Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: $pp$-waves as exact solutions to ghost-free infinite derivative gravity   We construct exact pp-wave solutions of ghost-free infinite derivative
gravity and demonstrate that the sourceless theory does not bring any pp-wave
solutions save for that of Einstein's gravity. These waves described in the
Kerr-Schild form also solve the linearized field equations of the theory. We
also find an exact gravitational shock wave with non-singular curvature
invariants and with a finite limit in the ultraviolet regime of non-locality
which is in contrast to the divergent limit in Einstein's theory.

37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
29
The categories of this paper are: funct-an
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin