Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: $\hat p \cdot \hat A$ vs $\hat x \cdot \hat E$: Gauge invariance in
  quantum optics
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
26
The categories of this paper are: bayes-an
5
The categories of this paper are: gr-qc
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
0
The categories of this paper are: hep-ph
16
The categories of this paper are: stat
3
The categories of this paper are: physics
2
The categories of this paper are: math
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio