Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The existence of a quantum phase transition in a Hubbard model on a
  square lattice   A novel canonical transformation is offered as the mean for studying
properties of a system of strongly correlated electrons. As an example of the
utility of the transformation, it is used to demonstrate the existence of a
quantum phase transition in a Hubbard model on a square lattice. An Appendix
presents two cases with a negative result.

31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
11
The categories of this paper are: hep-lat
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
5
The categories of this paper are: gr-qc
10
The categories of this paper are: q-bio