human: Given a node-centered hypergraph: <hypergraph>, where nodes represent authors and hyperedges represent groups of authors who co-authored the same paper.

Details of the hypergraph:
Center node: "On optimal orientations of cartesian products with a bipartite graph"
  Abstract: Abstract   For a graph  G , let    D  (G)    be the family of strong orientations of  G . Define    d  →  (G)=  min  {d(D)    |    D∈  D  (G)}    and    ρ(G)=  d  →  (G)−d(G),    where  d ( D ) (resp.,  d ( G )) denotes the diameter of the digraph  D  (resp., graph  G ). Let  G × H  denote the cartesian product of the graphs  G  and  H . In this paper, we show that  ρ ( G × A  1 × A  2 ×⋯× A   k  )=0, where  G  is a bipartite graph fulfilling certain weak conditions and    {A     i     |    1⩽i⩽k}    is certain combination of graphs.

Connected through hyperedge HE-1 (induced by source paper "On optimal orientations of cartesian products of graphs (II): complete graphs and even cycles"):
  with members:
    (none)

Question: Please tell me which research area the center author belongs to. The 4 classes are: Computer science, Mathematics, Engineering, Economics, Psychology, Business. Directly output the class name.

gpt: Mathematics