1. Introduction
Domination and its variations have been extensively studied (see, for example, [Reference Chen and Xu1, Reference Dong, Shan, Kang and Li2, Reference Haynes, Hedetniemi and Slater4, Reference Henning6, Reference Henning and Yeo11]). A subset D of vertices in a graph G is a dominating set of G if every vertex of
$V(G)\setminus D$
is adjacent to a vertex in D. The minimum cardinality
$\gamma (G)$
of a dominating set is called the dominating number of G. A subset D of vertices in a graph G is a total dominating set of G if every vertex of
$V(G)$
is adjacent to a vertex in D. The minimum cardinality
$\gamma _{t}(G)$
of a total dominating set is called the total dominating number of G. It is worth noting that the study of total dominating sets is meaningful only on an isolate-free graph.
Semitotal domination, introduced by Goddard et al. [Reference Goddard, Henning and McPillan3] in 2014, is a relaxed form of total domination. A subset D of vertices in an isolate-free graph G is a semitotal dominating set, abbreviated semi-TD-set, of G if it is a dominating set of G and every vertex in D is within distance 2 of another vertex of D. The semitotal domination number of G, denoted by
$\gamma _{t2}(G)$
, is the minimum cardinality of a semi-TD-set in G. We refer to a minimum semi-TD-set of G as a
$\gamma _{t2}(G)$
-set. Since every total dominating set is a semi-TD-set and every semi-TD-set is a dominating set,
$\gamma (G)\leqslant \gamma _{t2}(G)\leqslant \gamma _{t}(G)$
. However, the semitotal domination number is very different from the domination and total domination number. For example, the total domination number cannot be compared with the matching number, while the semitotal domination number is comparable with the matching number and cannot be greater than the matching number plus one (see [Reference Henning, Kang, Shan and Yeo7, Reference Henning and Marcon8]). That makes the study of semitotal domination interesting.
There is much interest in bounds for the semitotal domination number of graphs. For example, Goddard et al. [Reference Goddard, Henning and McPillan3] proved that if G is a connected graph of order
$n\geqslant 4$
, then
$\gamma _{t2}(G)\leqslant \tfrac 12 n$
and proposed Conjecture 1.1 below. Henning and Marcon [Reference Henning and Marcon9] proved that if G is a connected claw-free cubic graph of order
$n\geqslant 10$
, then
$\gamma _{t2}(G)\leq \frac{4}{11}n$
, and conjectured that this bound can be improved to
$\tfrac 13n$
if
$G\notin \{K_4,N_2\}$
, where
$N_2$
is a graph shown in Figure 1(a). This conjecture was solved by Zhu et al. [Reference Zhu, Shao and Xu13] and they proposed Conjecture 1.2 below. Zhu and Liu [Reference Zhu and Liu12] proved that Conjectures 1.1 and 1.2 hold for line graphs with minimum degree 3 and 4, respectively. In [Reference Henning5], Henning established the tight upper bounds on the upper semitotal domination number of a regular graph using edge weighting functions. For algorithmic aspects of semitotal domination in graphs, Henning and Pandey [Reference Henning and Pandey10] showed the semitotal domination problem is NP-complete for planar graphs, chordal bipartite graphs and split graphs.
Two graphs:
$N_2$
and
$N^{\prime }_2$
, where the black vertices form a minimum semi-TD-set of their respective graphs.
Conjecture 1.1 [Reference Goddard, Henning and McPillan3]
If
$G\neq K_4$
is a graph of order n with minimum degree
$\delta (G)\geqslant 3$
, then
$\gamma _{t2}(G)\leqslant \tfrac 25n$
.
Conjecture 1.2 [Reference Zhu, Shao and Xu13]
If
$G\neq N_2$
is a connected claw-free graph of order
$n\geq 6$
with minimum degree
$\delta (G)\geq 3$
, then
$\gamma _{t2}(G)\leq \tfrac 13n$
.
Inspired by [Reference Henning5], using edge weighting functions, we establish the tight upper bound on the semitotal domination number of a connected claw-free graph with minimum degree at least 3. In Section 2, we give some basic definitions and a lemma as preliminaries. In Section 3, we prove that if
$G\neq N_2$
is a connected claw-free graph of order
$n\geq 6$
with minimum degree
$\delta (G)\geq 3$
, then
$\gamma _{t2}(G)\leq \frac{4}{11}n$
. Also, we construct a graph attaining this bound and thus disprove Conjecture 1.2.
2. Preliminaries
In this section, we introduce some basic definitions and a useful lemma.
Let
$G=(V(G),E(G))$
be a connected finite simple undirected graph with vertex set
$V(G)$
and edge set
$E(G)$
of order
$n=|V(G)|$
. For a vertex
$v\in V(G)$
, we denote by
$N_G(v)=\{u\in V(G) \mid uv\in E(G)\}$
the neighbourhood of v and by
$N_G[v]=N_G(v)\cup \{v\}$
the closed neighbourhood of v. The degree of v is
$d_G(v)=|N_G(v)|$
and the number
$\delta (G)=\min \{d_G(v)\mid v\in V(G)\}$
is the minimum degree of G. We call a path connecting vertices u and v a
$(u,v)$
-path. The distance
$d_G(u,v)$
between u and v is the length of a shortest
$(u,v)$
-path in G. For a subset S of
$V(G)$
, we denote by
$N_S(v)$
the neighbourhood of v restricted on S and by
$G[S]$
the subgraph of G induced by S, while the graph
$G-S$
is the graph obtained from G by deleting the vertices in S and all edges incident with S. A graph is claw-free if it does not contain the complete bipartite graph
$K_{1,3}$
as an induced subgraph. If there is no confusion, then the subscript G is omitted in the notation, such as
$N(v), d(v),d(u,v)$
and so on.
Now consider
$S_1$
and
$S_2$
which are two disjoint subsets of
$V(G)$
. Let
$E[S_1, S_2]=\{u_1u_2\mid u_1\in S_1$
and
$u_2\in S_2\}$
. For a vertex v of S, the S-external private neighbourhood of v, denoted by
${\textit {epn}}(v,S)$
, is the set of all vertices in
$V(G)\setminus S$
that are adjacent to v but to no other vertex of S. In other words, if
$u\in {\textit {epn}}(v,S)$
, then
$u\in V(G)\setminus S$
and
$N_{G}(u)\cap S=\{v\}$
. The S-internal private
$2$
-neighbourhood of v, denoted by
${\textit {ipn}}_2(v,S)$
, is the set of all vertices in
$S\setminus \{v\}$
that are within distance 2 of v in G but at a distance greater than
$2$
from every other vertex of S. In other words, if
$u\in {\textit {ipn}}_2(v,S)$
, then
$u\in S\setminus \{v\}$
,
$d(v,u)\leqslant 2$
and
$d(u,w)>2$
for any vertex
$w\in S\setminus \{u,v\}$
.
A semi-TD-set in a graph G is a minimal semi-TD-set if it contains no semi-TD-set of G as a proper subset. The following result in [Reference Henning and Marcon8] provides a characterisation of minimal semi-TD-sets.
Lemma 2.1 [Reference Henning and Marcon8]
Let S be a semi-TD-set in a graph G. Then, S is a minimal semi-TD-set of G if and only if every vertex
$v\in S$
satisfies at least one of the following three properties:
-
(a) the vertex v is isolated in
$G[S]$
; -
(b)
${\textit {ipn}}_2(v,S)\neq \emptyset $
; -
(c)
${\textit {epn}}(v,S)\neq \emptyset $
.
3. Main result
In this section, we establish the tight upper bound on the semitotal domination number of a connected claw-free graph with minimum degree at least 3 using edge weighting functions. Before that, we define two graphs
$N_2$
and
$N^{\prime }_2$
as in Figure 1. Note that
$N^{\prime }_2$
is a graph attaining the bound of Theorem 3.1. This shows that Conjecture 1.2 is not true.
Theorem 3.1. If
$G\neq N_2$
is a connected claw-free graph of order
$n\geq 6$
with minimum degree
$\delta (G)\geq 3$
, then
$\gamma _{t2}(G)\leq \frac{4}{11}n$
, and this bound is sharp.
Proof. Suppose that the theorem is false. Let G be a counterexample such that
$|V(G)|$
is as small as possible. By the choice of G,
$G\neq N_2$
is a connected claw-free graph of order
$n\geq 6$
with
$\delta (G)\geq 3$
such that
$\gamma _{t2}(G)> \frac{4}{11}n$
, and any connected claw-free graph
$G'\neq N_2$
of order
$n'<n$
with
$\delta (G')\geq 3$
has
$\gamma _{t2}(G')\leq \frac{4}{11}n'$
, where
$n'\geq 6$
.
For a
$\gamma _{t2}(G)$
-set S, set
$\overline {S}=V(G)\setminus S$
. Define sets
$A^{S}=\{v\in S\mid {\textit {ipn}}_2(v,S)\neq \emptyset \}$
,
$A^S_1=\{v\in A^S \mid v\in {\textit {ipn}}_2(v',S)$
for some vertex
$v'\in S\}$
and
$A^S_2=A^S\setminus A^S_1$
. Let
$v\in A^S_1$
and
$v\in {\textit {ipn}}_2(v',S)$
. Then
$v'$
is the only vertex of S within distance
$2$
from v in G. Since
${\textit {ipn}}_2(v,S)\neq \emptyset $
,
$v'\in {\textit {ipn}}_2(v,S)$
and
$v'\in A^S_1$
. Further,
${\textit {ipn}}_2(v,S)=\{v'\}$
and
${\textit {ipn}}_2(v',S)=\{v\}$
. This implies that the vertices in
$A^S_1$
are paired off. For each vertex
$u\in A^S_2$
, let
$S_u={\textit {ipn}}_2(u,S)\cup \{u\}$
. If
$u'\in {\textit {ipn}}_2(u,S)$
, then
$u'\notin A^S$
. Otherwise,
$u'\in A^S$
and
$u\in {\textit {ipn}}_2(u',S)$
, for u is the only vertex of S within distance 2 from
$u'$
in G, which contradicts the fact that
$u\in A^S_2$
. We note that if
$u_1$
and
$u_2$
are two distinct vertices in
$A^S_2$
, then
${\textit {ipn}}_2(u_1,S)\cap {\textit {ipn}}_2(u_2,S)=\emptyset $
. Hence,
$S_{u_1}\cap S_{u_2}=\emptyset $
for each pair of different vertices
$u_1,u_2\in A^S_2$
. Let
$B^S=\bigcup _{u\in A^S_2}S_u$
and
$C^S=S\setminus (A^S_1\cup B^S)$
. Further, we partition
$C^S$
into three subsets:
$C^S_0=\{z\mid z\in ~C^S$
and
$|{\textit {epn}}(z,S)|=0\}$
,
$C^S_1=\{z\mid z\in C^S$
and
$|{\textit {epn}}(z,S)|=1\}$
, and
$C^S_2=\{z\mid z\in C^S$
and
$|{\textit {epn}}(z,S)|\geq 2\}$
. Then
$S=A^S_1\cup B^S\cup C^S_0\cup C^S_1\cup C^S_2$
.
In particular, a vertex u of
$A^S_2$
is special if
$|S_u|=2$
,
$d(u')=3$
and
$|N_{\overline {S}}(u)\setminus N_{\overline {S}}(u')|=1$
, where
$\{u'\}=S_v\setminus \{u\}$
. Further, we define sets
$A^S_{\widetilde {2}}=\{u\mid u\in A^S_2$
and u is special
$\}$
and
$C^S_{\widetilde {i}}=\{z\mid z\in C^S_i$
and
$d(z)=3\}$
for
$i\in \{0,1\}$
.
A diamond in G is an induced graph of G isomorphic to
$K_4-e$
. We call a diamond of G a special diamond if each of its vertices has degree 3 in G. Let
$\mathcal {D}$
be the set of vertices in a special diamond. Among all
$\gamma _{t2}(G)$
-set, we choose a
$\gamma _{t2}(G)$
-set S satisfying the following conditions:
-
(1) the number of edges in
$G[S]$
, denoted by
$\lambda (S)$
, is minimised; -
(2) subject to condition (1),
$|\mathcal {D}\cap S|$
is minimised; -
(3) subject to condition (2),
$|C^S_{\widetilde {0}}|$
is minimised; -
(4) subject to condition (3),
$|C^S_0|$
is minimised; -
(5) subject to condition (4),
$|C^S_{\widetilde {1}}|$
is minimised.
We prove the following claim about the set S.
Claim 1. S is an independent set of G.
Suppose to the contrary that there exist two adjacent vertices
$v_1$
and
$v_2$
in S. If
${\textit {epn}}(v_1,S)\neq \emptyset $
or
${\textit {epn}}(v_2,S)\neq \emptyset $
, then without loss of generality, consider
${\textit {epn}}(v_1,S)\neq \emptyset $
. Let
$x_1$
be a vertex in
${\textit {epn}}(v_1,S)$
. Since G is claw-free, each vertex of
$N(v_1)\setminus \{x_1,v_2\}$
is adjacent to either
$x_1$
or
$v_2$
. Thus,
$S_1=(S\setminus \{v_1\})\cup \{x_1\}$
is a
$\gamma _{t2}(G)$
-set. However,
$\lambda (S_1)<\lambda (S)$
, for
$x_1$
is adjacent to no vertex of
$S\setminus \{v_1\}$
, which contradicts the choice of S. Hence,
${\textit {epn}}(v_1,S)=\emptyset $
and
${\textit {epn}}(v_2,S)=\emptyset $
. By Lemma 2.1,
${\textit {ipn}}_2(v_1,S)\neq \emptyset $
and
${\textit {ipn}}_2(v_2,S)\neq \emptyset $
.
If
${\textit {ipn}}_2(v_1,S)\neq \{v_2\}$
, then there exists a vertex
$v_3\in {\textit {ipn}}_2(v_1,S)\setminus \{v_2\}$
. Combined with
$v_1v_2\in E(G)$
,
$d(v_1,v_3)=2$
. As
$v_3\in {\textit {ipn}}_2(v_1,S)$
, any vertex of
$N(v_3)$
belongs to
$\overline {S}$
and is adjacent to no vertex of
$S\setminus \{v_1,v_3\}$
. Let
$x_2$
be a vertex connecting
$v_1$
and
$v_3$
. Then
$x_2v_2\notin E(G)$
. Since G is claw-free, each vertex of
$N(v_1)\setminus \{x_2,v_2\}$
is adjacent to either
$x_2$
or
$v_2$
. When all vertices of
$N(v_3)\setminus \{x_2\}$
are adjacent to
$x_2$
,
$(S\setminus \{v_1,v_3\})\cup \{x_2\}$
is a semi-TD-set of G, which contradicts the minimality of S. However, when there exists a vertex
$x_3\in N(v_3)\setminus \{x_2\}$
such that
$x_2x_3\notin E(G)$
, each vertex of
$N(v_3)\setminus \{x_2,x_3\}$
is adjacent to either
$x_2$
or
$x_3$
as G is claw-free. Then
$S_1=(S\setminus \{v_1,v_3\})\cup \{x_2,x_3\}$
is a
$\gamma _{t2}(G)$
-set. But,
$\lambda (S_1)<\lambda (S)$
, which is a contradiction.
