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 only one vertex of in-degree zero. A vertex x of T is internal if its out-degree is positive. We design randomized and deterministic algorithms for deciding whether an input digraph contains a given out-tree with k vertices. The algorithms are of running time O^*(5.704^k) and O^*(6.14^k), respectively. We apply the deterministic algorithm to obtain a deterministic 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) [9].

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