Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetic energy harvesting and concentration at distance by
  transformation optics   Magnetic energy is one the main agents powering our society: generating
energy in power plants, keeping information in magnetic memories, moving our
devices with motors. All of these applications require a certain spatial
distribution of magnetic energy, for example concentrating it in a transformer
core or in a magnetic sensor. We introduce in this work a way to collect
magnetic energy and distribute it in space with unprecedented efficiency and
flexibility, allowing very large concentration of magnetic energy in a free
space region, an enhanced magnetic coupling between two magnetic sources, and
the transfer of magnetic energy from a source to a given distant point
separated by empty space. All these features are achieved with a single device,
a magnetic shell designed by transformation optics.

6
The categories of this paper are: astro-ph
31
The categories of this paper are: ao-sci
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
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The categories of this paper are: q-fin
16
The categories of this paper are: stat
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
21
The categories of this paper are: adap-org