Hence,
${\textit {ipn}}_2(v_1,S)= \{v_2\}$
. Similarly,
${\textit {ipn}}_2(v_2,S)= \{v_1\}$
. Further, all vertices in
$N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)$
are adjacent to no vertex of
$S\setminus \{v_1,v_2\}$
. Recall that
${\textit {epn}}(v_1,S)=\emptyset $
and
${\textit {epn}}(v_2,S)=\emptyset $
. Thus,
$N_{\overline {S}}(v_1)=N_{\overline {S}}(v_2)$
. Since
$n\geq 6$
,
$\gamma _{t2}(G)\geq \frac{4}{11} n>2$
. This implies that
$\{v_1,v_2\}$
is not a
$\gamma _{t2}(G)$
-set. As G is connected, there exists a vertex
$x_4$
in
$\overline {S}\setminus N_{\overline {S}}(v_1)$
such that
$x_4$
is adjacent to a vertex
$x_5$
in
$N_{\overline {S}}(v_1)$
. We note that
$x_4$
has a neighbour in
$S\setminus \{v_1,v_2\}$
. When all vertices of
$N_{\overline {S}}(v_1)$
are adjacent to
$x_5$
,
$S_1=(S\setminus \{v_1,v_2\})\cup \{x_5\}$
is a semi-TD-set of G with
$|S_1|< |S|$
, which is a contradiction. When there exists a vertex
$x_6\in N_{\overline {S}}(v_1)$
such that
$x_5x_6\notin E(G)$
, each vertex of
$N_{\overline {S}}(v_1)\setminus \{x_5,x_6\}$
is adjacent to either
$x_5$
or
$x_6$
as G is claw-free. Then
$S_1=(S\setminus \{v_1,v_2\})\cup \{x_5,x_6\}$
is a
$\gamma _{t2}(G)$
-set with
$\lambda (S_1)<\lambda (S)$
, which is a contradiction. This completes the proof of Claim 1.
Combining Claim 1 and the claw-freeness of G, we see that x has at most two neighbours in S for any vertex x of
$\overline {S}$
. We define an edge weighting function w on G:
$[\overline {S},S]\rightarrow [0,1]$
. For each vertex
$x\in \overline {S}$
, the function w assigns weight for each edge
$e\in [\{x\},S]$
as follows.
-
• If x is an S-external private neighbour, then for the unique edge
$e\in [\{x\},S]$
,
$w(e)=1$
. -
• If x is not an S-external private neighbour and
$N_{C^S_{\widetilde {0}}}(x)=\emptyset $
, then
$w(e)=\tfrac 12$
for each edge
$e\in [\{x\},S]$
. -
• Assume that x is not an S-external private neighbour and
$N_{C^S_{\widetilde {0}}}(x)\neq \emptyset $
. Let
$N_{\overline {S}}(x)=\{y_1,y_2\}$
, where
$y_1\in N_{C^S_{\widetilde {0}}}(x)$
. It follows from the partition of S that
$y_2\in A^S_2\cup C^S_0\cup C^S_1\cup C^S_2$
.-
• If
$y_2\in A^S_{\widetilde {2}}\cup C^S_{\widetilde {0}}$
, then
$w(xy_1)=w(xy_2)=\tfrac 12$
. -
• If either
$y_2\in (A^S_2\setminus A^S_{\widetilde {2}})\cup (C^S_1\setminus C^S_{\widetilde {1}})$
, or
$y_2\in C^S_{\widetilde {1}}$
and
$|\{u\mid u\in N_{\overline {S}}(y_2) \mbox { and} N_{C^S_{\widetilde {0}}}(u)\neq \emptyset \}|=1$
, then
$w(xy_1)=\tfrac 34$
and
$w(xy_2)=\tfrac 14$
. -
• If either
$y_2\in C^S_0\setminus C^S_{\widetilde {0}}$
and
$|\{u\mid u\in N_{\overline {S}}(y_2) \mbox { and } N_{C^S_{\widetilde {0}}}(u)\neq \emptyset \}|\leq 2$
, or
$y_2\in C^S_{\widetilde {1}}$
and
$|\{u\mid u\in N_{\overline {S}}(y_2) \mbox { and } N_{C^S_{\widetilde {0}}}(u)\neq \emptyset \}|=2$
, then
$w(xy_1)=\tfrac 58$
and
$w(xy_2)=\tfrac 38$
. -
• If
$y_2\in C^S_0\setminus C^S_{\widetilde {0}}$
and
$|\{u\mid u\in N_{\overline {S}}(y_2) \mbox { and } N_{C^S_{\widetilde {0}}}(u)\neq \emptyset \}|\geq 3$
, then
$w(xy_1)= \frac{9}{16}$
and
$w(xy_2)= \frac{7}{16}$
. -
• If
$y_2\in C^S_2$
, then
$w(xy_1)=1$
and
$w(xy_2)=0$
.
-
From the definition of the edge weighting functions, the sum of the weights assigned to the edges joining x to S is 1. For any subset
$S_1$
of S, we define a weighting function f on
$S_1$
with
$f(S_1)=\sum _{e\in [\overline {S},S_1]}w(e)$
. We prove the following claims.
Claim 2.
$f(A^S_1)> \tfrac 74|A^S_1|$
.
Recall that the vertices in
$A^S_1$
are paired off. Let
$v_1$
and
$v_2$
be a pair of vertices in
$A^S_1$
. Then
${\textit {ipn}}_2(v_1,S)=\{v_2\}$
and
${\textit {ipn}}_2(v_2,S)=\{v_1\}$
. This implies that all vertices in
$N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)$
are adjacent to no vertex of
$S\setminus \{v_1,v_2\}$
. Further, we have
$f(\{v_1,v_2\})=|N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)|$
. Combining Claim 1 and
$\delta (G)\geq 3$
, we have
$|N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)|\geq 3$
. If
$|N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)|= 3$
, then
$N_{\overline {S}}(v_1)=N_{\overline {S}}(v_2)$
and
$|N_{\overline {S}}(v_1)|=3$
. In this case,
$n=5$
as G is claw-free and G is connected, which is a contradiction. Thus,
$|N_{\overline {S}}(v_1)\cup N_{\overline {S}}(v_2)|\geq 4$
. Hence
$f(\{v_1,v_2\})\geq 4>\tfrac 72$
and then
$f(A^S_1)> \tfrac 74|A^S_1|$
.
Claim 3.
$f(B^S)\geq \tfrac 74|B^S|$
.
Note that
$B^S=\bigcup _{u\in A^S_2}S_u$
and
$S_u\cap S_{u'}=\emptyset $
for any two different vertices
$u,u'\in A^S_2$
. We show that for any vertex
$u_1$
of
$A^S_2$
,
$f(S_{u_1})\geq \tfrac 74|S_{u_1}|$
. Let
$S_{u_1}=\{u_1,\ldots ,u_{r}\}$
, where
$r=|S_{u_1}|\geq 2$
. Since
$\{u_2,\ldots ,\ u_{r}\}\subseteq {\textit {ipn}}_2(u_1)$
, all neighbours of
$u_i$
in
$\overline {S}$
are adjacent to no vertex of
$S\setminus \{u_1,u_i\}$
, where
$i\in \{2,\ldots ,r\}$
. Combined with Claim 1,
$f(S_{u_1})\geq \sum _{i\in \{2,\ldots ,r\}}d(u_i)$
. If
$u_1\in A^S_{\widetilde {2}}$
, then
$f(S_{u_1})=f(\{u_1,u_2\})=w(x_1u_1)+d(u_2)=w(x_1u_1)+3$
, where
$\{x_1\}=N_{\overline {S}}(u_1)\setminus N_{\overline {S}}(u_2)$
. Since
$u_1\notin {\textit {ipn}}_2(u_2,S)$
,
$x_1$
has a neighbour in S other than
$u_1$
. Thus,
$w(x_1u_1)=\tfrac 12$
. Further,
$f(\{u_1,u_2\})=\tfrac 72$
and
$f(S_{u_1})=\tfrac 74|S_{u_1}|$
, as desired. Thus, we may assume that
$u_1\in A^S_2\setminus A^S_{\widetilde {2}}$
. Then either
$r\geq 3$
, or
$r=2$
and
$d(u_2)\geq 4$
, or
$r=2$
and
$d(u_2)=3$
and
$|N_{\overline {S}}(u_1)\setminus N_{\overline {S}}(u_2)|\geq 2$
.
If
$r\geq 3$
, then
${3r-3}>\tfrac {7}{4}r$
. Since
$\delta (G)\geq 3$
,
$f(S_{u_1})\geq \sum _{i\in \{2,\ldots ,r\}}d(u_i)\geq 3(r-1)=3r-3$
. Further,
$f(S_{u_1})>\tfrac 74r$
. When
$r=2$
and
$d(u_2)\geq 4$
,
$f(S_{u_1})=f(\{u_1,u_2\})\geq d(u_2)\geq 4>\tfrac 74r$
. When
$r=2$
,
$d(u_2)=3$
and
$|N_{\overline {S}}(u_1)\setminus N_{\overline {S}}(u_2)|\geq 2$
, let
$x_1$
be a vertex in
$N_{\overline {S}}(u_1)\setminus N_{\overline {S}}(u_2)$
. From the definition of the edge weighting functions, we have
$w(x_1u_1)\geq \tfrac 14$
. Thus,
$f(S_{u_1})=f(\{u_1,u_2\})\geq 2w(x_1u_1)+d(u_2)\geq \tfrac 12+3=\tfrac 72\geq \tfrac 74r$
. This completes the proof of Claim 3.
Claim 4.
$f(C^S_0\setminus C^S_{\widetilde {0}})\geq \tfrac 74|C^S_0\setminus C^S_{\widetilde {0}}|$
.
Let
$z_1$
be a vertex in
$C^S_0\setminus C^S_{\widetilde {0}}$
and let
$N_{\overline {S}}(z_1)=\{x_1,\ldots ,x_r\}$
, where
$r\geq 4$
. If we have
$|\{x\mid x\in N_{\overline {S}}(z_1) \mbox { and } N_{C^S_{\widetilde {0}}}(x)\neq \emptyset \}|\leq 2$
, then
$|\{x\mid x\in N_{\overline {S}}(z_1) \mbox { and } N_{C^S_{\widetilde {0}}}(x)=\emptyset \}|\geq r-2\geq 2$
. Without loss of generality, consider
$N_{C^S_{\widetilde {0}}}(x_1)=\emptyset $
and
$N_{C^S_{\widetilde {0}}}(x_2)=\emptyset $
. By the definition of the edge weighting functions,
$w(x_1z_1)= \tfrac 12$
,
$w(x_2z_1)=\tfrac 12$
and
$w(x_iz_1) \geq \tfrac 38$
for any
$i\in \{3,\ldots ,r\}$
. Hence,
$f(\{z_1\})\geq w(x_1z_1)+w(x_2z_1)+\sum _{i\in \{3,\ldots ,r\}}w(x_iz_1)\geq 1+\tfrac 38(r-2)\geq \tfrac 74$
. When
$|\{x\mid x\in N_{\overline {S}}(z_1) \mbox { and } N_{C^S_{\widetilde {0}}}(x)\neq \emptyset \}|\geq 3$
,
$w(x_iz_1)\geq \frac{7}{16}$
for any
$i\in \{1,\ldots ,r\}$
. Then
$f(\{z_1\})\geq \frac{7}{16} r\geq \tfrac 74$
. In all cases, we have
$f(\{z_1\})\geq \tfrac 74$
. Therefore,
$f(C^S_0\setminus C^S_{\widetilde {0}})\geq \tfrac 74|C^S_0\setminus C^S_{\widetilde {0}}|$
.
Claim 5.
$f(C^S_1)\geq \tfrac 74|C^S_1|$
.
Let
$z_1$
be a vertex in
$C^S_1$
and
$N_{\overline {S}}(z_1)=\{x_1,x_2,\ldots ,x_r\}$
, where
$\{x_1\}={\textit {epn}}(z_1,S)$
and
$r\geq 3$
. According to the definition of the edge weighting functions,
$w(x_1z_1)=1$
. When
$z_1\in C^S_1\setminus C^S_{\widetilde {1}}$
, we have
$r\geq 4$
and
$w(x_iz_1)\geq \tfrac 14$
for any
$i\in \{2,\ldots ,r\}$
. Thus,
$f(\{z_1\})=w(x_1z_1)+\sum _{i\in \{2,\ldots ,r\}}w(x_iz_1)\geq 1+\tfrac 14(r-1)\geq \tfrac 74$
. When
$z_1\in C^S_{\widetilde {1}}$
and either
$N_{C^S_{\widetilde {0}}}(x_2)=\emptyset $
or
$N_{C^S_{\widetilde {0}}}(x_3)=\emptyset $
, without loss of generality, we can take
$N_{C^S_{\widetilde {0}}}(x_2)=\emptyset $
. Then
$w(x_2z_1)=\tfrac 12$
and
$w(x_3z_1)\geq \tfrac 14$
. Further,
$f(z_1)=w(x_1z_1)+w(x_2z_1)+w(x_3z_1)\geq 1+\tfrac 12+\tfrac 14=\tfrac 74$
. When both
$z_1\in C^S_{\widetilde {1}}$
and
$N_{C^S_{\widetilde {0}}}(x_2)\neq \emptyset $
and
$N_{C^S_{\widetilde {0}}}(x_3)\neq \emptyset $
, we have
$w(x_2z_1)=w(x_3z_1)=\tfrac 38$
. Further,
$f(\{z_1\})=w(x_1z_1)+w(x_2z_1)+w(x_3z_1)=\tfrac 74$
. In both cases,
$f(\{z_1\})\geq \tfrac 74$
. Therefore,
$f(C^S_1)\geq \tfrac 74|C^S_1|$
.
Claim 6.
$f(C^S_2)> \tfrac 74|C^S_2|$
.
Let
$z_1$
be a vertex in
$C^S_2$
and
$x_1,x_2$
be two vertices in
${\textit {epn}}(z_1,S)$
. Then
$w(x_1z_1)=w(x_2z_1)=1$
and further
$f(\{z_1\})\geq 2$
. Hence,
$f(C^S_2)\geq 2|C^S_2|> \tfrac 74|C^S_2|$
.
If
$f(C^S_{\widetilde {0}})\geq \tfrac 74|C^S_{\widetilde {0}}|$
, then
$f(S)\geq \tfrac 74|S|$
by Claims 2–6. From the definition of the edge weighting functions,
$f(S)=n-|S|$
. It follows that
$|S|\leq \frac{4}{11}n$
, which is a contradiction. Thus,
$f(C^S_{\widetilde {0}})< \tfrac 74|C^S_{\widetilde {0}}|$
and there exists a vertex
$y_1\in C^S_{\widetilde {0}}$
such that
$f(\{y_1\})< \tfrac 74$
. Suppose that
$N_{\overline {S}}(y_1)=\{x_1,x_2,x_3\}$
and
$N_S(x_i)=\{y_1,y_{i+1}\}$
for any
$i\in \{1,2,3\}$
. If
$\{y_2,y_3,y_4\}\cap ((A^S_2\setminus A^S_{\widetilde {2}})\cup (C^S_1\setminus C^S_{\widetilde {1}})\cup C^S_2)\neq \emptyset $
, then at least one edge of
$\{x_1y_1,x_2y_1,x_3y_1\}$
has a weight of at least
$\tfrac 34$
. Further,
$f(\{y_1\})=w(x_1y_1)+w(x_2y_1)+w(x_3y_1)\geq \tfrac 34+\tfrac 12+\tfrac 12\geq \tfrac 74$
, which is a contradiction. Thus,
$\{y_2,y_3,y_4\}\cap ((A^S_2\setminus A^S_{\widetilde {2}})\cup (C^S_1\setminus C^S_{\widetilde {1}})\cup C^S_2)=\emptyset $
. Next, we prove two claims about the set
$\{y_2,y_3,y_4\}$
.
Claim 7.
$\{y_2,y_3,y_4\}\cap A^S_{\widetilde {2}}=\emptyset $
.
In contrast, we may assume that
$y_2\in A^S_{\widetilde {2}}$
. Let
$S_{y_2}=\{y_2,y_5\}$
and
$N(y_5)=\{x_4,x_5,x_6\}$
, where
$x_4$
is a vertex connecting
$y_2$
and
$y_5$
. According to the definition of
$A^S_{\widetilde {2}}$
,
$N(y_2)\subseteq \{x_1,x_4,x_5,x_6\}$
. Note that
${\textit {ipn}}_2(y_1,S)=\emptyset $
and
${\textit {epn}}(y_1,S)=\emptyset $
. If
$d(x_1,y_5)\leq 2$
, then
$(S\setminus \{y_1,y_2\})\cup \{x_1\}$
is a semi-TD-set of G, which contradicts the minimality of S. Thus,
$d(x_1,y_5)\geq 3$
which implies that
$x_1x_i\notin E(G)$
for any
$i\in \{4,5,6\}$
. Combining
$\delta (G)\geq 3$
and the claw-freeness of G, we have
$x_1x_2\in E(G)$
or
$x_1x_3\in E(G)$
and
$x_4x_5\in E(G)$
or
$x_4x_6\in E(G)$
. Without loss of generality, consider
$x_1x_2\in E(G)$
and
$x_4x_5\in E(G)$
.
If
$x_4x_6\in E(G)$
, then
$S_1=(S\setminus \{y_1,y_2,y_5\})\cup \{x_1,x_4\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$x_4x_6\notin E(G)$
. Since G is claw-free,
$y_2x_6\notin E(G)$
. Then
$y_2x_5\in E(G)$
as
$N(y_2)\subseteq \{x_1,x_4,x_5,x_6\}$
and
$d(y_2)\geq 3$
. Further,
$d(y_2)=3$
. If
$x_5x_6\in E(G)$
, then
$S_1=(S\setminus \{y_1,y_2,y_5\})\cup \{x_1,x_5\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$x_5x_6\notin E(G)$
. Since G is claw-free,
$N(x_4)=\{y_2,y_5,x_5\}$
and
$N(x_5)=\{y_2,y_5,x_4\}$
. We note that
$d(x_4)=d(x_5)=3$
and then
$G[\{y_2,y_5,x_4,x_5\}]$
is a special diamond.
Since
$y_5\in {\textit {ipn}}_2(y_2,S)$
and
$x_6y_2\notin E(G)$
,
$x_6$
is adjacent to no vertex of
$S\setminus \{y_5\}$
. If
$x_6$
is not in a special diamond, then
$S_1=(S\setminus \{y_2,y_5\})\cup \{x_4,x_6\}$
is a
$\gamma _{t2}(G)$
-set with
$\lambda (S_1)=\lambda (S)$
but with
$|\mathcal {D}\cap S_1|<|\mathcal {D}\cap S|$
, which contradicts our choice of S. Thus,
$x_6$
is in a special diamond D. Observe that
$x_6x_2\notin E(G)$
and
$x_6x_3\notin E(G)$
. Let
$V(D)=\{x_6,x_7,x_8,y_6\}$
, where
$x_6y_6$
is the missing edge in the special diamond D. Then
$\{x_7,x_8\}\subseteq \overline {S}$
. To dominate
$x_7$
and
$x_8$
, we must have
$y_6\in S$
. If
$y_6x_2\in E(G)$
, then
$y_6\in {\textit {ipn}}_2(y_1,S)$
which contradicts
$y_1\in C^S_{\widetilde {0}}$
. Thus,
$y_6x_2\notin E(G)$
. Similarly,
$y_6x_3\notin E(G)$
.
Let
$N(y_6)=\{x_7,x_8,x_9\}$
. We note that
$y_6\in {\textit {ipn}}_2(v,S)$
for some vertex
$v\in S$
so call
$v = y_7$
. Then
$y_7x_9\in E(G)$
. Recall that
$\{y_3,y_4\}\cap (A^S_2\setminus A^S_{\widetilde {2}})=\emptyset $
. If
$y_7x_2\in E(G)$
or
$y_7x_3\in E(G)$
, then
$y_7=y_3$
or
$y_4$
and
$y_7\in A^S_2\setminus A^S_{\widetilde {2}}$
, which is a contradiction. Thus,
$y_7x_2\notin E(G)$
and
$y_7x_3\notin E(G)$
. If
$y_7\notin {\textit {ipn}}_2(y_6,S)$
, then
$S_1=(S\setminus \{y_1,y_2,y_6\})\cup \{x_1,x_7\}$
is a semi-TD-set of G, which is a contradiction. Hence,
$y_7\in {\textit {ipn}}_2(y_6,S)$
. Let
$S_1=(S\setminus \{y_5\})\cup \{x_6\}$
. Clearly,
$S_1$
is a
$\gamma _{t2}(G)$
-set,
$\lambda (S_1)=\lambda (S)$
and
$|\mathcal {D}\cap S_1|=|\mathcal {D}\cap S|$
. We note that
$y_2\in {\textit {ipn}}_2(y_1,S_1)$
and
$\{x_6,y_7\}\subset {\textit {ipn}}_2(y_6,S_1)$
. Further,
$\{y_1,y_2,x_6,y_6,y_7\}\subseteq B^{S_1}$
and
$|C^{S_1}_{\widetilde {0}}|<|C^S_{\widetilde {0}}|$
, which contradicts the choice of S.
By Claim 7, each vertex of
$\{y_2,y_3,y_4\}$
belongs to
$C^S_0\cup C^S_{\widetilde {1}}$
.
Claim 8.
$\{y_2,y_3,y_4\}\cap C^S_{\widetilde {1}}\neq \emptyset $
.
Suppose to the contrary that
$\{y_2,y_3,y_4\}\cap C^S_{\widetilde {1}}=\emptyset $
. Then
$\{y_2,y_3,y_4\}\subseteq C^S_0$
. Since G is claw-free, there exists an edge in
$G[\{x_1,x_2,x_3\}]$
, say
$x_1x_2$
. If
$y_2\neq y_3$
, then
$x_3y_2\notin E(G)$
or
$x_3y_3\notin E(G)$
, as
$x_3$
has only one neighbour in
$S\setminus \{y_1\}$
. By symmetry, consider
$y_2x_3\notin E(G)$
(that is,
$y_2\neq y_4$
). Then
$S_1=(S\setminus \{y_1,y_2\})\cup \{x_1\}$
is a dominating set of G as
$y_2\in C^S_0$
and
$y_1\in C^S_{\widetilde {0}}$
. Since
$|S_1|<|S|$
,
$S_1$
cannot be a semi-TD-set of G. Combined with
$\{y_1,y_2\}\subseteq C^S_0$
,
$y_1$
and
$y_2$
are the only two vertices of S within distance 2 from
$y_4$
in G, and
$y_4\neq y_3$
. Thus, all vertices of
$N_{\overline {S}}(y_4)\setminus \{x_3\}$
are adjacent to
$y_2$
. Further,
$S_1=(S\setminus \{y_1,y_3\})\cup \{x_2\}$
is a semi-TD-set of G as
$y_3\in C^S_0$
and
$y_1\in C^S_{\widetilde {0}}$
, which contradicts the minimality of S. Hence,
$y_2= y_3$
.
Since
$y_1\notin {\textit {ipn}}_2(y_2,S)$
,
$y_2x_3\notin E(G)$
(that is,
$y_2\neq y_4$
). Let
$S_2=(S\setminus \{y_1,y_4\})\cup \{x_3\}$
. As
$y_4\in C^S_0$
,
$S_2$
is a dominating set of G. If
$d(y_2,x_3)\leq 2$
, then
$S_2$
is a semi-TD-set of G, which is a contradiction. Thus,
$d(y_2,x_3)> 2$
. Further,
$x_1x_3\notin E(G)$
and
$x_2x_3\notin E(G)$
. Combining
$d(x_3)\geq 3$
and the claw-freeness of G, there exists a vertex
$x_4$
such that
$x_4x_3\in E(G)$
and
$x_4y_4\in E(G)$
. By Claim 1,
$x_4\in \overline {S}$
. We note that
$y_2x_4\notin E(G)$
as
$d(y_2,x_3)> 2$
. Since
$y_4\in C^S_0$
,
$x_4\notin {\textit {epn}}(y_4,S)$
and
$x_4$
has a neighbour
$y_5$
in S other than
$y_4$
. As
$S_2$
cannot be a semi-TD-set of G,
$y_1$
and
$y_4$
are the only two vertices of S within distance 2 from
$y_2$
in G. Thus, all vertices of
$N_{\overline {S}}(y_2)\setminus \{x_1,x_2\}$
are adjacent to
$y_4$
.
Let
$x_5$
be a vertex in
$N_{\overline {S}}(y_2)\setminus \{x_1,x_2\}$
. Then
$x_5y_4\in E(G)$
. If
$x_1x_5\in E(G)$
, then
$(S\setminus \{y_1,y_2\})\cup \{x_1\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$x_1x_5\notin E(G)$
. Recall that
$x_1x_3\notin E(G)$
. This implies that
$d(x_1)=3$
. Similarly,
$d(x_2)=3$
. Note that
$x_5x_3\notin E(G)$
as
$d(y_2,x_3)> 2$
. Since G is claw-free,
$x_5x_4\notin E(G)$
and each vertex of
$N(y_4)\setminus \{x_3,x_5\}$
is adjacent to either
$x_3$
or
$x_5$
. Let
$S_3=(S\setminus \{y_1,y_2,y_4\})\cup \{x_1,x_3,x_5\}$
. Then
$S_3$
is a
$\gamma _{t2}(G)$
-set and
$\lambda (S_3)=\lambda (S)$
. If
$d(y_2)=3$
, then
$G[\{y_1,y_2,x_1,x_3\}]$
is a special diamond. Further,
$|\mathcal {D}\cap S_2|<|\mathcal {D}\cap S|$
, which contradicts the choice of S. Thus,
$d(y_2)\geq 4$
. Let
$x_6$
be a vertex in
$N_{\overline {S}}(y_2)\setminus \{x_1,x_2,x_5\}$
. Then
$x_6y_4\in E(G)$
. We note that
$\{y_2,y_4\}\in C^S_0\setminus C^S_{\widetilde {0}}$
and
$|\{x\mid x\in N_{\overline {S}}(y_2) \mbox { and } N_{C^S_{\widetilde {0}}}(x)\neq \emptyset \}|\leq 2$
. From the definition of the edge weighting functions, we have
$w(x_1y_1)=w(x_2y_1)=\tfrac 58$
and
$w(x_3y_1)\geq \frac{9}{16}$
. Further,
$f(\{y_1\})=w(x_1y_1)+w(x_2y_1)+w(x_3y_1)>\tfrac 74$
, which contradicts the choice of
$y_1$
.
By Claim 8, we may assume that
$y_2\in C^S_{\widetilde {1}}$
. Then
$w(x_1y_1)\geq \tfrac 58$
. If
$y_3\in C^S_{\widetilde {1}}$
, then
$w(x_2y_1)\geq \tfrac 58$
and further
$f(\{y_1\})=w(x_1y_2)+w(x_2y_2)+w(x_3y_2)\geq \tfrac 58+\tfrac 58+\tfrac 12\geq \tfrac 74$
, which is a contradiction. Thus,
$y_3\in C^S_0$
. Similarly,
$y_4\in C^S_0$
. This implies that
$y_2\neq y_3$
and
$y_2\neq y_4$
. Let
$N(y_2)=\{x_1,x_4,x_5\}$
, where
$\{x_4\}= {\textit {epn}}(y_2,S)$
. If
$N_{C^S_{\widetilde {0}}}(x_5)=\emptyset $
, then by the definition of the edge weighting functions, we have
$w(x_1y_1)=\tfrac 34$
. Further,
$f(\{y_1\})=w(x_1y_2)+w(x_2y_2)+w(x_3y_2)\geq \tfrac 34+\tfrac 12+\tfrac 12\geq \tfrac 74$
, which is a contradiction. Hence,
$N_{C^S_{\widetilde {0}}}(x_5)\neq \emptyset $
. We proceed with a series of claims that culminate in a contradiction.
Claim 9.
$|E(G[\{x_1,x_2,x_3\}])|= 1$
.
Since G is claw-free,
$|E(G[\{x_1,x_2,x_3\}])|\geq 1$
. If
$x_2x_1\in E(G)$
and
$x_2x_3\in E(G)$
, then
$(S\setminus \{y_1,y_3\})\cup \{x_2\}$
is a semi-TD-set of G as
$y_1\in C^S_{\widetilde {0}}$
and
$y_3\in C^S_{0}$
, which contradicts the minimality of S. Thus,
$x_2x_1\notin E(G)$
or
$x_2x_3\notin E(G)$
. Similarly,
$x_3x_1\notin E(G)$
or
$x_3x_2\notin E(G)$
. Suppose that
$|E(G[\{x_1,x_2,x_3\}])|\geq 2$
. Then
$x_1x_2\in E(G)$
,
$x_1x_3\in E(G)$
and
$x_2x_3\notin E(G)$
. This means G has a claw, which contradicts the claw-freeness of G. Hence,
$|E(G[\{x_1,x_2,x_3\}])|= 1$
.
Claim 10.
$y_3=y_4$
.
