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On k-prime graphs

  • Omar A. Abughneim EMAIL logo and Baha’ Abughazaleh
Published/Copyright: December 6, 2024

Abstract

In the context of a simple undirected graph G , a k -prime labeling refers to assigning distinct integers from the set { k , k + 1 , , V ( G ) + k 1 } to its vertices, such that adjacent vertices in G are labeled with numbers that are relatively prime to each other. If G has a k -prime labeling, we say that G is a k -prime graph (k-PG). In this article, we characterize when a graph up to order 6 is a k-PG and characterize when a graph of order 7 is a k-PG whenever k and k + 1 are not divisible by 5. Also, we find a lower bound for the independence number of a k-PG. Finally, we study when a cycle is a k-PG.

MSC 2010: 05C78

1 Introduction

A set S of vertices of an undirected graph G is called an independent set if it has no adjacent pairs in G . The independence number of G is the maximum cardinality of an independent set of G , denoted by α ( G ) . A prime labeling (PL) of graph G is defined by a bijective map f from its vertex set to the set { 1 , 2 , , V ( G ) } such that f ( u ) and f ( v ) are relatively prime for every pair of adjacent vertices u and v in G and we say that G is a prime graph (PG) if it has a PL. In [1], Tout et al. introduced a PL, a concept defined by Entringer. Various families of graphs such as paths, cycles, stars, and spiders are proven to be prime [2,3]. More results on PL can be found in [48]. Vaidya and Prajapati [9] generalized the definition of PL as follows. A k -prime labeling (k-PL) of graph G is defined by a bijective map f from its vertex set to the set { k , k + 1 , , V ( G ) + k 1 } such that f ( u ) and f ( v ) are relatively prime for every pair of adjacent vertices u and v in G . Moreover, G is called a k -prime graph (k-PG). If k = 1 , then a k-PL is the same as PL. In this article, we characterize when a graph up to order 6 is a k-PG and characterize when a graph of order 7 is a k -PG whenever k and k + 1 are not divisible by 5. Also, we establish a lower bound for the independence number of a k-PG. Finally, we studied when a cycle is a k-PG and we generalized and modified some results in [10]. For notions and terminology not explicitly defined here, please refer to [1113].

2 k -prime graphs

It is clear that a spanning subgraph of a k-PG is a k-PG. We state this in the following lemma.

Lemma 2.1

If G is a k-PG, then every spanning subgraph of G is a k-PG.

Vaidya and Prajapati [9] proved the following two lemmas.

Lemma 2.2

Let G 1 be a PG of order n and G 2 be an ( n + 1 ) -PG. Then, G 1 G 2 is a PG.

Lemma 2.3

Any path P n is a k-PG for all k N .

By Lemma 2.3, it is obvious that every graph of order 1 or 2 is a k-PG for all k N .

In Lemma 2.4, we find a restriction on the independence number of a k-PG.

Lemma 2.4

If a graph G is a k-PG, then α ( G ) V ( G ) 2 if k is e v e n , V ( G ) 2 if k is o d d .

Proof

Let f be a k-PL of G . Let E be the set of all vertices of G whose labels are even. Then, E is an independent set with E = V ( G ) 2 if k is even , V ( G ) 2 if k is odd . Therefore, α ( G ) V ( G ) 2 if k is even , V ( G ) 2 if k is odd .

Corollary 2.5

A complete graph K n is a k-PG if and only if n 2 or ( n = 3 and k is odd).

Proof

If n 2 , then K n is a k-PG, because K 1 and K 2 are paths.

If n = 3 and k is odd, then the integers in the set { k , k + 1 , k + 2 } are mutually relatively prime. So, any bijection map from V ( K 3 ) to { k , k + 1 , k + 2 } is a k-PL. Thus, K 3 is a k-PG.

If n = 3 and k is even, then K 3 is not a k-PG because gcd ( k , k + 2 ) = 2 .

If n 4 , then

α ( K n ) = 1 < 2 V ( K n ) 2 .

So, using the previous lemma, K n is not a k-PG.□

In the following theorem, we provide a characterization when a graph of order 3 is a k-PG.

Theorem 2.6

A graph of order 3 is a k-PG if and only if it is a non-complete graph or k is odd.

Proof

Follows by Lemma 2.3 and Corollary 2.5.□

Corollary 2.7

Let G 1 be a PG of order m and G 2 be a graph of order 3. If G 2 is a non-complete graph or m is even, then G 1 G 2 is a P G .

Proof

Suppose m is even or G 2 is a non-complete graph of order 3. Then, m + 1 is odd or G 2 is a non-complete graph of order 3. By Theorem 2.6, we have G 2 is an ( m + 1 ) -PG. Therefore, by Lemma 2.2, we obtain G 1 G 2 is a PG.□

The previous corollary is a generalization of the following result in [7], and they prove that “if G 1 is a PG of order m where m is even and G 2 is a graph of order 3, then G 1 G 2 is a PG.”

Definition 2.8

A k-PG is called a maximal k-PG, if adding any edge to G yields a non- k -PG.

Remark 1

A graph is a k-PG if and only if it is a spanning subgraph of a maximal k-PG.

Remark 2

In order to determine the maximal k-PG of order n for a given value of k , we label the vertices of K n with the integers k , k + 1 , , k + n 1 . We remove all edges in which the labels on the end-vertices are not relatively prime. The resulting graph is a maximal k -PG of order n .

Definition 2.9

Let G 1 , G 2 , , G m be disjoint graphs. Then,

  1. The join of G 1 , G 2 , , G m , denoted by G 1 + G 2 , + G m , is the graph G 1 G 2 , G m with the addition of an edge from each vertex of G i to each vertex of G j for all i j .

  2. The pairwise join of G 1 , G 2 , , G m , denoted by G 1 + . G 2 , + . G m , is the graph G 1 G 2 , G m with the addition of an edge from each vertex of G i to each vertex of G i + 1 for i = 1 , 2 , , m 1 .

Consider the following figure to illustrate the differences between the graphs K 2 + K 1 + K 1 and K 2 + . K 1 + . K 1 (Figure 1).

Figure 1 
               
                  
                     
                        
                        
                           
                              
                                 K
                              
                              
                                 2
                              
                           
                           +
                           
                              
                                 K
                              
                              
                                 1
                              
                           
                           +
                           
                              
                                 K
                              
                              
                                 1
                              
                           
                        
                        {K}_{2}+{K}_{1}+{K}_{1}
                     
                   and 
                     
                        
                        
                           
                              
                                 K
                              
                              
                                 2
                              
                           
                           
                              
                                 +
                              
                              
                                 .
                              
                           
                           
                              
                                 K
                              
                              
                                 1
                              
                           
                           
                              
                                 +
                              
                              
                                 .
                              
                           
                           
                              
                                 K
                              
                              
                                 1
                              
                           
                        
                        {K}_{2}\mathop{+}\limits^{.}{K}_{1}\mathop{+}\limits^{.}{K}_{1}
                     
                  .
Figure 1

K 2 + K 1 + K 1 and K 2 + . K 1 + . K 1 .

In the following theorem, we provide a characterization when a graph of order 4 is a maximal k-PG.

Theorem 2.10

A graph of order 4 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. K 2 + . K 1 + . K 1 whenever k is divisible by 3.

  2. K 4 e whenever k is not divisible by 3, where e is an edge of K 4

Proof

Let G be a maximal k-PG of order 4. The labels of the vertices are k , k + 1 , k + 2 , k + 3 . Since the largest difference between these integers is 3, the only possible common prime factors between any pair of these integers are 2 and 3. The integers k and k + 2 are divisible by 2 whenever k is even and the integers k + 1 and k + 3 are divisible by 2 whenever k is odd.

  1. If k is divisible by 3, then k + 3 is also divisible by 3. So, G must have a vertex of degree 1. Thus, G = K 2 + . K 1 + . K 1 .

  2. If k is not divisible by 3, then no pair of these integers is divisible by 3. So, we have only two vertices that are not adjacent in G and hence G = K 4 e , where e is an edge of K 4 .

According to Theorem 2.10, it is easy to characterize k -PGs of order 4. We state this in the following corollary.

Corollary 2.11

A graph of order 4 is a k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of K 2 + . K 1 + . K 1 .

  2. A non-complete graph whenever k is not divisible by 3.

Remark 3

In the first condition in Corollary 2.11, we need not to say k is divisible by 3, because K 2 + . K 1 + . K 1 is a subgraph of K 4 e .

Corollary 2.12

Let G 1 be a PG of order m and G 2 be a graph of order 4. If G 2 is a subgraph of K 2 + . K 1 + . K 1 or ( m + 1 is not divisible by 3 and G 2 is a-non complete graph), then G 1 G 2 is a PG.

Proof

Follows by Corollary 2.11 and Lemma 2.2.□

Remark 4

A graph with no edges on i vertices is called the null graph, denoted by N i .

In Theorem 2.13, we characterize when a graph of order 5 is a maximal k-PG.

Theorem 2.13

A graph of order 5 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. K 5 e whenever k 1 ( mod 6 ), where e is an edge of K 5 .

  2. K 2 + . K 2 + . K 1 whenever k 3 or 5 ( mod 6 ) .

  3. K 2 + N 3 whenever k 4   ( mod 6 ) .

  4. P 3 + . K 1 + . K 1 whenever k 0 or 2 ( mod 6 ) .

Proof

Let G be a maximal k-PG of order 5. The labels of the vertices are k , k + 1 , , k + 4 and the only possible common prime factors between any pair of these integers are 2 and 3. In all cases, we will mention when pairs of integers are not relatively prime.

  1. If k 1 ( mod 6 ), then the integers k + 1 and k + 3 are even and are the only pair of integers that are not relatively prime. By Remark 2, G = K 5 e , where e is an edge of K 5 .

  2. If k 3 or 5 ( mod 6 ), then the integers k and k + 3 are not relatively prime whenever k is divisible by 3 and the integers k + 1 and k + 4 are not relatively prime whenever k + 1 is divisible by 3. Also, k + 1 and k + 3 are not relatively prime. By Remark 2, G = K 2 + . K 2 + . K 1 .

  3. If k 4 ( mod 6 ), then the integers k , k + 2 , k + 4 are mutually not relatively prime. By Remark 2, G = K 2 + N 3 .

  4. If k 0 or 2 ( mod 6 ), then the integers k and k + 3 are not relatively prime whenever k is divisible by 3 and the integers k + 1 and k + 4 are not relatively prime whenever k + 1 is divisible by 3. Also, the integers k , k + 2 , k + 4 are mutually not relatively prime. By Remark 2, G = P 3 + . K 1 + . K 1 .□

According to Theorem 2.13, we obtain the following corollary.

Corollary 2.14

A graph of order 5 is a k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 1 + . K 1 .

  2. A subgraph of K 2 + N 3 whenever k is odd or k + 2 is divisible by 3.

  3. A subgraph of K 2 + . K 2 + . K 1 whenever k is odd.

  4. A non-complete graph whenever k 1 ( mod 6 ) .

Proof

Since K 2 + . K 2 + . K 1 is a non-complete graph, P 3 + . K 1 + . K 1 is a subgraph of K 2 + N 3 , and K 2 + N 3 is a subgraph of K 2 + . K 2 + . K 1 , the proof follows by Theorem 2.13 and Remark 1.□

Corollary 2.15

If G is a k-PG of order 5 whenever k is even, then G is a k-PG of order 5 whenever k is odd.

Corollary 2.16

Let G 1 be a PG of order m and G 2 be a graph of order 5. Then, G 1 G 2 is a PG whenever G 2 exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 1 + . K 1 .

  2. A subgraph of K 2 + N 3 whenever m is even or m is divisible by 3.

  3. A subgraph of K 2 + . K 2 + . K 1 whenever m is even.

  4. A non-complete graph whenever m 0 ( mod 6 ) .

In Theorems 2.17 and 2.18, we characterize when a graph of order 6 is a maximal k-PG.

Theorem 2.17

The only maximal k-PG of order 6 is P 3 + . K 2 + . K 1 whenever k is not divisible by 5.

Proof

Let G be a maximal k-PG of order 6. The labels of the vertices are k , k + 1 , , k + 5 . Since k is not divisible by 5, the only possible common prime factors between any pair of these integers are 2 and 3. It is clear that three of these integers are divisible by 2 and two of them are divisible by 3. Furthermore, only one of these integers is divisible by 2 and 3 at the same time. Thus, G has exactly two maximal independent sets of cardinality greater than 1. One of them is of cardinality 3 and the other of cardinality 2 and these sets have a common vertex. By Remark 2, G = P 3 + . K 2 + . K 1 .

In the following theorems and corollaries, let H be the following graph (Figure 2).

Figure 2 
               
                  H.
Figure 2

H.

Theorem 2.18

If k is divisible by 5, then a graph of order 6 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. ( P 3 + K 1 ) + . K 1 + . K 1 whenever k 0 or 1 ( mod 6 ) .

  2. K 2 + . K 2 + . N 2 whenever k 3 or 4 ( mod 6 ) .

  3. H whenever k 2 or 5 ( mod 6 ).

Proof

Let G be a maximal k-PG of order 6. The labels of the vertices are k , k + 1 , , k + 5 . Since k is divisible by 5, the integers k and k + 5 are not relatively prime. In all cases, we will state all pairs of these integers that are not relatively prime.

  1. Suppose k 0 or 1 ( mod 6 ). Then, either k , k + 2 , and k + 4 are mutually not relatively prime and k and k + 3 are not relatively prime or k + 1 , k + 3 , and k + 5 are mutually not relatively prime and k + 2 and k + 5 are not relatively prime. By Remark 2, G = ( P 3 + K 1 ) + . K 1 + . K 1 .

  2. Suppose k 3 or 4 ( mod 6 ). Then, either k , k + 2 , and k + 4 are mutually not relatively prime and k + 2 and k + 5 are not relatively prime) or k + 1 , k + 3 , and k + 5 are mutually not relatively prime and k and k + 3 are not relatively prime). By Remark 2, G = K 2 + . K 2 + . N 2 .

  3. Suppose k 2 or 5 ( mod 6 ), then k + 1 and k + 4 are not relatively prime. Also, either k , k + 2 , and k + 4 are mutually not relatively prime or k + 1 , k + 3 , and k + 5 are mutually not relatively prime. By Remark 2, G = H .□

According to Theorems 2.17 and 2.18 and since K 2 + . K 2 + . N 2 , ( P 3 + K 1 ) + . K 1 + . K 1 and H are subgraphs of P 3 + . K 2 + . K 1 , it is easy to characterize k -PGs of order 6. We state this in the following corollary.

Corollary 2.19

A graph of order 6 is a k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 2 + . K 1 whenever k is not divisible by 5.

  2. A subgraph of ( P 3 + K 1 ) + . K 1 + . K 1 whenever k 0 or 1 ( mod 6 ) .

  3. A subgraph of K 2 + . K 2 + . N 2 whenever k 3 or 4 ( mod 6 ) .

  4. A subgraph of H whenever k 2 or 5 ( mod 6 ) .

According to Corollary 2.19, we have the following two corollaries.

Corollary 2.20

Let G be a graph of order 6. Then

  1. If G is a k-PG whenever k is divisible by 5, then G is a k-PG whenever k is not divisible by 5.

  2. If G is a subgraph of K 3 + . K 1 + . N 2 , then G is a k-PG for all k N .

Proof

(ii) Because K 3 + . K 1 + . N 2 is the maximal common subgraph of K 2 + . K 2 + . N 2 , ( P 3 + K 1 ) + . K 1 + . K 1 and H .□

Corollary 2.21

Let G 1 be a PG of order m and G 2 be a graph of order 6. Then, G 1 G 2 is a PG whenever G 2 exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 2 + . K 1 whenever m + 1 is not divisible by 5.

  2. A subgraph of ( P 3 + K 1 ) + . K 1 + . K 1 whenever m 0 or 5 ( mod 6 ) .

  3. A subgraph of K 2 + . K 2 + . N 2 whenever m 2 or 3 ( mod 6 ) .

  4. A subgraph of H whenever m 1 or 4 ( mod 6 ) .

To characterize when a graph of order 7 is a maximal k-PG, we have many cases. In the following theorem, we will study when k and k + 1 are not divisible by 5.

Theorem 2.22

If k and k + 1 are not divisible by 5, then a graph of order 7 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. P 3 + . K 2 + . N 2 whenever k 0 ( mod 6 ) .

  2. K 1 , 3 + . K 2 + K 1 whenever k 2 or 4 ( mod 6 ) .

  3. C 4 + . K 2 + . K 1 whenever k 3 ( mod 6 ) .

  4. P 3 + . K 3 + . K 1 whenever k 1 or 5 ( mod 6 ) .

Proof

Let G be a maximal k-PG of order 7. The labels of the vertices are k , k + 1 , , k + 6 . In all cases, we will state all pairs of these integers that are not relatively prime.

  1. If k 0 ( mod 6 ) , then k , k + 2 , k + 4 , and k + 6 are mutually not relatively prime. Also, k , k + 3 , and k + 6 are mutually not relatively prime. By Remark 2, G = P 3 + . K 2 + . N 2 .

  2. If k 2 or 4 ( mod 6 ) , then k , k + 2 , k + 4 , and k + 6 are mutually not relatively prime. Also, either k + 1 and k + 4 are relatively prime or k + 2 and k + 5 are relatively prime. By Remark 2, G = K 1 , 3 + . K 2 + . K 1 .

  3. If k 3 ( mod 6 ) , then k + 1 , k + 3 , and k + 5 are mutually not relatively prime. Also k , k + 3 , and k + 6 are mutually not relatively prime. By Remark 2, G = C 4 + . K 2 + . K 1 .

  4. If k 1 or 5 ( mod 6 ) , then k + 1 , k + 3 , and k + 5 are mutually not relatively prime. Also, either k + 1 and k + 4 are relatively prime or k + 2 and k + 5 are relatively prime. By Remark 2, G = P 3 + . K 3 + . K 1 .□

According to Theorem 2.22 and since the graph P 3 + . K 2 + . N 2 is a subgraph of K 1 , 3 + . K 2 + . K 1 and C 4 + . K 2 + . K 1 which are subgraphs of P 3 + . K 3 + . K 1 , it is easy to characterize k -PGs of order 7 whenever k and k + 1 are not divisible by 5. This is articulated in the following corollary.

Corollary 2.23

If k and k + 1 are not divisible by 5, then a graph of order 7 is a k -PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 2 + . N 2 .

  2. A subgraph of K 1 , 3 + . K 2 + . K 1 whenever k is not divisible by 3.

  3. A subgraph of C 4 + . K 2 + . K 1 whenever k is odd.

  4. A subgraph of P 3 + . K 3 + . K 1 whenever k is odd and k is not divisible by 3.

Corollary 2.24

Let G 1 be a PG of order m and G 2 be a graph of order 7 where m + 1 and m + 2 are not divisible by 5. Then, G 1 G 2 is a PG whenever G 2 exhibits an isomorphism with one of the following graphs:

  1. A subgraph of P 3 + . K 2 + . N 2 .

  2. A subgraph of K 1 , 3 + . K 2 + . K 1 whenever m + 1 is not divisible by 3.

  3. A subgraph of C 4 + . K 2 + . K 1 whenever m is even.

  4. A subgraph of P 3 + . K 3 + . K 1 whenever m is even and m + 1 is not divisible by 3.

To study when a graph of order 7 is a maximal k-PG whenever k is divisible by 5 or whenever k + 1 is divisible by 5, we need the six graphs in the following figures (Figures 3, 4, 5, 6, 7, 8).

Figure 3 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 1
                              
                           
                        
                        {J}_{1}
                     
                  .
Figure 3

J 1 .

Figure 4 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 2
                              
                           
                        
                        {J}_{2}
                     
                  .
Figure 4

J 2 .

Figure 5 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 3
                              
                           
                        
                        {J}_{3}
                     
                  .
Figure 5

J 3 .

Figure 6 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 4
                              
                           
                        
                        {J}_{4}
                     
                  .
Figure 6

J 4 .

Figure 7 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 5
                              
                           
                        
                        {J}_{5}
                     
                  .
Figure 7

J 5 .

Figure 8 
               
                  
                     
                        
                        
                           
                              
                                 J
                              
                              
                                 6
                              
                           
                        
                        {J}_{6}
                     
                  .
Figure 8

J 6 .

Theorem 2.25

If k is divisible by 5, then a graph of order 7 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. J 1 whenever k 0 ( mod 6 ) .

  2. J 2 whenever k 1 ( mod 6 ) .

  3. J 3 whenever k 2 ( mod 6 ) .

  4. J 4 whenever k 3 ( mod 6 ) .

  5. J 5 whenever k 4 ( mod 6 ) .

  6. J 6 whenever k 5 ( mod 6 ) .

Theorem 2.26

If k + 1 is divisible by 5, then a graph of order 7 is a maximal k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. J 1 whenever k 0 ( mod 6 ) .

  2. J 6 whenever k 1 ( mod 6 ) .

  3. J 5 whenever k 2 ( mod 6 ) .

  4. J 4 whenever k 3 ( mod 6 ) .

  5. J 3 whenever k 4 ( mod 6 ) .

  6. J 2 whenever k 5 ( mod 6 ) .

Remark 5

J 1 is a subgraph of J 3 and J 5 , J 5 is a subgraph of J 4 , J 3 , and J 4 are subgraphs of J 2 and J 6 .

According to Theorems 2.25 and 2.26 and Remark 5, we have the following corollaries:

Corollary 2.27

If k is divisible by 5, then a graph of order 7 is a k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of J 1 .

  2. A subgraph of J 3 whenever k 1 , 2 or 5 ( mod 6 )

  3. A subgraph of J 5 whenever k is odd or k 4 ( mod 6 ) .

  4. A subgraph of J 4 whenever k is odd.

  5. A subgraph of J 2 whenever k 1 ( mod 6 ) .

  6. A subgraph of J 6 whenever k 5 ( mod 6 ) .

Corollary 2.28

If k + 1 is divisible by 5, then a graph of order 7 is a k-PG if and only if it exhibits an isomorphism with one of the following graphs:

  1. A subgraph of J 1 .

  2. A subgraph of J 3 whenever k 1 , 4 or 5 ( mod 6 )

  3. A subgraph of J 5 whenever k is odd or k 2 ( mod 6 ) .

  4. A subgraph of J 4 whenever k is odd.

  5. A subgraph of J 2 whenever k 5 ( mod 6 ) .

  6. A subgraph of J 6 whenever k 1 ( mod 6 ) .

Corollary 2.29

Let G 1 be a PG of order m and G 2 be a graph of order 7 where m + 1 is divisible by 5. Then G 1 G 2 is a PG whenever G 2 exhibits an isomorphism with one of the following graphs:

  1. A subgraph of J 1 .

  2. A subgraph of J 3 whenever m 0 , 1 or 4 ( mod 6 )

  3. A subgraph of J 5 whenever m is even or m 3 ( mod 6 ) .

  4. A subgraph of J 4 whenever m is even.

  5. A subgraph of J 2 whenever m is divisible by 6.

  6. A subgraph of J 6 whenever m 4 ( mod 6 ) .

Corollary 2.30

Let G 1 be a PG of order m and G 2 be a graph of order 7 where m + 2 is divisible by 5. Then, G 1 G 2 is a PG whenever G 2 exhibits an isomorphism with one of the following graphs:

  1. A subgraph of J 1 .

  2. A subgraph of J 3 whenever m 0 , 3 or 4 ( mod 6 ) .

  3. A subgraph of J 5 whenever m is even or m 1 ( mod 6 ) .

  4. A subgraph of J 4 whenever m is even.

  5. A subgraph of J 2 whenever m 4 ( mod 6 ) .

  6. A subgraph of J 6 whenever m is divisible by 6.

Finally, we will study when a cycle is a k-PG. In Theorem 2.31, we prove that an odd cycle is not a k-PG whenever k is even.

Theorem 2.31

An odd cycle C 2 m + 1 is not a k-PG whenever k is even.

Proof

Let G be any k-PG of order 2 m + 1 whenever k is even. Then, by Lemma 2.4

α ( G ) V ( G ) 2 = 2 m + 1 2 = m + 1 > m = α ( C 2 m + 1 ) .

Thus, C 2 m + 1 is not a k - P G .

Now, we will present some cases in which C m is a k-PG.

Theorem 2.32

A cycle C m is a k-PG whenever gcd ( k , k + m 1 ) = 1 .

Proof

Let C m be the following cycle v 1 v 2 v m v 1 . Label the vertices of C m successively with the integers k , k + 1 , , k + m 1 . Since gcd ( k , k + m 1 ) = 1 and successive integers are relatively prime, we obtain C m is a k-PG.□

In [10], the authors proved that “an odd cycle C 2 m + 1 is a k-PG whenever k is odd and m = 1 , 2 , 4 .” In Theorem 2.33, we generalized their result.

Theorem 2.33

A cycle C 2 m + 1 is a k-PG whenever k is odd.

Proof

Since k is odd,

gcd ( k , k + ( 2 m + 1 ) 1 ) = gcd ( k , k + 2 m ) = 1 .

So, by Theorem 2.32 C 2 m + 1 is a k-PG.□

Also, in [10], the authors said “an even cycle C 2 m is a k-PG for all m 2 .” Nevertheless, it is crucial to emphasize that this result does not hold in general, for example, C 4 is not 3 - P G . In the following theorem, we add a condition on k to make the previous result correct.

Theorem 2.34

An even cycle C 2 m is a k-PG whenever g c d ( k , 2 m 1 ) = 1 .

Proof

Let d = gcd ( k , k + 2 m 1 ) . Then, d divides k and d divides

( k + 2 m 1 ) k = 2 m 1 .

But k and 2 m 1 are relatively prime and hence d = 1 . Use Theorem 2.32 to obtain C 2 m is a k-PG.□

Acknowledgement

We sincerely thank the reviewers for their precise evaluation and insightful comments on the manuscript. Their valuable input is gratefully acknowledged.

  1. Funding information: The authors state no funding involved.

  2. Author contributions: All authors listed have significantly contributed to the development and the writing of this article.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: No data were used for the research described in the article.

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Received: 2024-04-21
Revised: 2024-10-28
Accepted: 2024-11-08
Published Online: 2024-12-06

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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