Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Electromagnetic Helicity in Classical Physics   This pedagogical note revisits the concept of electromagnetic helicity in
classical systems. In particular, magnetic helicity and its role in mean field
dynamo theories is briefly discussed highlighting the major mathematical
inconsistency in most of these theories---violation of magnetic helicity
conservation. A short review of kinematic dynamo theory and its classic
theorems is also presented in the Appendix.

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The categories of this paper are: econ
34
The categories of this paper are: chem-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: q-bio
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The categories of this paper are: q-alg
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The categories of this paper are: supr-con
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The categories of this paper are: hep-ex
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The categories of this paper are: quant-ph
0
The categories of this paper are: hep-ph
2
The categories of this paper are: math
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The categories of this paper are: plasm-ph
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The categories of this paper are: solv-int
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The categories of this paper are: cs
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The categories of this paper are: chao-dyn
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-th