Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The two-time Green's Function and screened Self-Energy for two-electron
  quasi-degenerate States   Precise predictions of atomic energy levels require the use of QED,
especially in highly-charged ions, where the inner electrons have relativistic
velocities. We present an overview of the two-time Green's function method;
this method allows one to calculate level shifts in two-electron highly-charged
ions by including in principle all QED effects, for any set of states
(degenerate, quasi-degenerate or isolated).

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The categories of this paper are: comp-gas
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The categories of this paper are: econ
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The categories of this paper are: q-bio
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The categories of this paper are: astro-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-fin
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-lat
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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: cmp-lg
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-th
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The categories of this paper are: math-ph
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The categories of this paper are: stat