Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the classification of (2,1) heterotic strings   We classify all untwisted (2,1) heterotic strings. The only solutions are the
three already known cases, having massless spectra consisting either of 24
chiral fermions, or of 24 bosons, or of 8 scalars and 8 fermions of each
chirality.

22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ
16
The categories of this paper are: stat
37
The categories of this paper are: supr-con
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
27
The categories of this paper are: comp-gas
30
The categories of this paper are: q-alg
2
The categories of this paper are: math
5
The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg