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: "A Push–Pull Class-E Inverter With Improved Efficiency"
  Abstract: This paper presents a concept for improving the efficiency of a push-pull Class-E inverter. The concept consists of lowering the root-mean-square transistor current (in reducing the transistor power losses) only by adding a differential capacitor to a push-pull Class-E topology. Theoretical explanations and confirming experimental results are given.

Connected through hyperedge HE-1 (induced by source paper "High-Efficiency Class E, $hboxEF_2$ , and $hboxE/F_3$ Inverters"):
  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: Engineering