Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ferromagnetism in a hard-core boson model   The problem of ferromagnetism -- associated with a ground state with maximal
total spin -- is discussed in the framework of a hard-core model, which forbids
the occupancy at each site with more than one particle. It is shown that the
emergence of ferromagnetism on finite square lattices crucially depends on the
statistics of the particles. Fermions (electrons) lead to the well-known
instabilities for finite hole densities, whereas for bosons (with spin)
ferromagnetism appears to be stable for all hole densities.

37
The categories of this paper are: supr-con
6
The categories of this paper are: astro-ph
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
13
The categories of this paper are: quant-ph
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
7
The categories of this paper are: hep-th
15
The categories of this paper are: nucl-ex
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
2
The categories of this paper are: math
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an