Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Difficulties with the Klein-Gordon Equation   Relying on the variational principle, it is proved that new contradictions
emerge from an analysis of the Lagrangian density of the Klein-Gordon field:
normalization problems arise and interaction with external electromagnetic
fields cannot take place. By contrast, the Dirac equation is free of these
problems. Other inconsistencies arise if the Klein-Gordon field is regarded as
a classical field.

17
The categories of this paper are: q-fin
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
19
The categories of this paper are: eess
16
The categories of this paper are: stat
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
10
The categories of this paper are: q-bio