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 constant time algorithm for middle levels gray codes"
  Abstract: For any integer $n\geq 1$ a middle levels Gray code is a cyclic listing of all $n$-element and $(n+1)$-element subsets of $\{1,2,\ldots,2n+1\}$ such that any two consecutive subsets differ in adding or removing a single element. The question whether such a Gray code exists for any $n\geq 1$ has been the subject of intensive research during the last 30 years, and has been answered affirmatively only recently [T. M\"utze. Proof of the middle levels conjecture. To appear in Proc. London Math. Soc., 2014]. In a follow-up paper [T. M\"utze and J. Nummenpalo. An efficient algorithm for computing a middle levels Gray code. Proc. ESA, 2015] this existence proof was turned into an algorithm that computes each new set in the Gray code in time $\mathcal{O}(n)$ on average. In this work we complete this line of research by presenting an algorithm for computing a middle levels Gray code in optimal time and space: Each new set is generated in time $\mathcal{O}(1)$, and the required space is $\mathcal{O}(n)$.

Connected through hyperedge HE-1 (induced by source paper "optimal quasi gray codes does the alphabet matter"):
  with members:
    - "generalized gray codes with prescribed ends"
    - "a linear lower bound for incrementing a space optimal integer representation in the bit probe model"
    - "loopless gray code enumeration and the tower of bucharest"
    - "efficient computation of middle levels gray codes"
    - "snake in the box codes for rank modulation"
    - "improved methods for generating quasi gray codes"

Connected through hyperedge HE-2 (induced by source paper "loopless gray code enumeration and the tower of bucharest"):
  with members:
    - "counting and enumerating crossing free geometric graphs"

Connected through hyperedge HE-3 (induced by source paper "efficient computation of middle levels gray codes"):
  with members:
    - "a note on the middle levels conjecture"

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.DM(Discrete Mathematics)