Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Shifts of optical frequency references based on spectral-hole burning in
  Eu:Y2SiO5
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
2
The categories of this paper are: math
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org