Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Novel Time-Independent Model for Determining the Critical Mass of a
  Fissile Nuclide   A straightforward, time-independent model for determining the critical mass
of a spherical sample of a fissile nuclide is developed from basic neutron
collision theory and elementary probability arguments. This model is suitable
for presentation to even undergraduate students and gives results in excellent
accord with published values.

17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
3
The categories of this paper are: physics
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
21
The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
13
The categories of this paper are: quant-ph
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
14
The categories of this paper are: math-ph