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: "Algorithm for Finding k-Vertex Out-trees and Its Application to k-Internal Out-branching Problem"
  Abstract: An  out-tree   T  is an oriented tree with exactly one vertex of in-degree zero and a vertex  x  of  T  is called  internal  if its out-degree is positive. We design randomized and deterministic algorithms for deciding whether an input digraph contains a subgraph isomorphic to a given out-tree with  k  vertices. Both algorithms run in  O  *(5.704  k  ) time. We apply the deterministic algorithm to obtain an algorithm of runtime  O  *( c   k  ), where  c  is a constant, for deciding whether an input digraph contains a spanning out-tree with at least  k  internal vertices. This answers in affirmative a question of Gutin, Razgon and Kim (Proc. AAIM'08).

Connected through hyperedge HE-1 (induced by source paper "Algorithm for finding k-vertex out-trees and its application to k-internal out-branching problem"):
  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