Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the chromatic number of some $P_5$-free graphs   Let $G$ be a graph. We say that $G$ is perfectly divisible if for each
induced subgraph $H$ of $G$, $V(H)$ can be partitioned into $A$ and $B$ such
that $H[A]$ is perfect and $\omega(H[B])<\omega(H)$. We use $P_t$ and $C_t$ to
denote a path and a cycle on $t$ vertices, respectively. For two disjoint
graphs $F_1$ and $F_2$, we use $F_1\cup F_2$ to denote the graph with vertex
set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup E(F_2)$, and use $F_1+F_2$ to
denote the graph with vertex set $V(F_1)\cup V(F_2)$ and edge set $E(F_1)\cup
E(F_2)\cup \{xy\;|\; x\in V(F_1)\mbox{ and } y\in V(F_2)\}$. In this paper, we
prove that (i) $(P_5, C_5, K_{2, 3})$-free graphs are perfectly divisible, (ii)
$\chi(G)\le 2\omega^2(G)-\omega(G)-3$ if $G$ is $(P_5, K_{2,3})$-free with
$\omega(G)\ge 2$, (iii) $\chi(G)\le {3\over 2}(\omega^2(G)-\omega(G))$ if $G$
is $(P_5, K_1+2K_2)$-free, and (iv) $\chi(G)\le 3\omega(G)+11$ if $G$ is $(P_5,
K_1+(K_1\cup K_3))$-free.

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The categories of this paper are: physics
33
The categories of this paper are: plasm-ph
11
The categories of this paper are: hep-lat
32
The categories of this paper are: atom-ph
6
The categories of this paper are: astro-ph
20
The categories of this paper are: acc-phys
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
5
The categories of this paper are: gr-qc
22
The categories of this paper are: chao-dyn
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The categories of this paper are: ao-sci