Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Aut\`omats de Stallings, un cam\'i d'anada i tornada   In this paper we review some of the fundamental properties of the free group
and give a detailed account of Stallings's theory of automata, a geometric
interpretation of its subgroups that has been (and still is) immensely
fruitful, both as a means of understanding classical results, as as a source of
new results. We review some of the most important ones.
  --
  En aquest article revisem algunes de les propietats fonamentals del group
lliure i fem una exposici\'o detallada de la teoria dels aut\`omats de
Stallings, una interpretaci\'o geom\`etrica dels seus subgrups que ha estat (i
segueix essent) immensament fruct\'ifera, tant com a mitj\`a per entendre
resultats cl\`assics, com com a font de nous resultats. N'expliquem alguns dels
m\'es rellevants.

5
The categories of this paper are: gr-qc
3
The categories of this paper are: physics
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
11
The categories of this paper are: hep-lat
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
32
The categories of this paper are: atom-ph
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an