Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Normal ordering problem and the extensions of the Stirling grammar   The purpose of this paper is to investigate the connection between
context-free grammars and normal ordering problem, and then to explore various
extensions of the Stirling grammar. We present grammatical characterizations of
several well known combinatorial sequences, including the generalized Stirling
numbers of the second kind related to the normal ordering problem and the
$r$-Dowling polynomials. Also, possible avenues for future research are
described.

37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
30
The categories of this paper are: q-alg
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas