M-Polynomials and Degree-Based Topological Indices of Triangular, Hourglass, and Jagged-Rectangle Benzenoid Systems
Abstract
Chemical graph theory is a branch of mathematical chemistry which has an important effect on the development of the chemical sciences. The study of topological indices is currently one of the most active research fields in chemical graph theory. Topological indices help to predict many chemical and biological properties of chemical structures under study. The aim of this report is to study the molecular topology of some benzenoid systems. M-polynomial has wealth of information about the degree-based topological indices. We compute M-polynomials for triangular, hourglass, and jagged-rectangle benzenoid systems, and from these M-polynomials, we recover nine degree-based topological indices. Our results play a vital role in pharmacy, drug design, and many other applied areas.
1. Introduction
Mathematical chemistry provides tools such as polynomials and functions to capture information hidden in the symmetry of molecular graphs and thus predict properties of compounds without using quantum mechanics. A topological index is a numerical parameter of a graph and depicts its topology. Topological indices describe the structure of molecules numerically and are used in the development of qualitative structure activity relationships (QSARs). Most commonly known invariants of such kinds are degree-based topological indices. These are actually the numerical values that correlate the structure with various physical properties, chemical reactivity, and biological activities [1–5]. It is an established fact that many properties such as heat of formation, boiling point, strain energy, rigidity, and fracture toughness of a molecule are strongly connected to its graphical structure.
Hosoya polynomial, Wiener polynomial [6], plays a pivotal role in distance-based topological indices. A long list of distance-based indices can be easily evaluated from Hosoya polynomial. A similar breakthrough was obtained recently by Deutsch and Klavžar [7], in the context of degree-based indices. Deutsch and Klavžar [7] introduced M-polynomial in, 2015, to play a role, parallel to Hosoya polynomial to determine closed form of many degree-based topological indices [8–11]. The real power of M-polynomial is its comprehensive nature containing healthy information about degree-based graph invariants. These invariants are calculated on the basis of symmetries present in the 2d-molecular lattices and collectively determine some properties of the material under observation. Benzenoid hydrocarbons play a vital role in our environment and in the food and chemical industries.
Benzenoid molecular graphs are systems with deleted hydrogens. It is a connected geometric figure obtained by arranging congruent regular hexagons in a plane so that two hexagons are either disjoint or have a common edge.
This figure divides the plane into one infinite (external) region and a number of finite (internal) regions. All internal regions must be regular hexagons. Benzenoid systems are of considerable importance in theoretical chemistry because they are the natural graph representation of benzenoid hydrocarbons. A vertex of a hexagonal system belongs to, at most, three hexagons. A vertex shared by three hexagons is called an internal vertex [12].
In this paper, we study three benzenoid systems, namely, triangular, hourglass, and jagged-rectangle benzenoid systems.
2. Basic Definitions and Literature Review
Throughout this article, we assume G to be a connected graph, V (G) and E (G) are the vertex set and the edge set, respectively, and dv denotes the degree of a vertex v.
Definition 1. The M-polynomial of G is defined as M(G, x, y) = ∑δ≤i≤j≤Δmij(G)xiyj where δ = Min{dv | v ∈ V(G)}, Δ = Max{dv | v ∈ V(G)} and mij(G) is the edge vu ∈ E(G) such that where ≤j [7].
Wiener Index and its various applications are discussed in [13–15]. Randić Index, R−1/2(G), was introduced by Milan Randić in 1975, defined as . For general details about R−1/2(G) and its generalized Randić Index, refer [16–20], and the Inverse Randić Index is defined as . Clearly, R−1/2(G) is a special case of Rα(G) where α = −(1/2). This index has many applications in diverse areas. Many papers and books such as [21–23] are written on this topological index as well. Gutman and Trinajstić introduced two indices defined as M1(G) = ∑uv∈E(G)(dudv) and M2(G) = ∑uv∈E(G)(dudv). The modified second Zagreb Index is defined as mM2(G) = ∑uv∈E(G)(1/d(u)d(v)). We refer [24–28] to the readers for comprehensive details of these indices. Other famous indices are Symmetric Division Index: SDD(G) = ∑uv∈E(G){(min(du, dv)/max(du, dv)) + (max(du, dv)/min(du, dv))}, Harmonic Index: H(G) = ∑vu∈E(G)(2/du + dv), Inverse Sum Index: I(G) = ∑vu∈E(G)(dudv/du + dv), and Augmented Zagreb Index: [29, 30].