Assume, to the contrary, that
$y_3\neq y_4$
. By Claim 9, without loss of generality, we consider
$E(G[\{x_1,x_2,x_3\}])= \{x_1x_2\}$
or
$\{x_2x_3\}$
. Let
$S_1=(S\setminus \{y_1,y_3\})\cup \{x_2\}$
. Since
$y_3\in C^S_0$
,
$S_1$
is a dominating set of G. Note that
$S_1$
cannot be a semi-TD-set of G. Thus, there exists a vertex y such that
$y_1$
and
$y_3$
are the only two vertices of S within distance 2 from y in G and
$d(y,x_2)>2$
. If
$E(G[\{x_1,x_2,x_3\}])= \{x_2x_3\}$
, then
$d(x_2,y_4)\leq 2$
and
$y=y_2$
. This implies that
$x_5y_3\in E(G)$
. However, then
$(S\setminus \{y_1,y_4\})\cup \{x_3\}$
is a semi-TD-set of G as
$y_4\in C^S_0$
, which contradicts the minimality of S. Hence,
$E(G[\{x_1,x_2,x_3\}])= \{x_1x_2\}$
. In this case,
$d(y_2,x_2)\leq 2$
and
$y=y_4$
. Thus, all vertices of
$N_{\overline {S}}(y_4)\setminus \{x_3\}$
are adjacent to
$y_3$
. Let x be a vertex in
$N_{\overline {S}}(y_4)\setminus \{x_3\}$
. Then
$xy_3\in E(G)$
. Recall that
$d(y,x_2)>2$
. Thus,
$d(y_4,x_2)>2$
and
$x_2x\notin E(G)$
. Since G is claw-free,
$N_{\overline {S}}(y_4)\setminus \{x_3\}$
is a clique of G. Combining
$d(x_3)\geq 3$
and the claw-freeness of G, there exists a vertex
$x'$
in
$N_{\overline {S}}(y_4)\setminus \{x_3\}$
such that
$x_3x'\in E(G)$
. Then
$(S\setminus \{y_3,y_4\})\cup \{x'\}$
is a semi-TD-set of G as
$y_3\in C^S_0$
, which is a contradiction.
If
$y_3(=y_4)\in C^S_0\setminus C^S_{\widetilde {0}}$
, then
$w(x_2y_1)\geq \frac{9}{16}$
and
$w(x_3y_1)\geq \frac{9}{16}$
. Further,
$f(\{y_1\})=w(x_1y_2)+w(x_2y_1)+w(x_3y_1)\geq \tfrac 58+ \frac{9}{16} + \frac{9}{16} \geq \tfrac 74$
, which contradicts the choice of
$y_1$
. Thus,
$y_3\in C^S_{\widetilde {0}}$
.
Claim 11.
$x_2x_3\notin E(G)$
.
For the sake of contradiction, suppose that
$x_2x_3\in E(G)$
. If
$d(y_2,x_2)\leq 2$
, then
$(S\setminus \{y_1,y_3\})\cup \{x_2\}$
is a semi-TD-set of G as
$y_3\in C^S_{\widetilde {0}}$
, which contradicts the minimality of S. Thus,
$d(y_2,x_2)\geq 3$
. Similarly,
$d(y_2,x_3)\geq 3$
. This implies that
$E[\{x_1,x_4,x_5\}, \{x_2,x_3\}]=\emptyset $
. If
$d(x_2)> 3$
, then there exists a vertex x in
$\overline {S}$
adjacent to
$x_2$
. Since G is claw-free,
$xy_3\in E(G)$
. Thus, x has a neighbour in S different from
$y_3$
as
$y_3\in C^S_{\widetilde {0}}$
. Combined with
$N_{C^S_{\widetilde {0}}}(x_5)\neq \emptyset $
,
$(S\setminus \{y_1,y_3\})\cup \{x_2\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$d(x_2)=3$
. Similarly,
$d(x_3)=3$
. Observe that
$y_1$
and
$y_3$
are in the same special diamond of G.
If
$x_1x_4\in E(G)$
, then
$S_1=(S\setminus \{y_1,y_2,y_3\})\cup \{x_1,x_2\}$
is a dominating set of G. Otherwise,
$x_5$
is not dominated by
$S_1$
, and further
$x_5x_1\notin E(G)$
and
$x_5y_3\in E(G)$
as
$y_2\in C^S_{\widetilde {1}}$
and
$y_3\in C^S_{\widetilde {0}}$
. Since
$d(x_5)\geq 3$
and G is claw-free,
$N(x_5)=\{y_2,y_3,x_4\}$
. In this case,
$G=N_2$
, which is a contradiction. As
$N_S(x_5)\setminus \{y_2\}\subseteq C^S_{\widetilde {0}}$
and
$y_3\in C^S_{\widetilde {0}}$
, there does not exist a vertex in
$S\setminus \{y_1\}$
such that
$y_2$
and
$y_3$
are the only two vertices of S within distance 2 from it in G. Thus,
$S_1$
is a semi-TD-set of G which contradicts the minimality of S. Hence,
$x_1x_4\notin E(G)$
. Since
$d(x_1)\geq 3$
and G is claw-free,
$x_1x_5\in E(G)$
. Note that
$d(x_1)=3$
. If
$x_4x_5\in E(G)$
, then G has a claw, for
$x_5$
has a neighbour in S other than
$y_2$
, which is a contradiction. Thus,
$x_4x_5\notin E(G)$
. This implies that
$x_1$
is not in a special diamond.
Let
$S_2=(S\setminus \{y_1,y_2,y_3\})\cup \{x_1,x_2,x_4\}$
. Observe that
$S_2$
is a
$\gamma _{t2}(G)$
-set and
$\lambda (S_2)=\lambda (S)$
. If
$x_4$
is not in a special diamond, then
$|\mathcal {D}\cap S_2|<|\mathcal {D}\cap S|$
, which contradicts the choice of S. Thus,
$x_4$
is in a special diamond D of G. Let
$V(D)=\{x_4,x_6,x_7,y_5\}$
, where
$x_4y_5$
is the missing edge in the special diamond D. Clearly,
$\{x_6,x_7\}\subseteq \overline {S}$
and
$y_5\in S$
. We note that
$y_5$
is an S-internal private neighbour. Let
$y_5\in {\textit {ipn}}_2(y_6,S)$
and
$x_8$
be the vertex of
$\overline {S}$
connecting
$y_5$
and
$y_6$
.
If
${\textit {epn}}(y_6,S)=\emptyset $
, then
$S_3=(S\setminus \{y_1,y_2,y_5,y_6\})\cup \{x_1,x_4,x_8\}$
is a dominating set of G. Since
$d(x_8)\geq 3$
, there exists a vertex x adjacent to
$xx_8\in E(G)$
. Further,
$xy_6\in E(G)$
as G is claw-free. Combined with
${\textit {epn}}(y_6,S)=\emptyset $
, x has a neighbour in
$S\setminus \{y_1,y_5,y_6\}$
. Thus, there exists a vertex in
$S_3$
within distance 2 from
$x_8$
. It follows from the minimality of S that
$S_3$
cannot be a semi-TD-set of G. Thus,
$y_3$
is at a distance greater than 2 from every other vertex of
$S_3$
. This implies that
$y_3x_5\notin E(G)$
,
$x'y_6\in E(G)$
and
$x'x_8\notin E(G)$
, where
$\{x'\}=N(y_3)\setminus \{x_2,x_3\}$
. We note that neither
$x_8$
nor
$x'$
are in a special diamond. Otherwise,
${\textit {epn}}(y_6,S)\neq \emptyset $
or G has a claw, which is a contradiction. Further,
$(S\setminus \{y_1,y_2,y_3,y_5,y_6\})\cup \{x_1,x_2,x_6,x_8,x'\}$
is a semi-TD-set of G with
$\lambda (S_3)=\lambda (S)$
but with
$|\mathcal {D}\cap S_3|<|\mathcal {D}\cap S|$
, which contradicts the choice of S. Hence,
${\textit {epn}}(y_6,S)\neq \emptyset $
.
