa"
    - "branching bisimilarity of normed bpa processes is in nexptime"
    - "branching bisimilarity checking for prs"
    - "bisimilarity on basic process algebra is in 2 exptime an explicit proof"
    - "equivalence checking on infinite state systems techniques and results"

Connected through hyperedge HE-5 (induced by source paper "bisimulation equivalence of first order grammars"):
  with members:
    - "complexity hierarchies beyond elementary"
    - "dividing line between decidable pda s and undecidable ones"
    - "branching bisimilarity checking for prs"
    - "bisimilarity on basic process algebra is in 2 exptime an explicit proof"
    - "on the complexity of the equivalence problem for probabilistic automata"
    - "the bisimulation problem for equational graphs of finite out degree"

Connected through hyperedge HE-6 (induced by source paper "branching bisimilarity of normed bpa processes is in nexptime"):
  with members:
    - "branching bisimilarity on normed bpa is exptime complete"
    - "a polynomial time algorithm for deciding branching bisimilarity on totally normed bpa"
    - "branching bisimilarity checking for prs"
    - "bisimilarity on basic process algebra is in 2 exptime an explicit proof"

Connected through hyperedge HE-7 (induced by source paper "bisimilarity on basic process algebra is in 2 exptime an explicit proof"):
  with members:
    - "bisimilarity of pushdown systems is nonelementary"
    - "beyond language equivalence on visibly pushdown automata"
    - "the bisimulation problem for equational graphs of finite out degree"

Connected through hyperedge HE-8 (induced by source paper "bisimilarity of probabilistic pushdown automata"):
  with members:
    - "bisimilarity on basic process algebra is in 2 exptime an explicit proof"
    - "beyond language equivalence on visibly pushdown automata"
    - "the bisimulation problem for equational graphs of finite out degree"

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)