3. Computational Results
In this section, we give our computational results. In terms of chemical graph theory and mathematical chemistry, we associate a graph with the molecular structure where vertices correspond to atoms and edges to bonds. The triangular benzenoid system is shown in Figure 1. In the following theorem, we compute M-polynomial of the triangular benzenoid system.

Theorem 1. Let Tpbe a Triangular benzenoid system where p shows the number of hexagons in the base graph and total no. of hexagons inTpis(1/2)p(p + 1). Then,
Proof. Let Tp be a triangular benzenoid. Then from Figure 1, we have
Now, we derive formulas for many degree-based topological indices using M-polynomial.
Proposition 2. Let Tpbe a triangular Benzenoid.
- (1)
M1(Tp) = 9p2 + 21p − 6
- (2)
M2(Tp) = (27/2)p2 + 45p − 12
- (3)
mM 2(Tp) = (1/6)p2 + (5/6)p + (1/2)
- (4)
Rα(Tp) = (32α+1/2)p2 + (2α+1 · 3α+1 − (32α+1/2))p + (3 · 22α+1 − 2α+1 · 3α+1)
- (5)
RRα(Tp) = (1/2 · 32α−1)p2 + ((1/2α−1 · 3α−1) + (1/2 · 32α−1))p + ((3/22α−1) − (1/2α−1 · 3α−1))
- (6)
SSD(Tp) = 3p2 + 24p − 9
- (7)
H(Tp) = (1/2)p2 + (19/10)p + (3/5)
- (8)
I(Tp) = (9/4)p2 + (99/20)p − (6/5)
- (9)
A(Tp) = (2187/128)p2 + (46965/128)p − 336
Proof. Let
Then,
- (1)
First Zagreb Index:
(9) - (2)
Second Zagreb Index:
(10) - (3)
Modified second Zagreb Index:
(11) - (4)
Generalized Randic Index:
(12) - (5)
Inverse Randic Index:
(13) - (6)
Symmetric Division Index:
(14) - (7)
Harmonic Index:
(15) - (8)
Inverse Sum Index:
(16) - (9)
Augmented Zagreb Index:
(17)
Topological indices of Tp for specific values of p are given in Table 1.
Our next target is the benzenoid hourglass system which is obtained from two copies of a triangular benzenoid Tp by overlapping their external hexagons and shown in Figure 2. In Theorem 3, we compute M-polynomial of the benzenoid hourglass system.
