Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: BL Lacertae are probable sources of the observed ultra-high energy
  cosmic rays
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
9
The categories of this paper are: nlin
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
2
The categories of this paper are: math
16
The categories of this paper are: stat
34
The categories of this paper are: chem-ph