Let
$x_9$
be a vertex in
${\textit {epn}}(y_6,S)$
. Then
$|\mathcal {D}\cap S_2|=|\mathcal {D}\cap S|$
,
$\{x_1,x_2\}\cap C^{S_2}_{\widetilde {0}}=\emptyset $
and
$y_6\notin C^{S_2}_0$
. Further,
$|C^{S_2}_{\widetilde {0}}|\leq |C^S_{\widetilde {0}}|$
and
$|C^{S_2}_{0}|\leq |C^S_{0}|$
. If
$y_6\notin C^{S_2}_{\widetilde {1}}$
, then
$|C^{S_2}_{\widetilde {1}}|<|C^S_{\widetilde {1}}|$
, which contradicts the choice of S. Thus,
$y_6\in C^{S_2}_{\widetilde {1}}$
. Let
$N(y_6)=\{x_8,x_9,x_{10}\}$
(possibly,
$x_{10}=x_5$
). Then
$x_{10}$
has a neighbour in
$S_2\setminus \{y_6,y_5,x_4,x_2\}$
. Hence,
$x_{10}$
has a neighbour in
$S\setminus \{y_5,y_6\}$
. Let
$S_4=(S\setminus \{y_5\})\cup \{x_6\}$
. Then
$S_4$
is a
$\gamma _{t2}(G)$
-set. Now,
$\lambda (S_4)=\lambda (S)$
,
$|\mathcal {D}\cap S_4|=|\mathcal {D}\cap S|$
,
$x_6\in {\textit {epn}}_2(y_2,S_4)$
and
$y_6\in {\textit {ipn}}_2(y,S_4)$
for some vertex y of
$S_4$
. Thus,
$|C^{S_4}_{\widetilde {0}}|\leq |C^S_{\widetilde {0}}|$
,
$|C^{S_4}_0|\leq |C^S_0|$
and
$|C^{S_4}_{\widetilde {1}}|<|C^S_{\widetilde {1}}|$
, which contradicts the choice of S.
By Claims 9–11, we may assume that
$E(G[\{x_1,x_2,x_3\}])=\{x_1x_2\}$
. If
$x_1x_4\in E(G)$
, then
$(S\setminus \{y_1,y_2\})\cup \{x_1\}$
is a semi-TD-set of G as
$y_2\in C^S_{\widetilde {1}}$
, which contradicts the minimality of S. Thus,
$x_1x_4\notin E(G)$
.
Suppose first that
$x_5y_3\in E(G)$
. Since
$d(x_3)\geq 3$
and G is claw-free,
$x_5x_3\in E(G)$
. If
$x_5x_4\in E(G)$
, then
$(S\setminus \{y_1,y_2,y_3\})\cup \{x_1,x_5\}$
is a semi-TD-set of G, which contradicts the choice of S. Thus,
$x_5x_4\notin E(G)$
. Combining the claw-freeness of G and
$x_1x_4\notin E(G)$
, we have
$x_5x_1\in E(G)$
. Since G is claw-free,
$N(x_1)=\{y_1,y_2,x_2,x_5\}$
,
$N(x_2)=\{y_1,y_3,x_1\}$
,
$N(x_3)=\{y_1,y_3,x_5\}$
,
$N(x_5)=\{y_2,y_3,x_1,x_3\}$
and
$X_1:=N(x_4)\setminus \{y_2\}$
is a clique of G. We construct
$G'$
from G by removing all vertices of
$\{y_1,y_3,x_1,x_2,x_3,x_5\}$
and adding the edges between
$\{y_2\}$
and
$X_1$
such that
$\{y_2\}\cup X_1$
is a clique of
$G'$
. Since
$d(x_4)\geq 3$
, we have
$|X_1|\geq 2$
. Thus,
$G'\neq N_2$
is a connected claw-free graph of order
$n'=n-6$
with
$\delta (G')\geq 3$
. Note that
$X_1\subseteq \overline {S}$
as
$x_4\in {\textit {epn}}(y_2,S)$
. Since S is a semi-TD-set of G, there exist a vertex y of
$S\setminus \{y_1,y_2,y_3\}$
adjacent to some vertices of
$X_1$
and a vertex
$y'$
of
$S\setminus \{y_1,y_2,y_3,y\}$
within distance 2 from y in S. Hence,
$n'\geq 6$
. By the minimality of G,
$\gamma _{t2}(G')\leq \frac{4}{11}n'$
. Let
$S'$
be a
$\gamma _{t2}(G')$
-set. When
$y_2\notin S'$
,
$S'\cup \{y_1,x_5\}$
is a semi-TD-set of G. When
$y_2\in S'$
,
$(S'\setminus \{y_2\})\cup \{y_1,x_5,x_4\}$
is a semi-TD-set of G. In both cases,
$\gamma _{t2}(G)\leq \frac{4}{11}n'+2= \frac{4}{11}(n-6)+2< \frac{4}{11}n$
, which contradicts the choice of G.
Suppose next that
$x_5y_3\notin E(G)$
. Let
$N_S(x_5)\setminus \{y_2\}=\{y_5\}$
and
$N(y_3)\setminus \{x_2,x_3\}=\{x_6\}$
. Recall that
$N_{C^S_{\widetilde {0}}}(x_5)\neq \emptyset $
. Thus,
$y_5\in C^S_{\widetilde {0}}$
. Since
$d(x_3)\geq 3$
and G is claw-free,
$x_3x_6\in E(G)$
. If
$y_5x_6\in E(G)$
, then
$(S\setminus \{y_3,y_5\})\cup \{x_6\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$y_5x_6\notin E(G)$
. Let
$N(y_5)=\{x_5,x_7,x_8\}$
and
$N_S(x_6)\setminus \{y_3\}=\{y_6\}$
. If
$x_5x_1\notin E(G)$
, then
$x_4x_5\in E(G)$
as G is claw-free and
$x_1x_4\notin E(G)$
. In this case,
$(S\setminus \{y_1,y_2,y_5\})\cup \{x_1,x_5\}$
is a semi-TD-set of G, which contradicts the minimality of S. Thus,
$x_5x_1\in E(G)$
. If
$x_5x_4\in E(G)$
, then
$(S\setminus \{y_2,y_5\})\cup \{x_5\}$
is a semi-TD-set of G, which is a contradiction. Thus,
$x_5x_4\notin E(G)$
. Since G is claw-free,
$d(x_1)=4$
,
$d(x_2)=3$
,
$d(x_3)=3$
,
$N(x_5)\subseteq \{y_2,y_5,x_1,x_7,x_8\}$
and
$X_2:=N(x_6)\setminus \{y_3,x_3\}$
is a clique of G. Let
$G'$
be the graph obtained from G by removing all vertices of
$\{x_1,x_2,x_3,x_6,y_1,y_3\}$
and adding the edges between
$\{y_2,x_5\}$
and
$X_2$
such that
$\{y_2,x_5\}\cup X_2$
is a clique of
$G'$
. We observe that
$G'\neq N_2$
is a connected claw-free graph of order
$n'=n-6\geq 6$
with
$\delta (G')\geq 3$
. Then
$G'$
has a
$\gamma _{t2}(G')$
-set
$S'$
with at most
$\frac{4}{11}n'$
vertices by the minimality of G. When
$X_2\cap S'\neq \emptyset $
,
$S'\cup \{x_1,y_3\}$
is a semi-TD-set of G. When
$X_2\cap S'=\emptyset $
,
$S'\cup \{y_1,x_6\}$
is a semi-TD-set of G. In either case,
$\gamma _{t2}(G)\leq \frac{4}{11}n'+2= \frac{4}{11} (n-6)+2 < \frac{4}{11}n$
, which is a contradiction.
Acknowledgement
The authors thank the referees for their careful review and helpful suggestions.
