Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Uniqueness in Relativistic Hydrodynamics   The choice of the 4-velocity in the relativistic hydrodynamics proposed by
Landau and Lifshitz is demonstrated to be the only one satisfying all general
principles. Especially the far more widely adopted Eckart choice has to be
abandoned.

2
The categories of this paper are: math
25
The categories of this paper are: patt-sol
10
The categories of this paper are: q-bio
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph
23
The categories of this paper are: solv-int
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom