Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Towards an understanding of hole superconductivity   From the very beginning K. Alex M\"uller emphasized that the materials he and
George Bednorz discovered in 1986 were $hole$ superconductors. Here I would
like to share with him and others what I believe to be $the$ key reason for why
high $T_c$ cuprates as well as all other superconductors are hole
superconductors, which I only came to understand a few months ago. This paper
is dedicated to Alex M\"uller on the occasion of his 90th birthday.

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The categories of this paper are: patt-sol
5
The categories of this paper are: gr-qc
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The categories of this paper are: acc-phys
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The categories of this paper are: econ
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The categories of this paper are: eess
13
The categories of this paper are: quant-ph
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The categories of this paper are: math-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: funct-an
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The categories of this paper are: chao-dyn
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: adap-org