Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the exact inverse problem of the calculus of variations   The article concerns the problem if a~given system of differential equations
is identical with the Euler--Lagrange system of an~appropriate variational
integral. Elementary approach is applied. The main results involve the
determination of the first--order variational integrals related to the
second--order Euler--Lagrange systems.

6
The categories of this paper are: astro-ph
36
The categories of this paper are: cmp-lg
19
The categories of this paper are: eess
31
The categories of this paper are: ao-sci
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
20
The categories of this paper are: acc-phys
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
18
The categories of this paper are: econ
11
The categories of this paper are: hep-lat
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc