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Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn

Received: 12 May 2026     Accepted: 26 May 2026     Published: 27 June 2026
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Abstract

Graph labeling is an important and rapidly developing area in graph theory due to its numerous applications in communication networks, frequency assignment, channel allocation, and coding theory. Among the various labeling methods, radial radio labeling has gained attention because of its connection with distance-based constraints and graph structure. In this paper, we study the radial radio labeling of certain connected subgraphs derived from complete graphs. We introduce the concepts of radial radio number and radial radio sequence and examine their behavior for specific graph classes obtained from complete graphs through edge deletions. The main objective of this work is to determine the radial radio sequence of connected graphs formed by deleting a perfect matching and an edge covering from the complete graph. Using fundamental graph-theoretic techniques and structural analysis, we derive exact values for the radial radio sequence of these graph families. The obtained results provide insight into the influence of graph structure and neighborhood properties on radial radio labeling. This study extends existing research on distance-based graph labeling and contributes to a better understanding of how structural modifications in graphs affect labeling parameters, which may further support applications in communication network optimization and interference reduction problems.

Published in American Journal of Applied Mathematics (Volume 14, Issue 3)
DOI 10.11648/j.ajam.20261403.17
Page(s) 168-173
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution and reproduction in any medium or format, provided the original work is properly cited.