Name of index | p = 1 | p = 2 | p = 3 | p = 4 | p = 5 |
---|---|---|---|---|---|
M-polynomial | 6x2y2 | 6x2y2 + 6x2y3 + 3x3y3 | 6x2y2 + 12x2y3 + 9x3y3 | 6x2y2 + 18x2y3 + 18x3y3 | 6x2y2 + 24x2y3 + 30x3y3 |
First Zagreb Index | 24 | 72 | 138 | 222 | 324 |
Second Zagreb Index | 46.50000000 | 132 | 244.5000000 | 384 | 550.5000000 |
Modified second Zagreb Index | 1.500000000 | 2.833333333 | 4.500000000 | 6.500000000 | 8.833333333 |
Randić Index for α = 1/2 | 17.39387692 | 41.09081537 | 73.78775383 | 115.4846923 | 166.1816308 |
Inverse Randić Index for α = 1/2 | 2.898979486 | 7.348469229 | 12.79795897 | 19.24744872 | 26.69693846 |
Symmetric Division Index | 18 | 5 | 90 | 135 | 186 |
Harmonic Index | 3 | 6.400000000 | 10.80000000 | 16.20000000 | 22.60000000 |
Inverse Sum Index | 6 | 17.70000000 | 33.90000000 | 27.30000000 | 39.90000000 |
Augmented Zagreb Index | 48 | 466.1718750 | 918.5156250 | 1405.031250 | 1925.718750 |

Theorem 3. Let Xpdenotes the Benzenoid Hourglass. Then, its M-polynomial is
Proof. Let Xp denotes the benzenoid hourglass which is obtained from two copies of a triangular benzenoid Tp by overlapping their external hexagons. Then, we have
The edge set of Xp has the following three partitions:
Now,
Thus, the M-polynomial of Xp is
Now, we derive formulas for many degree-based topological indices using M-polynomial.
Proposition 4. Let Xpbe a Benzenoid Hourglass.
- (1)
M1(Xp) = 6(3p2 + 7p − 4)
- (2)
M2(Xp) = 27p2 + 45p − 28
- (3)
mM 2(Xp) = (1/3)p2 + (5/3)p − (2/9)
- (4)
Rα(Xp) = 32α+1p2 + (2α+2 · 3α+1 − 32α+1)p + (22α+3 − 2α+4 · 3α + 4 · 32α)
- (5)
RRα(Xp) = (1/32α−1)p2 + ((1/3α−1 · 2α−2) − (1/32α−1))p + ((1/22α−3) − (1/3 α⋅2α−4) + (4/32α))
- (6)
SSD(Xp) = 6p2 + 20p − 32/3
- (7)
H(Xp) = p2 + 19/5p − 16/15
- (8)
I(Xp) = 9/2p2 + 99/10p − 26/5
- (9)
A(Xp) = (2187/64)p2 + (3957/64)p − (295/16)
Topological indices of Xp for specific values of p are given in Table 2.
Name of index | p = 1 | p = 2 | p = 3 | p = 4 | p = 5 |
---|---|---|---|---|---|
M-polynomial | 8x2y2 − 4x2y3 + 4x3y3 | 8x2y2 + 8x2y3 + 10x3y3 | 8x2y2 + 20x2y3 + 22x3y3 | 8x2y2 + 32x2y3 + 40x3y3 | 8x2y2 + 44x2y3 + 64x3y3 |
First Zagreb Index | 36 | 132 | 264 | 432 | 636 |
Second Zagreb Index | 44 | 170 | 350 | 584 | 872 |
Modified second Zagreb Index | 1.777777778 | 4.444444444 | 7.777777778 | 11.77777778 | 16.44444444 |
Randić Index for α = 1/2 | 92.81601318 | 198.9976439 | 323.1792746 | 465.3609054 | 625.5425361 |
Inverse Randić Index for α = 1/2 | 3.648129059 | 8.097618797 | 14.54710855 | 22.99659829 | 33.44608803 |
Symmetric Division Index | 15.33333333 | 53.33333333 | 103.3333333 | 165.3333333 | 239.3333333 |
Harmonic Index | 3.733333333 | 10.53333333 | 19.33333333 | 30.13333333 | 44.26666667 |
Inverse Sum Index | 9.200000000 | 32.60000000 | 65 | 106.4000000 | 156.8000000 |
Augmented Zagreb Index | 77.56250000 | 241.9062500 | 474.5937500 | 775.6250000 | 1145 |
Now, we study benzenoid jagged-rectangle shown in Figure 3.

Theorem 5. Let Bp,qdenotes a Jagged-rectangle Benzenoid system for allp, q ∈ N − 1.
