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 hardness of solving simple word equations"
  Abstract: We investigate the class of regular-ordered word equations. In such equations, each variable occurs at most once in each side and the order of the variables occurring in both sides is the preserved (the variables can be, however, separated by potentially distinct constant factors). Surprisingly, we obtain that solving such simple equations, even when the sides contain exactly the same variables, is NP-hard. By considerations regarding the combinatorial structure of the minimal solutions of the more general quadratic equations we obtain that the satisfiability problem for regular-ordered equations is in NP. Finally, we also show that a related class of simple word equations, that generalises one-variable equations, is in P.

Connected through hyperedge HE-1 (induced by source paper "the theory of concatenation over finite models"):
  with members:
    - "dynamic complexity of document spanners"
    - "matching patterns with variables"
    - "weight annotation in information extraction"
    - "constant delay enumeration for nondeterministic document spanners"
    - "split correctness in information extraction"
    - "deterministic regular expressions with back references"
    - "constant delay algorithms for regular document spanners"
    - "decidability complexity and expressiveness of first order logic over the subword ordering"

Connected through hyperedge HE-2 (induced by source paper "matching patterns with variables"):
  with members:
    - "graph and string parameters connections between pathwidth cutwidth and the locality number"
    - "quadratic word equations with length constraints counter systems and presburger arithmetic with divisibility"
    - "the runs theorem"
    - "decidability complexity and expressiveness of first order logic over the subword ordering"
    - "detecting one variable patterns"
    - "solutions of word equations over partially commutative structures"
    - "a parameterized study of maximum generalized pattern matching problems"
    - "searching of gapped repeats and subrepetitions in a word"

Connected through hyperedge HE-3 (induced by source paper "on solving word equations using sat"):
  with members:
    - "recompression a simple and powerful technique for word equations"

Connected through hyperedge HE-4 (induced by source paper "quadratic word equations with length constraints counter systems and presburger arithmetic with divisibility"):
  with members:
    - "the satisfiability of extended word equations the boundary between decidability and undecidability"
    - "recompression a simple and powerful technique for word equations"
    - "algorithmic metatheorems for decidable ltl model checking over infinite systems"
    - "unary finite automata vs arithmetic progressions"

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)