Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the squared spin-orbit correction to the positronium fine-structure
  splitting   In the recent paper by Zhang the order $\alpha^4 R_{\infty}$ corrections to
the positronium P levels were reconsidered. Those calculations confirm our
corresponding results, except for the contribution due to the squared
spin-orbit interaction. We present here a new derivation of our previous result
for this last correction, this derivation being to our opinion both simple and
convincing.

21
The categories of this paper are: adap-org
12
The categories of this paper are: nucl-th
36
The categories of this paper are: cmp-lg
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
6
The categories of this paper are: astro-ph
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an