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 packing chromatic number of the square lattice is at least 12"
  Abstract: The packing chromatic number $\chi_\rho(G)$ of a graph $G$ is the smallest integer $k$ such that the vertex set $V(G)$ can be partitioned into disjoint classes $X_1, ..., X_k$, where vertices in $X_i$ have pairwise distance greater than $i$. For the 2-dimensional square lattice $\mathbb{Z}^2$ it is proved that $\chi_\rho(\mathbb{Z}^2) \geq 12$, which improves the previously known lower bound 10.

Connected through hyperedge HE-1 (induced by source paper "on packing chromatic number of subcubic outerplanar graphs"):
  with members:
    - "on packing colorings of distance graphs"
    - "s packing colorings of cubic graphs"

Connected through hyperedge HE-2 (induced by source paper "packing coloring of undirected and oriented generalized theta graphs"):
  with members:
    - "on packing colorings of distance graphs"
    - "dichotomies properties on computational complexity of s packing coloring problems"
    - "packing chromatic number of distance graphs"

Connected through hyperedge HE-3 (induced by source paper "the packing chromatic number of the infinite square lattice is between 13 and 15"):
  with members:
    - "subdivision into i packings and s packing chromatic number of some lattices"
    - "a sat attack on the erdos discrepancy conjecture"
    - "a new lower bound for the ramsey number r 4 8"

Connected through hyperedge HE-4 (induced by source paper "subdivision into i packings and s packing chromatic number of some lattices"):
  with members:
    - "on packing colorings of distance graphs"
    - "dichotomies properties on computational complexity of s packing coloring problems"

Connected through hyperedge HE-5 (induced by source paper "s packing colorings of cubic graphs"):
  with members:
    - "subdivision into i packings and s packing chromatic number of some lattices"
    - "dichotomies properties on computational complexity of s packing coloring problems"

Connected through hyperedge HE-6 (induced by source paper "packing chromatic number of distance graphs"):
  with members:
    - "on packing colorings of distance graphs"

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)