Proof. Let Bp,q denotes a benzenoid system jagged-rectangle for all p, q ∈ N − 1. A benzenoid jagged-rectangle forms a rectangle and the number of benzenoid called in each chain alternate p and p − 1.
The edge set of Bp,q has the following three partitions:
Now,
Thus, the M-polynomial of Bp,q is
Proposition 6. Let Bp,qdenotes a Jagged-rectangle Benzenoid system for allp, q ∈ N − 1.
- (1)
M1(Bp,q) = 2(13p − q + 18pq − 14)
- (2)
M2(Bp,q) = 33p − 13q − 44
- (3)
mM 2(Bp,q) = (7/9)p + (11/18)q + (2/3)pq − (1/9)
- (4)
Rα(Bp,q) = (3α · 2α+2 + 32α)p + (22α+1 + 3α · 2α+2 − 5 · 32α)q + 6 · 32αpq + (22α+2 − 3α · 2α+2 − 4 · 32α)
- (5)
RRα(Bp,q) = ((1/3α · 2α−2) + (1/32α))p + ((1/22α−1) + (1/3α · 2α−2) − (5/32α))q + (6/32α)pq + ((1/22α−2) − (1/3 α⋅2α−2) − (4/32α))
- (6)
SSD(Bp,q) = (32/3)p + (8/3)q + 12pq − (26/3)
- (7)
H(Bp,q) = (29/15)p + (14/15)q + 2pq − (14/15)
- (8)
I(Bp,q) = (63/10)p − (7/10)q + 9pq − (34/5)
- (9)
A(Bp,q) = (2777/64)p − (573/64)q + (2187/32)pq − 729/16.
Topological indices of Bp,q for specific values of p and q are given in Table 3.
Name of index | p = 1, q = 1 | p = 2, q = 11 | p = 3, q = 11 | p = 4, q = 11 | p = 5, q = 11 |
---|---|---|---|---|---|
M-polynomial | 6x2y2 + 4x2y3 − 2x3y3 | 6x2y2 + 8x2y3 + 4x3y3 | 6x2y2 + 12x2y3 + 12x3y3 | 6x2y2 + 16x2y3 + 16x3y3 | 6x2y2 + 20x2y3 + 22x3y3 |
First Zagreb Index | 12 | 54 | 96 | 138 | 180 |
Second Zagreb Index | −24 | 9 | 42 | 75 | 108 |
Modified second Zagreb Index | 1.944444444 | 3.388888889 | 4.833333333 | 6.277777778 | 7.722222222 |
Randic Index for α = 1/2 | 15.79795897 | 46.59591794 | 77.39387692 | 108.1918359 | 138.9897949 |
Inverse Randic Index for α = 1/2 | 3.966326495 | 7.932652990 | 11.89897949 | 15.86530598 | 19.83163248 |
Symmetric Division Index | 16.66666667 | 39.33333333 | 62 | 84.66666667 | 107.3333333 |
Harmonic Index | 3.933333333 | 7.866666667 | 11.80000000 | 15.73333333 | 19.66666667 |
Inverse Sum Index | 9.200000000 | 24.50000000 | 39.80000000 | 55.10000000 | 70.40000000 |
Augmented Zagreb Index | 57.21875000 | 168.9531250 | 280.6875000 | 392.4218750 | 504.1562500 |
4. Conclusion
In this paper, we computed M-polynomials for triangular, hourglass, and jagged-rectangle benzenoid systems. From these M-polynomials, we recover first Zagreb, second Zagreb, modified second Zagreb, Randić, inverse Randić, symmetric division, inverse sum, and harmonic and augmented Zagreb indices of triangular, hourglass, and jagged-rectangle benzenoid systems. Note that there are no benzenoid molecules having the triangular graphs and hourglass graphs as their skeleton. It is important to mention here that some of these topological indices are calculated directly by using formulas in the literature.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This study was supported by research funds from Dong-A University.
Open Research
Data Availability
The data used to support the findings of this study are included within the article.