Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Gauge-invariant observables in gravity and electromagnetism: black hole
  backgrounds and null dressings
29
The categories of this paper are: funct-an
13
The categories of this paper are: quant-ph
15
The categories of this paper are: nucl-ex
17
The categories of this paper are: q-fin
0
The categories of this paper are: hep-ph
34
The categories of this paper are: chem-ph
16
The categories of this paper are: stat
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
19
The categories of this paper are: eess
25
The categories of this paper are: patt-sol
20
The categories of this paper are: acc-phys
23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg