Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The two-fermion relativistic wave equations of Constraint Theory in the
  Pauli-Schr\"odinger form   The two-fermion relativistic wave equations of Constraint Theory are reduced,
after expressing the components of the $4\times 4$ matrix wave function in
terms of one of the $2\times 2$ components, to a single equation of the
Pauli-Schr\"odinger type, valid for all sectors of quantum numbers. The
potentials that are present belong to the general classes of scalar,
pseudoscalar and vector interactions and are calculable in perturbation theory
from Feynman diagrams. In the limit when one of the masses becomes infinite,
the equation reduces to the two-component form of the one-particle Dirac
equation with external static potentials. The Hamiltonian, to order $1/c^2$,
reproduces most of the known theoretical results obtained by other methods. The
gauge invariance of the wave equation is checked, to that order, in the case of
QED. The role of the c.m. energy dependence of the relativistic interquark
confining potential is emphasized and the structure of the Hamiltonian, to
order $1/c^2$, corresponding to confining scalar potentials, is displayed.

33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
24
The categories of this paper are: dg-ga
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
29
The categories of this paper are: funct-an
4
The categories of this paper are: cond-mat
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
21
The categories of this paper are: adap-org