On Hybrid Order-Sum Graphs of Finite Dihedral Groups  
Ibrahim, M.1,2, Isah, S. H.2  
1,2Department of Mathematic, Usmanu Danfodio University, Sokoto  
2Department of Mathematics, Nasarawa State University Keffi, Nasarawa.  
*Corresponding Author  
Received: 03 December 2025; Accepted: 09 December 2025; Published: 18 December 2025  
ABSTRACT  
This paper introduces a novel graph construction, the inverse-order sum graph, for the dihedral group 퐷₂ₙ. By  
merging the adjacency conditions of the inverse graph and the order sum graph, we define 훤ᵢᵥₒₙ(퐷₂ₙ) and  
investigate its fundamental graph properties. For n odd, we establish explicit formulas for vertex degrees, graph  
sizes, and completeness. The inverse graph 훤 (퐷₂ₙ) exhibits the highest connectivity with degrees 푛 − 1 in  
퐼푣  
푃₁ ∪ 푃₃ and 푛 − 2 푖푛 푃₂, while the order sum graph 훤 (퐷₂ₙ) is sparser with edges only between full-order  
푂푆  
rotations. The inverse-order sum graph 퐼푣푂(퐷₂ₙ) is the most restrictive, yielding 푛 − 3 degrees in 푃₃ and  
isolated vertices elsewhere. Our comparative analysis reveals strict inclusion relations and structural hierarchies  
among these graphs, demonstrating how combining algebraic conditions produces refined graphical  
representations of group elements. These results contribute to algebraic graph theory by providing new tools for  
analyzing finite group structures through hybrid graph constructions, with potential applications in group-based  
cryptography and network modeling.  
Keywords: inverse graph, order sum graph, inverse-order sum graph, dihedral group, hybrid algebraic graph;  
network modelling.  
INTRODUCTION  
This paper introduces a new graph construction, the inverse-order sum graph, for the dihedral group 2. Recent  
studies have explored various graph-theoretic concepts and their applications in group theory.  
Romdhini and Nawawi [1] derived the general formula for the degree sum energy of the non-commuting graph  
associated with dihedral groups 2of order 2푛 (where 푛 ≥ 3), defined as 퐸퐷ꢁ(훤퐺), with the degree sum  
matrix entries given by 푑ꢂ푝 + 푑ꢂ푞.  
Meanwhile, Ali et al. [2] established new upper bounds for the inverse sum 푖푛푑푒푔 (ꢃꢁꢃ) index of connected  
graphs, expressed as  
푗  
( )  
ꢃꢁꢃ 퐺 = ∑  
+ 푑푗  
ꢄ∼푗  
where 푖 ∼ ꢅdenotes adjacency between vertices and ꢅ.  
Further contributions include Naduvath's [3] introduction of the order sum graph of finite groups, analyzing its  
structural properties such as chromatic number, domination number, and spectral characteristics. Soto [4]  
investigated the irreducible representations of the dihedral group 2, determining that it has 푘 + 2 표푟 푘 + 3  
irreducible representations depending on whether n is odd or even. Tizard [5] expanded on domination theory in  
order sum graphs, examining variants like connected, global, and secure domination. Qasem et al. [6] studied  
the sum graph of {  
groups, extending graph theory applications to algebraic structures and computing  
}
ꢆ ꢈ  
Page 1460  
topological indices. Jamal and Rather [7] analyzed the inverse sum indeg energy of graphs, deriving bounds and  
constructing equienergetic graph pairs. Hasani [8] identified extremal molecular graphs with minimal and  
maximal ISI values, while Hafeez and Farooq [9] formulated ISI energy for specific graph classes and established  
related bounds. Gutman et al. [10] contributed by refining lower bounds for the ISI index through a  
comprehensive review of existing literature.  
A significant gap exists in the literature, as no graph structure has been developed to combine the properties of  
inverse graphs and order sum graphs, despite extensive research on graph structures associated with groups.  
Motivated by the combination of two or more properties of a single graph to form a new graph, as studied by  
authors such as Magami, Ibrahim, Ashafa, and Gana [11] and Bello, Ali, and Isah [12].  
