Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ghost-free higher-derivative theory   We present an example of the quantum system with higher derivatives in the
Lagrangian, which is ghost-free: the spectrum of the Hamiltonian is bounded
from below and unitarity is preserved.

5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat