Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Collision of two Hopfions   We study the collision of two hopfions or Hopf-Ra\~nada electromagnetic
fields. The superposition of two of such fields, travelling in opposite
directions, yields different topology for the electric and magnetic field
lines. Controlling the angular momentum of such fields, we can control the
topology of the flow associated to the field lines, and the energy
distribution. The concept of electromagnetic helicity and the exchange between
its magnetic and electric components are used to explain the different
behaviours observed when the angular momentum is reversed.

35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
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The categories of this paper are: cmp-lg
11
The categories of this paper are: hep-lat
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom
26
The categories of this paper are: bayes-an