Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: $R_{\rm e}$. I. Understanding galaxy sizes, associated luminosity
  densities, and the artificial division of the early-type galaxy population
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ
3
The categories of this paper are: physics
19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
5
The categories of this paper are: gr-qc
15
The categories of this paper are: nucl-ex
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat