Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The universal 'energy' operator   The "positive square" of any tensor is presented in a universal and unified
manner, valid in Lorentzian manifolds of arbitrary dimension, and independently
of any (anti)-symmetry properties of the tensor. For rank-m tensors, the
positive square has rank 2m. Positive here means future, that is to say,
satisfying the dominant property. The standard energy-momentum and super-energy
tensors are recovered as appropriate parts of the general square. A richer
structure of principal null directions arises.

17
The categories of this paper are: q-fin
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
15
The categories of this paper are: nucl-ex
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
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
19
The categories of this paper are: eess