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 complexity of grammar and related problems"
  Abstract: In [1] and [2] a complexity theory for formal languages and automata was developed. This theory implies most of the previously known results and yields many new results as well. Here we develop an analogous theory for several classes of more practically motivated problems. Two such classes, both closely related to formal language and automata theory, suggest themselves - grammar problems and program scheme problems. Here, our primary emphasis is on grammar problems of interest in parsing and compiling. Other problems considered include -   (1) possible techniques for proving non-trivial lower complexity bounds for problems in P;   (2) the relationship of the complexity of tree automaton equivalence, structural equivalence, and grammatical covering; and   (3) the complexity of the equivalence problem for schemes.   In each case we relate the computational complexity of a problem to its underlying combinatorial structure. The remainder of the paper is divided into four sections.

Connected through hyperedge HE-1 (induced by source paper "On the equivalence, containment, and covering problems for the regular and context-free languages"):
  with members:
    - "Finding Regular Simple Paths in Graph Databases"
    - "Commutative grammars: the complexity of uniform word problems"

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