Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Time-reversal symmetry in nonlinear optics   The applicability of time-reversal symmetry to nonlinear optics is discussed,
both from macroscopic (Maxwell equations) and microscopic (quantum theoretical)
point of view. We find that only spatial operations can be applied for the
symmetry classification of nonlinear optical processes in magnetic, in
particular antiferromagnetic, materials. An example is given where both
operations (time reversal and a spatial operation) can yield different results.

10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org
11
The categories of this paper are: hep-lat
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
23
The categories of this paper are: solv-int