Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Quantum field theory of degenerate systems   To set up a self-consistent quantum field theory of degenerate systems, the
unperturbed state should be described by a density matrix instead of a pure
state. This increases the combinatorial complexity of the many-body equations.
Hopf algebraic techniques are used to deal with this complexity and show that
the Schwinger-Dyson equations are modified in a non-trivial way. The hierarchy
of Green functions is derived for degenerate systems, and the case of a single
electron in a two-fold degenerate orbital is calculated in detail.

25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
3
The categories of this paper are: physics
5
The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph