Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Classical higher-order topological insulators   Topological states nurtures the emergence of devices with unprecedented
functions in photonics, plasmonics, acoustics and phononics. As one of the
recently discovered members, higher-order topological insulators (HOTIs) have
been increasingly explored, featuring lower-dimensional topological boundary
states, leading to rich mechanisms for topological manipulation, guiding and
trapping of classical waves. Here, we provide an overview of current
developments of HOTIs in classical waves including basic principles, unique
physical properties, various experimental realizations, novel phenomena and
potential applications. Based on these discussions, we remark on the trends and
challenges in this field and the impacts of higher-order topology on other
research fields.

21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
1
The categories of this paper are: cs
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
10
The categories of this paper are: q-bio
23
The categories of this paper are: solv-int
22
The categories of this paper are: chao-dyn
7
The categories of this paper are: hep-th