Volume 2018, Issue 1 8213950
Research Article
Open Access

M-Polynomials and Degree-Based Topological Indices of Triangular, Hourglass, and Jagged-Rectangle Benzenoid Systems

Young Chel Kwun

Young Chel Kwun

Department of Mathematics, Dong-A University, Busan 49315, Republic of Korea donga.ac.kr

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Ashaq Ali

Ashaq Ali

Department of Mathematics and Statistics, The University of Lahore, Lahore, Pakistan uol.edu.pk

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Waqas Nazeer

Corresponding Author

Waqas Nazeer

Division of Science and Technology, University of Education, Lahore 54000, Pakistan ue.edu.pk

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Maqbool Ahmad Chaudhary

Maqbool Ahmad Chaudhary

Department of Mathematics and Statistics, The University of Lahore, Lahore, Pakistan uol.edu.pk

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Shin Min Kang

Corresponding Author

Shin Min Kang

Department of Mathematics and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea gnu.ac.kr

Center for General Education, China Medical University, Taichung 40402, Taiwan cmu.edu.cn

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First published: 12 September 2018
Citations: 23
Academic Editor: Marjana Novic

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 [15]. 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 [811]. 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) = ∑δijΔmij(G)xiyj where δ = Min{dv | vV(G)}, Δ = Max{dv | vV(G)} and mij(G) is the edge vuE(G) such that where ≤j [7].

Wiener Index and its various applications are discussed in [1315]. 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 [1620], 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 [2123] are written on this topological index as well. Gutman and Trinajstić introduced two indices defined as M1(G) = ∑uvE(G)(dudv) and M2(G) = ∑uvE(G)(dudv). The modified second Zagreb Index is defined as  mM2(G) = ∑uvE(G)(1/d(u)d(v)). We refer [2428] to the readers for comprehensive details of these indices. Other famous indices are Symmetric Division Index: SDD(G) = ∑uvE(G){(min(du, dv)/max(du, dv))  + (max(du, dv)/min(du, dv))}, Harmonic Index: H(G) = ∑vuE(G)(2/du + dv), Inverse Sum Index: I(G) = ∑vuE(G)(dudv/du + dv), and Augmented Zagreb Index: [29, 30].

Tables presented in [710] relate some of these well-known degree-based topological indices with M-polynomial with the following reserved notations:
(1)

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.

Details are in the caption following the image
Triangular benzenoid.

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,

(2)

Proof. Let Tp be a triangular benzenoid. Then from Figure 1, we have

(3)

The edge set of Tp has the following three partitions:
(4)
Now,
(5)
Thus, the M-polynomial of Tp is
(6)

Now, we derive formulas for many degree-based topological indices using M-polynomial.

Proposition 2. Let Tpbe a triangular Benzenoid.

Then,
  • (1)

    M1(Tp) = 9p2 + 21p − 6

  • (2)

    M2(Tp) = (27/2)p2 + 45p − 12

  • (3)

    mM2(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

(7)

Then,

(8)
  • (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.

Table 1. Topological indices of triangular benzenoid for different values of p.
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
Details are in the caption following the image
Benzenoid hourglass system.

Theorem 3. Let Xpdenotes the Benzenoid Hourglass. Then, its M-polynomial is

(18)

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

(19)

The edge set of Xp has the following three partitions:

(20)

Now,

(21)

Thus, the M-polynomial of Xp is

(22)

Now, we derive formulas for many degree-based topological indices using M-polynomial.

Proposition 4. Let Xpbe a Benzenoid Hourglass.

Then,
  • (1)

    M1(Xp) = 6(3p2 + 7p − 4)

  • (2)

    M2(Xp) = 27p2 + 45p − 28

  • (3)

    mM2(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) − (1/3α⋅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.

Table 2. Topological indices of benzenoid hourglass for different values of p.
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.

Details are in the caption following the image
Benzenoid jagged-rectangle system.

Theorem 5. Let Bp,qdenotes a Jagged-rectangle Benzenoid system for allp, qN − 1.

Then,
(23)

Proof. Let Bp,q denotes a benzenoid system jagged-rectangle for all p, qN − 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:

(24)

Now,

(25)

Thus, the M-polynomial of Bp,q is

(26)

Proposition 6. Let Bp,qdenotes a Jagged-rectangle Benzenoid system for allp, qN − 1.

Then,
  • (1)

    M1(Bp,q) = 2(13pq + 18pq − 14)

  • (2)

    M2(Bp,q) = 33p − 13q − 44

  • (3)

    mM2(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) − (1/3α⋅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.

Table 3. Topological indices of jagged-rectangle benzenoid system for different values of p and q.
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.

    Data Availability

    The data used to support the findings of this study are included within the article.

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