4.1. Undeletable Subgraph and Classification of Tricyclic Graphs
We need the following important definition to start our analysis.
Definition 3 Suppose G is a cyclic graph, then the undeletable subgraph of G is defined as a maximum subgraph without a pendant vertex, i.e., the subgraph arising from G by deleting all pendant vertices recursively.
Obviously, is connected and . Moreover, an undeletable subgraph of a graph is unique. And, it is easy to verify that the undeletable subgraph of a unicyclic graph is a cycle.
With the definition and Theorem 2, we are able to prove the following crucial lemma:
Lemma 10. Suppose G is a cyclic -graph with an undeletable subgraph . Then, there exists a -graph such that if there is a vertex with .
Proof. Let by deleting all edges of . By the definition of , contains no cycle, i.e., is a forest.
Let T be the tree of containing w. We claim that T contains exactly one vertex of . First, assume that T contains no vertex of , then T is not connected with vertices of in G, thereby G is disconnected, which is a contradiction. Now, assume that T contains at least two vertices of , and denote two of which by x and y, then there is a unique path in T that connects them containing a vertex since . And, there exists a path in that connects u and v since is connected. Therefore, vertex z lies on a cycle of G, implying that it belongs to , which contradicts the fact that . So, T contains exactly one vertex of , say .
Let be the longest path from to all other vertices in T. Note that because a path from to a pendant vertex containing w is of length at least two. Hence, , , and is the only non-pendant neighbor of . Then, by (1) of Theorem 2, there is an -graph such that . □
For a graph G with an undeletable subgraph , if for each , i.e., each only has pendant neighbors in , it is said to be a pendant-maximized graph.
Lemma 11. Suppose G is a pendant-maximized tricyclic -graph with undeletable subgraph . If there is a vertex with exactly two non-adjacent neighbors in , then there is an -graph such that ; otherwise, must be one of the 15 graphs as shown in Figure 2, Figure 3 and Figure 4 up to isomorphism. Proof. (1) We first prove the “if” part. Without loss of generality, let the neighbors of v in be u and w with . Observe that because G is pendant-maximized and v has two neighbors in . Hence, since u and w are non-adjacent. Therefore, there is an -graph such that by (2) of Theorem 2.
(2) Now, the “otherwise” part. By the definition of undeletable subgraph, is a tricyclic graph, that is, and . In the remaining argument, all degree and neighbors are constrained in .
We claim that , where the lower bound is obvious by checking graphs of order one to four. We first show there are at most six vertices of degree two in . Notice that each vertex v of degree two must lie on a cycle of length three because the neighbors of v must be adjacent. Moreover, each cycle of length three contains at most two vertices of degree two, otherwise the cycle is disconnected with other parts of . Since there are at most three edge-disjoint cycles in a tricyclic graph, thereby at most three edge-disjoint cycles of length three. Hence, by Handshaking Lemma, we have , implying .
Therefore, we can list all possible graph structures for
with
, which are exactly those shown in
Figure 2,
Figure 3 and
Figure 4. Thus the proof is complete. □
Let be the set of n-vertex tricyclic graphs obtained from by attaching pendant vertices to . It is evident that graphs belonging to are pendant-maximized. On the other hand, if a pendant-maximized tricyclic graph G with , then .
4.2. Relations between
Suppose and are two graph sets, and if for any graph , there is a graph such that , then this relation is written as or . Our remaining task is to figure out the above described relations between all .
Lemma 12. .
Proof. We prove the results in the order of left to right.
(1) Suppose
G is a graph in
with undeletable subgraph labelled as
in
Figure 4. It may be assumed that
; otherwise, by Lemma 9, pendant neighbors of
can be moved to
without increasing
since
. Moreover, we may assume
by similarly reasoning. Then clearly,
.
Consider first when . If , by (1) of Theorem 3, moving pendant neighbors of to reduces . So we may assume . Since , and , therefore graph satisfies appealing to (3) of Theorem 2.
Now we turn to the case of , that is, . Note that , and , thus graph satisfies again by (3) of Theorem 2. It is not difficult to check that and both belong to . Thus we have .
(2) Let
be a graph with undeletable subgraph as
in
Figure 4. As before, we may assume
. Let
, and obviously
. Since
,
and
, then we get
by (4) of Theorem 2. Thus
holds.
(3) Using similar arguments as (2), there is a graph such that . Hence . □
Lemma 13. .
Proof. We prove the relations from left to right.
(1) Let
be a graph with
as
in
Figure 4. As before, we assume that
.
Let . If , then . And if , we may assume ; otherwise can be reduced by moving pendant neighbors of to according to (1) of Theorem 3. Then, we have . Moreover, , hence there is a graph with from (3) of Theorem 2. And, it is evident that , thus we obtain .
(2) Suppose
G is a graph in
with an undeletable subgraph as
in
Figure 4. Using analogous arguments as (1), we can show that
. Additionally, note that
. By (3) of Theorem 2, there is a graph
such that
. So, it follows
easily. □
Lemma 14. , , .
Proof. We prove the three relations in the order of left to right.
(1) Let
with the undeletable subgraph as
in
Figure 3. As before, we assume
, i.e.,
. And, notice that
. Then, by (4) of Theorem 2, graph
satisfies
. Thus,
holds clearly.
(2) Let
with an undeletable subgraph as
in
Figure 4. As before, we assume
. Note that
and
. If
, then by (2) of Theorem 3,
can be reduced by moving pendant neighbors of
to
. Similarly, it holds for
. So we may assume
.
Let , and we have and . Let , then . It is easy to check that . Thus, follows.
(3) Let
with the undeletable subgraph as
in
Figure 4. As before, we assume
. Moreover, we may assume
. Otherwise, note that
and
with
, then appealing to (2) of Theorem 3, moving pendant neighbors of
to
will decrease
.
Now, notice that , and , we can decrease by moving pendant neighbors of to if . Hence, we only have to consider the case .
Let , and notice that . Let , then It is easy to see that , thus holds. □
Lemma 15. , , .
Proof. We prove the assertions from left to right.
(1) Let
with the undeletable subgraph as
in
Figure 3. As before, we assume
. Observe that
. By Lemma 9, we can move pendant neighbors of
to
and do not increase
if
. Therefore, we may assume that
. Notice that
and
, hence there is
such that
from (3) of Theorem 2. Thus, we obtain
.
(2) Let
with the undeletable subgraph as
in
Figure 3. By analogous arguments as (1), we may assume that
, that is,
. And, note that
and
. Then, according to (2) of Theorem 3, moving pendant neighbors of
to
reduces
if
. So, we may assume that
.
Let , and let . Note that . Then, . It is easy to verify that . Therefore, holds.
(3) Let
with the undeletable subgraph as
in
Figure 3. Using similar arguments as (2), we may assume that
, i.e.,
.
Let , and let . Note that . Then, It is clear that . Therefore, we obtain as desired.
(4) Let
with the undeletable subgraph as
in
Figure 2. Let
. If
, i.e.,
, then clearly
. Otherwise, note that
, according to (1) of Theorem 3, moving pendant neighbors of
to
decreases
if
. So,
in this case. As a consequence, we have
by the above argument. Similarly,
, implying that
. And, notice that
and
. Appealing to (3) of Theorem 2, graph
satisfies
with
. Therefore, we have
. □
Before proceeding with more relations, let us define some essential functions and graph classes. Let
For , let be n-vertex graphs in with a vertex of degree . It is worth noting that all pendant vertices of are adjacent to a single vertex. Further, it can be verified easily that .
Lemma 16. If , then with equality if and only if .
Proof. If
, then clearly
. If
, at least two vertices of
have pendant neighbors. Suppose the undeletable subgraph
is labelled as
in
Figure 2. Without loss of generality, we may assume that
. Note that
. By Lemma 9, moving pendant neighbors of
to
will decrease
if
. Similarly, this holds for
and
. So, we can conclude that
if
. So, the proof is complete. □
Lemma 17. If , then with equality if and only if .
Proof. Suppose the undeletable subgraph
is labelled as
in
Figure 2. If
, i.e., one of
is adjacent to all pendant neighbors, then obviously
. Then, let us consider the case
.
- Case 1.
, .
It is easy to see that . By Lemma 9, can be reduced by moving pendant neighbors of to , implying .
- Case 2.
one of has pendant neighbors.
We may assume that . Notice that , and . By (2) of Theorem 3, we can move pendant neighbors of to to reduce . Thus, we have .
- Case 3.
at least two of have pendant neighbors.
Suppose without loss of generality. Note that . Then, again appealing to Lemma 9, pendant neighbors of can be moved to with decreased. Then, we arrive at Case 2, thus .
Therefore, it completes the proof. □
Lemma 18. If , then with equality if and only if .
Proof. Suppose the undeletable subgraph
is labelled as
in
Figure 2. If
, i.e.,
is adjacent to all pendant vertices, then obviously
. So, we suppose that
.
- Case 1.
.
Consider first . And, observe that and . Then, by (2) of Theorem 3, we can move pendant neighbors of to and get smaller. Likewise, this holds for . Therefore, we obtain .
- Case 2.
one of holds. Without loss of generality, suppose .
Subcase 2.1. . Let be obtained from G by moving pendant neighbors of to . Observe that . Let and let . So, , where the last inequality holds by Lemma 8. Thus, we obtain .
Subcase 2.2. . Observe that , moving pendant neighbors of to will reduce from (1) of Theorem 3. Then, we arrive at Subcase 2.1.
Subcase 2.3. . By analogous argument as Case 1, we can move pendant neighbors of to , so we get to Subcase 2.1.
Subcase 2.4. . Similarly, as Subcase 2.2, pendant neighbors of can be moved to and will decrease. Then, we arrive at Subcase 2.3.
According to the four subcases, we obtain in this case.
- Case 3.
.
Subcase 3.1. . Note that and . By Theorem 9, can be reduced by moving pendant neighbors of to . Then, we get the Subcase 2.1.
Subcase 3.2. . Notice that . By (1) of Theorem 3, pendant neighbors of can be moved to with decreased if . Analogously, this holds for . Thus, we arrive at Subcase 3.1.
Now, we can conclude that if vertices other than of have pendant neighbors. Thus, the proof is complete. □
Lemma 19. .
Proof. Suppose are two graph sets with . Let be a graph satisfying . Then, for any graph , we have , that is, .
So, by Lemmas 16–18, it suffices to show that . Observe that for or . Hence, . And, . Therefore, the assertion holds clearly. □
We draw all the relations mentioned here in
Figure 5, in which
represents
.