Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Particle Problem in Classical Gravity: A historical note on 1941   This note is based on a relatively unknown paper of Albert Einstein published
in 1941 in the Revista de la Universidad Nacional de Tucuman. That work can be
regarded as the prequel of the one written in 1943 in collaboration with
Wolfgang Pauli, where, along the same lines, a proof of the non-existence of
certain non-singular solutions in Kaluza-Klein theory was given. More
generally, disproving the existence of regular solutions in classical unified
field theories became, specially after 1930, an important criterion leading
Einstein's investigations on unified field theory. This is the context in which
Einstein's paper of 1941 and its generalizations of 1943 and 1948 become
important.

2
The categories of this paper are: math
10
The categories of this paper are: q-bio
12
The categories of this paper are: nucl-th
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
29
The categories of this paper are: funct-an
6
The categories of this paper are: astro-ph