Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Condensation of phonons in an ultracold Bose gas   We consider the generation of longitudinal phonons in an elongated
Bose-condensed gas at zero temperature due to parametric resonance as a result
of the modulation of the transverse trap frequency. The nonlinear temporal
evolution with account of the phonon-phonon interaction leads self-consistently
to the formation of the stationary state with the macroscopic occupation of a
single phonon quantum state.

19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
12
The categories of this paper are: nucl-th
21
The categories of this paper are: adap-org
13
The categories of this paper are: quant-ph
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
25
The categories of this paper are: patt-sol
29
The categories of this paper are: funct-an
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The categories of this paper are: q-alg
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The categories of this paper are: comp-gas
18
The categories of this paper are: econ
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
32
The categories of this paper are: atom-ph