Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Growth in the number of references in engineering journal papers during
  the 1972-2013 period   The number of references per paper, perhaps the best single index of a
journal's scholarliness, has been studied in different disciplines and periods.
In this paper we present a four decade study of eight engineering journals. A
data set of over 70000 references was generated after automatic data gathering
and manual inspection for errors. Results show a significant increase in the
number of references per paper, the average rises from 8 in 1972 to 25 in 2013.
This growth presents an acceleration around the year 2000, consistent with a
much easier access to search engines and documents produced by the
generalization of the Internet.

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The categories of this paper are: cond-mat
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The categories of this paper are: solv-int
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The categories of this paper are: quant-ph
22
The categories of this paper are: chao-dyn
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The categories of this paper are: physics
32
The categories of this paper are: atom-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: dg-ga
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The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
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The categories of this paper are: nlin
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The categories of this paper are: supr-con
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The categories of this paper are: astro-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-th