Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Action functionals for strings in four dimensions   All possible action functionals on the space of surfaces in ${\bf R}^4$ that
depend only on first and second derivatives of the functions, entering the
equation of the surface, and satisfy the condition of invariance with respect
to rigid motions are described.

10
The categories of this paper are: q-bio
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
31
The categories of this paper are: ao-sci
2
The categories of this paper are: math
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
5
The categories of this paper are: gr-qc
27
The categories of this paper are: comp-gas
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga