Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Gravitational Energy of a Black Hole   An exact energy expression for a physical black hole is derived by
considering the escape of a photon from the black hole. The mass of the black
hole within its horizon is found to be twice its mass as observed at infinity.
This result is important in understanding gravitational waves in black hole
collisions.

9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org
35
The categories of this paper are: mtrl-th
4
The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
24
The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
18
The categories of this paper are: econ
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math