Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Continuous theory of active matter systems with metric-free interactions   We derive a hydrodynamic description of metric-free active matter: starting
from self-propelled particles aligning with neighbors defined by "topological"
rules, not metric zones, -a situation advocated recently to be relevant for
bird flocks, fish schools, and crowds- we use a kinetic approach to obtain
well-controlled nonlinear field equations. We show that the density-independent
collision rate per particle characteristic of topological interactions
suppresses the linear instability of the homogeneous ordered phase and the
nonlinear density segregation generically present near threshold in metric
models, in agreement with microscopic simulations.

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The categories of this paper are: mtrl-th
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The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: nlin
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The categories of this paper are: q-bio
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The categories of this paper are: chem-ph
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The categories of this paper are: stat
5
The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs