Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Schwinger's Quantum Action Principle: From Dirac's formulation through
  Feynman's path integrals, the Schwinger-Keldysh method, quantum field theory,
  to source theory
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
24
The categories of this paper are: dg-ga
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
29
The categories of this paper are: funct-an
31
The categories of this paper are: ao-sci
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
8
The categories of this paper are: hep-ex
6
The categories of this paper are: astro-ph
11
The categories of this paper are: hep-lat
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
1
The categories of this paper are: cs
20
The categories of this paper are: acc-phys