Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Present understanding of the nucleon spin structure   The present understanding of the spin structure of the nucleon is briefly
reviewed. The main focus is on parton helicity distributions, orbital angular
momentum of partons as defined through generalized parton distributions, as
well as single spin asymmetries and time-reversal odd correlation functions.

28
The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
16
The categories of this paper are: stat
23
The categories of this paper are: solv-int
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
12
The categories of this paper are: nucl-th
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
37
The categories of this paper are: supr-con