This paper fills that gap by defining the inverse-order sum graph 퐼푂(퐺), where two distinct elements are  
adjacent if and only if they are inverses and the sum of their orders is at least |G|. We focus on the dihedral group  
2(n odd), the smallest non-abelian family containing elements of three distinct orders. The resulting hybrid  
graph exhibits dramatically lower density than its parents, offering a refined invariant for symmetry analysis,  
Cayley graph comparison, and group-based network modelling (e.g., sensor networks with reflection symmetry  
or cryptographic key predistribution exploiting inverse and order constraints).  
The remainder of this article is structured as follows: Section 2 outlines the fundamental graph-theoretic and  
algebraic concepts required. Subsequent sections present our main results on the properties of Γ퐼푣푂푆퐺(퐷2)  
including vertex degrees, connectivity, and completeness, followed by a conclusion.  
Fundamental Graph-Theory and Algebraic Scheme  
Fundamental Graph-Theory  
This section provides an overview of essential graph-theoretic concepts and previously established results that  
applied to achieve our study's main findings. These concepts facilitate understanding of our paper. We focus on  
simple graphs, which are undirected and no multiple edges or no loops.  
Definition 2.1.1 (Inverse Graph), [16]. The inverse graph of a group G, denoted by Γ (G), is a simple graph  
Iv  
with vertex set G. Two distinct vertices u and v are adjacent if u ∗ v ∈ TΓ or v ∗ u ∈ TΓ, where TΓ = {t ∈  
G | t ≠ t − 1}.  
Example 2.1.1: The dihedral group D2n = ⟨r, s ∶ rn = s2 = e, srs = r − 1⟩, with n ≥ 3, is an example of  
a group with no set of generators whose all elements are non-self-invertible. All non self-invertible elements of  
n
the group have the form ri, where i ∈ {1,2, . . . , n} and i =  
if n is even. The element s ∈ D2n cannot be  
2
expressed as a product of some finite ri or their inverses. Figure 2 shows the inverse graph of group 6.  
Definition 2.1.2 (Order Sum Graph) [3] Order sum Graph of a group is a simple graph whose vertices are the  
elements of , and two distinct vertices are adjacent if either the sum of their orders is greater than or equal to  
the order of . The order sum graph of group is denoted by ΓOS(G).  
Example 2.1.2: The dihedral group of D2n = ⟨r, s ∶ rn = s2 = e, srs = r − 1⟩, with n ≥ 3, where r3 =  
2
2
e, s2 = e and rs = sr−1 and e = 1, r = r  
= 3 and s = rs = r s = 2.  
| |  
| |  
| |  
| |  
| |  
|
|
Now compute all pairs x ≠ y such that ∣ x ∣ +∣ y ∣ 3 + 3 = 6 6: (r, r2).  
Definition 2.1.3. (Inverse-order Sum Graph of D2n), [3]. Let ΓD  
= {t ∈ D2n|t ≠ t−1}, The inverse-order sum  
2n  
graph of D2n denoted by ΓIvOSG(D2n) is a simple graph whose vertices are the elements of D2n, and two distinct  
vertices are adjacent if and only if u ⋅ v ΓD or v ⋅ u ∈ ΓD and o(u) + o(v) ≥ |G|, ∀ u ≠ v ∈ D2n.  
2n  
2n  
Definition 2.1.4 (Complete Graph) [17] A complete graph is a simple graph Γ such that every pair of vertices is  
joined by an edge. Any complete graph on n vertices is denoted Kn  
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Definition 2.1.5 (connected Graph), [14]. A graph Γ is said to be connected if any two distinct vertices of Γ  
are joined by a path. Γ is said to be disconnected if Γ is not connected,  
Definition 2.1.6. (Self Inverse Elements), [14]. Let (X,∗) be a finite Abelian group, an element 푥 ∈ 푋 is called  
self inverse element if x−1 = x. The set of all self inverse elements in X is denoted by Sivn  
Definition 2.1.7. (Mutual Inverse Elements). [14] Let (X,∗ ) be a finite Abelian group, an element y ∈ X is  
called mutual inverse element if ∃y′ ∈ X with y ≠ y′ such that y−1 = y′. The set of all mutual inverse  
elements in X is denoted by Mivn (Sowaity et al., 2020).  
Algebraic Schemes  
Lemma 2.2.1 Let 2= ⟨푟, 푠⟩ be the dihedral group of order 2푛, where 푛 ≥ 3 is a positive integer (even or  
odd). Then the elements of 2can be partitioned into three disjoint subsets based on element orders and  
corresponding cardnalities as follows:  
푃₁ = {푒}: Identity element of order 1.  
푃₂: Elements of order 2 (reflections), with |푃₂| = 푛.  
푃₃: Non-identity elements of order n (rotations), with |푃₃| = 푛 − 1.  
Proof  
Let 2= ⟨푟, 푠⟩ be defined by the relations = 푒, 2 = 푒, and 푠푟푠 = 푟1. The group consists of 푛  
rotations {푟0, 푟1, … , 푟−1} and n reflections {푠, 푠푟, 푠푟2 , … , 푠푟−1}, giving a total of 2푛 elements.  
Disjoint Subsets:  
We define the following three subsets:  
1 = {푒}, 2 = {푟∣ 1 ≤ 푘 ≤ 푛 − 1}, and 3 = {푠푟0 ≤ 푘 ≤ 푛 − 1}.  
Clearly, these subsets are pairwise disjoint. The identity element 푒 = 푟0 appears only in 1. The elements ꢉ  
2 are proper rotations and are distinct from both the identity and all reflections. The elements of 3 contain the  
generator , and thus cannot appear in 1 or 2. Therefore, 1 2 = ∅, 1 3 = ∅, and 2 3 = ∅.  
Since  
1 2 3 = {푟0 ≤ 푘 ≤ 푛 − 1} ∪ {푠푟0 ≤ 푘 ≤ 푛 − 1} = 퐷2,  
we conclude that {1, 2, 3} is a disjoint partition of the group.  
Cardinalities:  
|
|
The subset 1 consists of only the identity element, so 1 = 1. The set 2 contains all non-identity rotations,  
with ranging from 1 to 푛 − 1, so 2 = 푛 − 1. The set 3 contains all reflections 푠푟, with ranging from  
|
|
0 to 푛 − 1, giving exactly elements. Hence, the cardinalities are:  
|
|
|
|
|
|
1 = 1, 2 = 푛 − 1, 3 = 푛.  
Lemma 2.2.2  
Let 2= ⟨푟, 푠⟩ be the dihedral group of order 2푛, where n ≥ 3 is a positive integer (even or odd) Let the  
elements of 2be partitioned into the disjoint subsets 1 = {푒}, 2 = {푟∣ 1 ≤ 푘 ≤ 푛 − 1}, and 3 =  
{푠푟0 ≤ 푘 ≤ 푛 − 1} as defined in the preceding lemma, then for any 푥 ∈ 퐷2the order in these subsets  
are as follows:  
Page 1462  
1,  
If 푥 1,  
, where 푥 = 푟, If 푥 2,  
( )  
표 푥 = {  
푔푐푑(푘, 푛)  
2,  
for all 푥 = 푠푟If 푥 3  
Proof  
Let 푥 ∈ 퐷2. We consider three cases based on the partition of 2:  
Case 1: 푥 ∈ 1  
Then 푥 = 푒, the identity element. By definition of the identity, 1 = 푒, and for no smaller positive integer.  
Therefore, 표(푒) = 1.  
Case 2: 푥 ∈ 2  
Then 푥 = 푟, for some 1 ≤ 푘 ≤ 푛 − 1. Since generates a cyclic subgroup of order , we know that the  
(
)
order of in a cyclic group of order is given by 표 푟  
=
.
ꢊꢋꢌ(ꢉ,ꢀ)  
Hence, each 2 has order dividing , and its specific value depends on the value of relative to .  
Case 3: 푥 ∈ 3  
Then 푥 = 푠푟for some 0 ≤ 푘 ≤ 푛 − 1. We compute the square of as follows:  
2  
= 푠푟⋅ 푠푟= 푠 푟 푠 푟  
= 푠 푠푟 − 푘 푟  
= 푠2. 푟= 푒 ⋅ 푒 = 푒.  
(
)
(
)
(
)
푠푟  
Since 2 = 푒, the minimal positive integer m such that = 푒 푖푠 ꢎ = 2. Thus, 표(푥) = 2.  
RESULTS AND DISCUSSION  
This section establishes theorems for vertex degrees, graph sizes, and completeness, followed by a comparative  
analysis with concrete examples.  
Theorem 3.1 (Vertex Degree of 푰풗(푫₂ₙ) for n Odd)  
For 훤 (퐷₂ₙ) with n odd, the vertex degrees are:  
퐼푣  
푛 − 1 푓표푟 ꢂ 1 3  
( )  
{
If is odd, deg ꢂ =  
푛 − 2 푓표푟 ꢂ 2  
Proof  
For ꢂ ∈ 푃₁: The identity is adjacent to all elements in = 푃₃, so 푑푒푔(ꢂ) = |푃₃| = 푛 − 1.  
For ꢂ ∈ 푃₂: Any two distinct reflections have product in 푃₃ ⊆ , so each reflection is adjacent to all  
other 푛 − 1 reflections.  
For ꢂ ∈ 푃₃: A non-identity rotation is adjacent to the identity and to all other non-identity rotations except its  
inverse, giving 1 + (푛 − 2) − 1 = 푛 − 2 neighbors  
Theorem 3.2 (Size of (푫₂ₙ) for n Odd)  
(
)(  
)
−1 2−1  
| |  
The number of edges is 퐸 =  
2
Page 1463  
Proof  
By the Handshaking Lemma and Theorem 4.2.1: Sum of degrees = 1 · (푛 − 1) + 푛 · (푛 − 1) + (푛 − 1) ·  
(푛 − 2) = (푛 − 1)(2푛 − 1)  
(
)(  
)
−1 2−1  
| |  
Thus 퐸 =  
.
2
Theorem 3.3 (Completeness of 푰풗(푫₂ₙ) for Odd)  
훤 (퐷₂ₙ) is not complete.  
퐼푣  
Proof  
Vertices in 푃₁ and 푃₂ are not adjacent to each other, as their product is not in .  
Example  
for 푛 = 3 퐷₆ = {푒, 푟, 푟², 푠, 푠푟, 푠푟²}, = {푟, 푟²}.  
The edges are only 푒 ∼ 푟, 푒 ∼ 푟², 푠 ∼ 푠푟, 푠 ∼ 푠푟², 푠푟 ∼ 푠푟² (5 edges).  
(
)(  
)
3−1 6−1  
| |  
Degrees match the theorem, and 퐸 =  
= 5.  
2
Theorem 3,4 (Vertex Degree of 푶푺(푫₂ₙ) for n Odd)  
0 푓표푟 ꢂ 1 3  
( )  
deg ꢂ =  
{
푛 − 2 푓표푟 ꢂ 2  
Proof  
The condition |푥| + |푦| ≥ 2푛 holds only when both and have order , i.e., both are in 푃₃. Thus only  
vertices in 푃₃ have neighbors, and each is adjacent to all other 푛 − 2 vertices in 푃₃.  
Theorem 3.5 (Size of 푶푺(푫₂ₙ) for n Odd)  
(
)(  
)
−1 −2  
| |  
퐸 =  
2
Proof  
The graph is the complete graph on 푃₃ (which has 푛 − 1 vertices).  
Theorem 3.6 (Completeness of 푶푺(푫₂ₙ) for Odd)  
훤ₒₙ(퐷₂ₙ) is not complete (vertices in 푃₁ ∪ 푃₂ are isolated).  
Page 1464  
Example  
for 푛 = 3 Only the pair (푟, 푟²) satisfies the order-sum condition, giving one edge 푟 ∼ 푟² and |퐸| = 1, as  
predicted.  
Theorem 2.7 (Vertex Degree of 푰풗푶푺(푫₂ₙ) for n Odd)  
0 푓표푟 ꢂ 1 3  
( )  
deg ꢂ =  
{
푛 − 3 푓표푟 ꢂ 2  
Proof  
Adjacency is possible only inside 푃₃ (order-sum condition). Within P₃, the inverse condition additionally forbids  
an edge to the unique inverse, so each vertex loses one more neighbor compared to , yielding 푛 − 3 neighbors.  
푂푆  
(
)(  
)
−1 −3  
| |  
Theorem 3.8 (Size of 푰풗푶(푫₂ₙ) for Odd) 퐸 =  
2
Proof  
(
)(  
)
−1 −2  
−1  
Start from the complete graph on 푃₃ (which has  
edges) and remove the  
edges corresponding to  
2
2
(
)(  
−1 −3  
each rotation and its inverse, giving  
) edges.  
2
Theorem 3.9 (Completeness of 푰풗푶푺(푫₂ₙ) for Odd)  
퐼푣푂(퐷₂ₙ) is not complete (isolated vertices in 푃₁ ∪ 푃₂ and missing inverse edges in 푃₃).  
Example for 풏 = ퟑ The only candidate pair (푟, 푟²) fails the inverse condition (푟 · 푟² = 푒 ∉ ), so the graph  
has no edges (|퐸| = 0), matching the formula.  
Page 1465  
Table 3.1: Comparative Analysis for n Odd  
Graph Property  
Order  
Sum  
Graph  
Inverse-Order  
Sum  
Graph  
Inverse Graph (휞푰풗)  
(휞푶푺  
)
(휞푰풗푶푺)  
0
0
0
0
Vertex degree in 푃₁  
Vertex degree in 푃₂  
Vertex degree in 푃₃  
Number of edges |퐸|  
푛 − 1  
푛 − 1  
푛 − 2  
푛 − 2  
푛 − 3  
(
)(  
)
(
)(  
)
(
)(  
)
푛 − 1 2푛 − 1  
푛 − 1 푛 − 2  
푛 − 2 푛 − 4  
2
2
2
Completeness  
Not complete  
Not complete  
Not complete  
DISCUSSION  
The Inverse Graph exhibits high connectivity due to its purely algebraic (inverse) adjacency rule, creating edges  
across multiple partitions. The Order Sum Graph is considerably sparser, with edges restricted to high-order  
elements only. The Inverse-Order Sum Graph is the most restrictive, combining both conditions and producing  
the sparsest structure among the three. These differences illustrate how the choice of adjacency criteria  
dramatically affects connectivity, with potential applications in algebraic graph theory, group-based network  
modeling, and the study of Cayley-type graphs on dihedral groups.  
Applications and Implications  
The strict hierarchy 훤  
퐼푣  
⊃ 훤  
푂푆  
⊃ 훤 provides a tunable family of Cayley-like graphs on the same vertex set,  
퐼푂푆  
allowing precise control of edge density via algebraic constraints that is useful in symmetry-constrained network  
design and key predistribution in group-based cryptography. The almost-complete structure of on rotations  
퐼푂푆  
suggests applications in robust synchronisation protocols for systems with reflection symmetry.  
CONCLUSION  
This research successfully developed and analyzed the inverse-order sum graph 훤ꢃꢂꢐꢁ(퐷₂ₙ) for the dihedral  
group D₂ₙ with n odd by merging the adjacency conditions of the inverse graph and the order sum graph,  
(
)(  
) edges, 훤 (퐷₂ₙ) has  
−1 2−1  
revealing a hierarchy in graph density: 훤ꢃᵥ(퐷₂ₙ) is the most connected with  
푂푆  
2
(
)(  
)
)(  
edges, and 훤ꢃᵥꢐꢁ(퐷₂ₙ) is the sparsest with(  
) edges.  
−1 −2  
−1 −3  
2
2
By merging inverse and order-sum relations, the inverse-order sum graph 퐼푂(퐷2) (n odd) produces the  
sparsest non-trivial hybrid in the family while preserving algebraic significance, demonstrate the power of hybrid  
constructions for refining graphical invariants of non-abelian groups. The results open natural extensions to n  
even, dicyclic groups, and spectral or domination parameters.  
Author contributions: Conceptualization, Ibrahim, M., Isah, S. H.; Methodology, Ibrahim, M., Isah, S. H.;  
Software, Isah, S. H.; Validation, Ibrahim, M.; Formal Analysis, Ibrahim, M.; Investigation, Isah, S. H.; Writing  
original draft preparation, Isah, S. H.; Writing review and editing, Ibrahim, M.; Visualization, Isah, S. H.;  
Supervision, Ibrahim, M.; Project Administration, Ibrahim, M. All authors have read and agreed to the published  
version of the manuscript.  
Funding Statement: This research received no external funding. Data Availability: Not applicable.  
Acknowledgments: The authors thank the anonymous reviewers for their insightful suggestions.  
Page 1466  
Conflict of interest: The authors declare no conflict of interest.  
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