Copyright

Copyright © The Author(s), 2026. Published by Science Publishing Group

Keywords

Radio Number, Radial Radio Number, Radius, Diameter, Radial Radio Sequence

1. Introduction
Throughout this paper, we only consider the simple, connected and undirected graphs. The closed neighbourhood of a vertex v is a subset of vertices denoted by N [v] and is defined by Nv={v,u: uvE(G)}. The induced subgraph induced by the closed neighbourhood of v is denoted as <N [v] > and is defined by <N [v] >=(N [v], E(<N [v] >)), where E<Nv>=vx: vxEG, xN [v] {uw: u,wNvand uwE(G)}. A clique, S, is a subset of V(G) with maximum number of vertices such that <S> is complete. The clique number, ω or ω(G), is the number of vertices in the clique of G.
A perfect matching in a graph G is a set M, of edges such that every vertex in G is incident to exactly one edge in M. Equivalently, M is a matching of size |VG|2. Note that, a perfect matching exists only when the graph G has even number of vertices.
An edge cover or edge covering of a graph G is a set C of edges such that every vertex in G is incident to at least one edge in C. In other words, the union of end points of the edges in C covers the entire vertex set V(G). An edge cover is said to be minimum edge cover if it has the smallest of possible cardinality among all edge covers of G.
The distance between any two distinct vertices u and v is the length of the shortest uv-path in G and is denoted by d(u,v). The eccentricity of a vertex v, e(v), is the distance between v and the vertex farthest from v. The minimum eccentricity among all vertices is called as the radius of G and is denoted by rad(G). The maximum eccentricity among the vertices is called as the diameter of G and is denoted by diam(G).
The study of graph labeling has emerged as a significant area within graph theory, offering insights into combinatorial properties and their applications in network design, communication systems, and optimization problems. The concept labeling was introduced by Rosa, et. Al. . The detailed study of Frequency Assignment Problem by Chartrand et. Al. leads to the new concept of radio k coloring.
A function ς: V(G)N is said to be a radial radio labeling, if it satisfies the following condition: for any two distinct vertices u and υ,
du,ν+ςu+ςυ1+rad(G)(1)
where du,ν is the shortest distance between u and υ. The inequality (1) is known as the radial radio condition of G. The span of a radial radio labeling, ς, is the largest integer in the range of ς, and is denoted by span(ς). The minimum span taken over all possible radial radio labelings of G is known as the radial radio number of G.
Motivated by the concept, μ1v-rr sequence introduce by Selvam et. Al., , the concept radial radio sequence is defined in the following manner: The non-decreasing sequence, (rr<Nu>)uV(G), is called as the radial radio sequence of a graph G, where rr(<Nu>) is the radial radio number of the induced subgraph induced by the closed neighbourhood of the vertex u. Radial radio sequences of product of some standard graphs have been determined in .
The following lemma provides the necessary and sufficient condition for a function to be a radial radio labeling of a graph with radius 1.
Lemma
Let G be a connected graph with rad(G) = 1. A function ς: V(G)  N is a radial radio labelling of G if and only if |ς(u) + ς(v)| ≥ 1 whenever uv is an edge.
Proof
If radG=1, then the radial radio condition becomes
dμ,ν+ςμ+ςυ1+radG
dμ,ν+ςμ+ςυ2
ςμ+ςυ2-dμ,ν
ςμ+ςυ2-dμ,ν
ςμ+ςυ1, if dμ,ν=10, if dμ,ν=2
This completes the proof.
Remark
To verify the radial radio condition for a graph with radius 1, by above lemma, it is enough to show that the label difference between adjacent vertices is at least 1 and the label difference between non-adjacent vertices is at least 0.
Equivalently, the non-adjacent vertices may receive the same positive integers as labels.
The following theorem is useful in establishing the results of this paper.
Theorem A: For any simple connected graph G, rr(G)ω(G), where ω(G) is the clique number of G. .
For further basic concepts, one may refer and . For more results on radio number and the radial radio number, one can refer and . In this research paper, we study the radial radio sequence of the subgraphs of the complete graph Kn which are obtained by deleting some of the subgraphs such as perfect matching and minimum edge covering. The radial radio numbers of these graphs are already determined in .
2. Radial Radio Sequence of K2n-M, n ≥ 2
The graph K2n-Μ is obtained by deleting the perfect matching, M, from the complete graph K2n. Let VK2n=αi: 1in and let M=α2i-1α2i: 1in. After deleting M, we observe the following:
a) VK2n-M=VK2n
b) E(K2n-M)=E(K2n)-M
c) degαi=2n-2, for each i, 1i2n.
d) radK2n-M=2
For, the graph K8-M is illustrated in Figure 1.
Theorem 2.1 provides the radial radio sequence of the graph K2n-M.
Theorem 2.1
The radial radio sequence of K2n-M is  n,n,n, ,n 2n times, for n3.
Proof
