Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Electromagnetic corrections to $\pi ^-$p scattering length from pionic
  hydrogen
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
1
The categories of this paper are: cs
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol