Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Effective delta sources and regularity in higher-derivative and
  ghost-free gravity
3
The categories of this paper are: physics
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs
18
The categories of this paper are: econ
2
The categories of this paper are: math
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin