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 훤퐼푂푆(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
퐷2ꢀ (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(퐷2ꢀ)
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 퐷6.
Definition 2.1.2 (Order Sum Graph) [3] Order sum Graph of a group 퐺 is a simple graph whose vertices are the
elements of 퐺, and two distinct vertices are adjacent if either the sum of their orders is greater than or equal to
the order of 퐺. The order sum graph of group 퐺 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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