We have Nαi=αi: 1ii+12n-1 and Nα2n=VK2n-M-{α2n-1}. We observe that, the induced subgraphs induced by Nαi and Nαj, 1ij2n are isomorphic. This forces that, rr<Nαi>=rr(<Nαj>), 1ij2n. So that, it is enough to find radial radio number of Nαi, for some, i, 1i2n. Without loss of generality, assume that, i=1.
We note that, Kn is a clique in <Nα1> and so, by Theorem A,
rr(<Nα1>)n(2)
Define ζ: Nα1{1,2,3,} such that ζα1=1; ζα2i-1=ζα2i=i, 2in.
The induced subgraph <Nα1> and its corresponding labeling under ζ are presented in Figure 2.
Figure 2. a <Nα1>, b Labeling ζ of <Nα1>.
Since rad<Nα1>=1, by Lemma, we can say that ζ is a radial radio labeling for the graph <Nα1>. Hence spanζ=n which implies that,
rr(<Nα1>)n(3)
From the inequalities (2) and (3), rr<Nα1>=n. Thus rr<Nαi>=n, for all i, 1in.
3. Radial Radio Sequence of K2n+1-T, Where T Is a Minimum Edge Cover of K2n+1
Assume that, VK2n+1={βi: 1i2n+1}. If we take T={β1β2n+1, β1β2,β2i-1β2i: 2in}, then T is the minimum edge cover of K2n+1. We observe that,
i) K2n+1-T is connected.
ii) EK2n+1-T=EK2n+1-T
iii) radK2n+1-T=2
The graph K11-T is illustrated in Figure 3.
In this section, we determine the radial radio sequence of K2n+1-T, for n3.
Theorem 3.1
For n3, the radial radio sequence of K2n+1-T is (n+1, n+1,, n+1n times, n).
Proof
Here, we identify three types of induced subgraphs arising from the deletion of an edge covering in the graph K2n+1. These types are characterized as follows:
1) Induced subgraph induced by β1.
2) Induced subgraph induced by the vertices which are not adjacent to β1, that is, β2 and β2n+1.
3) Induced subgraphs induced by the vertices which are adjacent to β1.
We determine the radial radio numbers of each of these three types of induced subgraphs separately.
Case 1: <N [β1] >
Subcase 1a:
Figure 4. a <N [β1] > in K7-T, 4b Labeling ξof <N [β1] > in K7-T.
For n=3, the sets {β1, β4, β5} and {β1, β3, β6} induce subgraphs both are isomorphic to K3. Define ξ: Nβ1{1,2,3,} by ξβ1=1; ξβ3=2; ξβ6=3; ξβ4=2; ξβ5=3. Figure 4 illustrates this labeling:
From Figure 4, we note that, adjacent vertices receive distinct positive integers as labels and K3 is a clique, which proves that, rr<Nβ1>=3.
Subcase 1b:
For n4, the vertices in the sets {β1, β2i-1: 2in} and {β1, β2i: 2in} induce subgraph isomorphic to Kn. Also, Kn is the clique in <N [β1] >, which implies that,
rr(<N [β1] >)n(4)
Define ξ': Nu1{1,2,3,} by ξ'β1=1; ξ'β2i-1=i, 1in; ξ'β2i=i, 2in. The induced subgraph <Nβ1> of K11-T is illustrated in Figure 5.
Figure 5. a <N [β1] > in K11-T, b Labeling ξ' of <N [β1] > in K11-T.
By Lemma, we confirm that ξ' is a radial radio labeling for<N [β1] >, as the non-adjacent vertices get the distinct positive integers as labels. Thus span ξ'=n and so
rr(<N [β1] >)n(5)
By inequalities (4) and (5), we have rr<Nβ1>=n.
Case 2: The vertices which are not adjacent to β1, that is, β2 and β2n+1.
We observe the following:
1) <N [β2] ><N [β2n+1] > this implies that, rr<Nβ2>=rr(<N [β2n+1] >).
2) Nβ2={βi: 2i2n+1}.
3) rad<Nβ2>=1.
Assume the induced subgraph induced by β2. Consider the sets of vertices:
i) {β2, β2n+1, β2i-1: 2in}
ii) {β2, β2n+1, β2i: 2in}
Both the vertex sets (i) and (ii) induce subgraphs isomorphic to Kn+1, which is a clique in Nβ2 and hence by Theorem A,
rr<Nβ2>n+1(6)
Define ς: Nβ2{1,2,3,} such that ςβ2=1; ςβ2i-1=i, 2in+1; ςβ2i=i, 2in. The illustration of the function ς is shown in Figure 6.
(a)
Figure 6. a: <N [β2] > in K11-T; b: Labeling ς of <N [β2] > in K11-T.
Since dβ2i-1, β2i=2, 1in; dβ2, βi=1, 3in+1 and all adjacent vertices receive distinct positive integers as labels, by Lemma, we conclude that, ς is a radial radio labeling of <N [β2] >. Hence spanς=n+1 and so
rr<Nβ2>n+1(7)
From inequalities, (6) and (7), we conclude that, rr<Nβ2>=n+1. Also, rr<Nβ2>=rr<Nβ2n+1>=n+1.
Case 3: The vertices which are adjacent to β1. That is, the vertices belong to {βi: 3i2n}.
Note that: <Nβi><Nβj> and rr(<Nβi>)rr(<Nβj>), for every 3ij2n. Here, rad<Nβi>=1. Without loss of generality, assume that, i=3. It is observed that, the set of vertices in (i) and (ii) induce subgraphs which are isomorphic to Kn+1, which is a clique in <Nβ3>:
i) {β1, β2, β3, β2i-1: 3in+1}
ii) {β1, β2, β3, β2n+1,β2i: 3in}
By Theorem A,
rr<Nβ3>n+1(8)
Now, Define φ: Nβ3{1,2,3,} such that, φβ2i-1=φβ2i=i, 1in; φβ2n+1=n+1. Figure 7 shows the optimal radial radio labeling of the induced subgraph.
(a)
Figure 7. a: <N [β3] > in K11-T; b: Labeling φ of <N [β3] > in K11-T.
By Lemma, we can prove that, φ is a radial radio labeling of <Nβ3> and spanφ=n+1 and hence
rr<Nβ3>n+1(9)
Inequalities (8) and (9), proves that rr<Nβ3>=n+1. Hence r<Nβi>=n+1, 3i2n.
Cases 1, 2 and 3 concludes that the radial radio sequence of K2n+1-T is (n+1, n+1,, n+1n times, n). This completes the proof.
4. Conclusion
In this paper, we determined the radial radio sequences of the connected induced subgraphs of the complete graph Kn, which serves as our primary graph of interest. These results may be extended in future work to other specific families of graphs. It is important to note that the resulting graph must remain connected after the deletion of a perfect matching or an edge covering in order to define the radial radio sequence.
Abbreviations

Kn

Complete Graph on n Vertices

rad(G)

Radius of Graph G

rr(G)

Radial Radio Number of Graph G

d(u,v)

Distance Between Vertices u and v

N [u]

Closed Neighbourhood of Vertex u

rr(N [u] )

Radial Radio Number of the Induced Subgraph by Closed Neighbourhood of u

Acknowledgments
The author sincerely thanks the Management of Ayya Nadar Janaki Ammal College (Autonomous), Sivakasi, Tamil Nadu, India, for their constant support and encouragement. The authors also gratefully acknowledge the Postgraduate Department of Data Science, Computer Science Lab and the Science Instrumentation Centre of Ayya Nadar Janaki Ammal College for providing the necessary facilities and support to carry out this research work.
Author Contributions
Vimalajenifer Selvaraj: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing
Conflicts of Interest
The author declares that she has no conflicts of interest.
References
[1] Selvam Avadayappan, M. Bhuvaneshwari, S. Vimalajenifer, The Radial Radio Number and the Clique Number of a Graph, International Journal of Engineering and Advanced Technology (IJEAT), Volume 9, Issue 1S4, pp. 1002-1006, December 2019.
[2] Selvam Avadayappan, M. Bhuvaneshwari, S. Vimalajenifer, A Note on Radial Radio Number of a Graph, International Journal of Applied and Advanced Scientific Research, pp. 62–68, 2017.
[3] Selvam Avadayappan, M. Bhuvaneshwari, S. Vimalajenifer, Radial Radio Sequence of a Graph, International Journal of Mathematical Combinatorics, Special Issue, pp. 1–8, 2018.
[4] Selvam Avadayappan, M. Bhuvaneshwari and S. Vimalajenifer, rr-sequence product graphs, Advances in Mathematics: Scientific Journal, no. 3, pp. 927-936, 2020.
[5] R. Balakrishnan and K. Ranganathan, A Textbook of Graph Theory, Second Edition, Springer, New York, 2012.
[6] G. Chartrand, D. Erwin, F. Harary and P. Zhang, Radio labeling of graphs, Bulletin of the Institute of Combinatorics and its Applications, vol. 33, pp. 77-85, 2001.
[7] W. K. Hale, Frequency Assignment: Theory and Applications, Proceedings of the IEEE, vol. 68, no. 12, pp. 1497-1514, 1980.
[8] M. K. A. Kaabar, et al., Radio and Radial Radio Numbers of Certain Sunflower Extended Graphs, Journal of Mathematics, vol. 2022, Article ID 9229409, 2022.
[9] B. Mari, Further results on the radio number for some construction of the path, complete, and complete bipartite graphs, Heliyon, vol. 10, no. 15, e35678, 2024.
[10] K. M. Paramasivam, K. Yenoke, B. S. K. Muralidharan, Radial radio number of chess board graph and king’s graph, TELKOMNIKA Telecommunication Computing Electronics and Control, vol. 20, no. 1, pp. 189–196, 2022.
[11] R. Ponraj, S. Sathish Narayanan, Radio Mean Number of Certain Graphs, International Journal of Mathematical Combinatorics, vol. 4, pp. 45–53, 2021.
[12] A. Rosa, On certain valuations of the vertices of a graph, Theory of Graphs (Internat. Symposium, Rome, July 1966), Gordon and Breach, N. Y. and Dunod Paris, pp. 349-355, 1967.
[13] S. Vimalajenifer, Radial Radio Sequence of Path-Deleted, Cycle-Deleted and Star-deleted subgraphs of the complete Graph. (Submitted).
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    Selvaraj, V. (2026). Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn. American Journal of Applied Mathematics, 14(3), 168-173. https://doi.org/10.11648/j.ajam.20261403.17

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    Selvaraj, V. Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn. Am. J. Appl. Math. 2026, 14(3), 168-173. doi: 10.11648/j.ajam.20261403.17

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    Selvaraj V. Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn. Am J Appl Math. 2026;14(3):168-173. doi: 10.11648/j.ajam.20261403.17

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  • @article{10.11648/j.ajam.20261403.17,
      author = {Vimalajenifer Selvaraj},
      title = {Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn},
      journal = {American Journal of Applied Mathematics},
      volume = {14},
      number = {3},
      pages = {168-173},
      doi = {10.11648/j.ajam.20261403.17},
      url = {https://doi.org/10.11648/j.ajam.20261403.17},
      eprint = {https://article.sciencepublishinggroup.com/pdf/10.11648.j.ajam.20261403.17},
      abstract = {Graph labeling is an important and rapidly developing area in graph theory due to its numerous applications in communication networks, frequency assignment, channel allocation, and coding theory. Among the various labeling methods, radial radio labeling has gained attention because of its connection with distance-based constraints and graph structure. In this paper, we study the radial radio labeling of certain connected subgraphs derived from complete graphs. We introduce the concepts of radial radio number and radial radio sequence and examine their behavior for specific graph classes obtained from complete graphs through edge deletions. The main objective of this work is to determine the radial radio sequence of connected graphs formed by deleting a perfect matching and an edge covering from the complete graph. Using fundamental graph-theoretic techniques and structural analysis, we derive exact values for the radial radio sequence of these graph families. The obtained results provide insight into the influence of graph structure and neighborhood properties on radial radio labeling. This study extends existing research on distance-based graph labeling and contributes to a better understanding of how structural modifications in graphs affect labeling parameters, which may further support applications in communication network optimization and interference reduction problems.},
     year = {2026}
    }
    

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    T1  - Radial Radio Sequences of Perfect Matching-deleted and Minimum Edge Covering-deleted Subgraphs of the Complete Graph Kn
    AU  - Vimalajenifer Selvaraj
    Y1  - 2026/06/27
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    DO  - 10.11648/j.ajam.20261403.17
    T2  - American Journal of Applied Mathematics
    JF  - American Journal of Applied Mathematics
    JO  - American Journal of Applied Mathematics
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    PB  - Science Publishing Group
    SN  - 2330-006X
    UR  - https://doi.org/10.11648/j.ajam.20261403.17
    AB  - Graph labeling is an important and rapidly developing area in graph theory due to its numerous applications in communication networks, frequency assignment, channel allocation, and coding theory. Among the various labeling methods, radial radio labeling has gained attention because of its connection with distance-based constraints and graph structure. In this paper, we study the radial radio labeling of certain connected subgraphs derived from complete graphs. We introduce the concepts of radial radio number and radial radio sequence and examine their behavior for specific graph classes obtained from complete graphs through edge deletions. The main objective of this work is to determine the radial radio sequence of connected graphs formed by deleting a perfect matching and an edge covering from the complete graph. Using fundamental graph-theoretic techniques and structural analysis, we derive exact values for the radial radio sequence of these graph families. The obtained results provide insight into the influence of graph structure and neighborhood properties on radial radio labeling. This study extends existing research on distance-based graph labeling and contributes to a better understanding of how structural modifications in graphs affect labeling parameters, which may further support applications in communication network optimization and interference reduction problems.
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