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 the hardness of approximating spanners"
  Abstract: A k -spanner of a connected graph G=(V,E) is a subgraph G' consisting of all the vertices of V and a subset of the edges, with the additional property that the distance between any two vertices in G' is larger than the distance in G by no more than a factor of k . This paper concerns the hardness of finding spanners with a number of edges close to the optimum. It is proved that for every fixed k , approximating the spanner problem is at least as hard as approximating the set-cover problem.

Connected through hyperedge HE-1 (induced by source paper "A fast parametric maximum flow algorithm and applications"):
  with members:
    - "A new approach to the maximum flow problem"
    - "Image Restoration with Discrete Constrained Total Variation Part I: Fast and Exact Optimization"

Connected through hyperedge HE-2 (induced by source paper "Geodesic Spanners on Polyhedral Surfaces"):
  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