Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Four-wave interaction in gas and vacuum. Definition of a third order
  nonlinear effective susceptibility in vacuum   Semiclassical methods are used to study the nonlinear interaction of light in
vacuum in the context of four wave mixing. This study is motivated by a desire
to investigate the possibility of using recently developed powerful ultrashort
(femtosecond) laser pulses to demonstrate the existence of nonlinear effects in
vacuum, predicted by quantum electrodynamics (QED). An approach, similar to
classical nonlinear optics in a medium, is developed in this article. A third
order nonlinear effective susceptibility of vacuum is then introduced .

7
The categories of this paper are: hep-th
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The categories of this paper are: chao-dyn
25
The categories of this paper are: patt-sol
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
1
The categories of this paper are: cs
2
The categories of this paper are: math
6
The categories of this paper are: astro-ph
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an
21
The categories of this paper are: adap-org