Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Quadratic Density Response Function of a Two Dimensional Electron Gas   A closed-form expression of the retarded quadratic density response function
of a two dimensional non-relativistic electron gas at zero temperature is
written in terms of a non-analytic complex function. A careful analysis is made
of the mathematical mechanism of exact static features such as vanishing,
discontinuity and peaks. Special attention is paid to an ambiguity in treating
the phase of the non-analytic complex function.

15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
22
The categories of this paper are: chao-dyn
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
36
The categories of this paper are: cmp-lg
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
14
The categories of this paper are: math-ph
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess