Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Simulations of surface charge density changes during the untreated solid
  tumor growth
5
The categories of this paper are: gr-qc
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
16
The categories of this paper are: stat
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
26
The categories of this paper are: bayes-an
9
The categories of this paper are: nlin
32
The categories of this paper are: atom-ph