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Akbar Ali

Publications and source records attributed to Akbar Ali.

32 records · Page 2Linked to original sources
arXiv2019

On the Complementary Equienergetic Graphs

Energy of a simple graph $G$, denoted by $\mathcal{E}(G)$, is the sum of the absolute values of the eigenvalues of $G$. Two graphs with the same order and energy are called equienergetic graphs. A graph $G$ with the property $G\cong \overline{G}$ is called self-complementary graph, where $\overline{G}$ denotes the complement of $G$. Two non-self-complementary equienergetic graphs $G_1$ and $G_2$ satisfying the property $G_1\cong \overline{G_2}$ are called complementary equienergetic graphs. Recently, Ramane et al. [Graphs equienergetic with their complements, MATCH Commun. Math. Comput. Chem. 82 (2019) 471-480] initiated the study of the complementary equienergetic regular graphs and they asked to study the complementary equienergetic non-regular graphs. In this paper, by developing some computer codes and by making use of some software like Nauty, Maple and GraphTea, all the complementary equienergetic graphs with at most 10 vertices as well as all the members of the graph class $Ω=\{G \ : \ \mathcal{E}(L(G)) = \mathcal{E}(\overline{L(G)}) \text{, the order of $G$ is at most 10}\}$ are determined, where $L(G)$ denotes the line graph of $G$. In the cases where we could not find the closed forms of the eigenvalues and energies of the obtained graphs, we verify the graph energies using a high precision computing (2000 decimal places) of Maple. A result about a pair of complementary equienergetic graphs is also given at the end of this paper.

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arXiv2019

Two Irregularity Measures Possessing High Discriminatory Ability

An $n$-vertex graph whose degree set consists of exactly $n-1$ elements is called antiregular graph. Such type of graphs are usually considered opposite to the regular graphs. An irregularity measure ($IM$) of a connected graph $G$ is a non-negative graph invariant satisfying the property: $IM(G) = 0$ if and only if $G$ is regular. The total irregularity of a graph $G$, denoted by $irr_t(G)$, is defined as $irr_t(G)= \sum_{\{u,v\} \subseteq V(G)} |d_u - d_v|$ where $V(G)$ is the vertex set of $G$ and $d_u$, $d_v$ denote the degrees of the vertices $u$, $v$, respectively. Antiregular graphs are the most nonregular graphs according to the irregularity measure $irr_t$; however, various non-antiregular graphs are also the most nonregular graphs with respect to this irregularity measure. In this note, two new irregularity measures having high discriminatory ability are devised. Only antiregular graphs are the most nonregular graphs according to the proposed measures.

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arXiv2019

A Note on the Modified Albertson Index

The modified Albertson index, denoted by $A\!^*\!$, of a graph $G$ is defined as $A\!^*\!(G)=\sum_{uv\in E(G)} |(d_{u})^{2}- (d_{v})^{2}|$, where $d_u$, $d_v$ denote the degrees of the vertices $u$, $v$, respectively, of $G$ and $E(G)$ is the edge set of $G$. In this note, a sharp lower bound of $A\!^*$ in terms of the maximum degree for the case of trees is derived. The $n$-vertex trees having maximal and minimal $A\!^*$ values are also characterized here. Moreover, it is shown that $A\!^*\!(G)$ is non-negative even integer for every graph $G$ and that there exist infinitely many connected graphs whose $A\!^*$ value is $2t$ for every integer $t\in\{0,3,4,5\}\cup\{8,9,10,\cdots\}$.

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arXiv2018

Estimating Some General Molecular Descriptors of Saturated Hydrocarbons

Three general molecular descriptors, namely the general sum-connectivity index, general Platt index and ordinary generalized geometric-arithmetic index, are studied here. Best possible bounds for the aforementioned descriptors of arbitrary saturated hydrocarbons are derived. These bounds are expressed in terms of number of carbon atoms and number of carbon-carbon bonds of the considered hydrocarbons.

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arXiv2018

A note on polyomino chains with extremum general sum-connectivity index

The general sum-connectivity index of a graph $G$ is defined as $χ_α(G)= \sum_{uv\in E(G)} (d_u + d_{v})^α$ where $d_{u}$ is degree of the vertex $u\in V(G)$, $α$ is a real number different from $0$ and $uv$ is the edge connecting the vertices $u,v$. In this note, the problem of characterizing the graphs having extremum $χ_α$ values from a certain collection of polyomino chain graphs is solved for $α<0$. The obtained results together with already known results (concerning extremum values of polyomino chain graphs) give the complete solution of the aforementioned problem.

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arXiv2017

On the zeroth-order general Randić index, variable sum exdeg index and trees having vertices with prescribed degree

The zeroth-order general Randić index (usually denoted by $R_α^{0}$) and variable sum exdeg index (denoted by $SEI_{a}$) of a graph $G$ are defined as $R_α^{0}(G)= \sum_{v\in V(G)} (d_{v})^α$ and $SEI_{a}(G)= \sum_{v\in V(G)}d_{v}a^{d_{v}}$ where $d_{v}$ is degree of the vertex $v\in V(G)$, $a$ is a positive real number different from 1 and $α$ is a real number other than $0$ and $1$. A segment of a tree is a path $P$, whose terminal vertices are branching or pendent, and all non-terminal vertices (if exist) of $P$ have degree 2. For $n\ge6$, let $\mathbb{PT}_{n,n_1}$, $\mathbb{ST}_{n,k}$, $\mathbb{BT}_{n,b}$ be the collections of all $n$-vertex trees having $n_1$ pendent vertices, $k$ segments, $b$ branching vertices, respectively. In this paper, all the trees with extremum (maximum and minimum) zeroth-order general Randić index and variable sum exdeg index are determined from the collections $\mathbb{PT}_{n,n_1}$, $\mathbb{ST}_{n,k}$, $\mathbb{BT}_{n,b}$. The obtained extremal trees for the collection $\mathbb{ST}_{n,k}$ are also extremal trees for the collection of all $n$-vertex trees having fixed number of vertices with degree 2 (because it is already known that the number of segments of a tree $T$ can be determined from the number of vertices of $T$ with degree 2 and vise versa).

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arXiv2017

On the Extremal Graphs with Respect to Bond Incident Degree Indices

Many existing degree based topological indices can be classified as bond incident degree (BID) indices, whose general form is $BID(G)=\sum_{uv\in E(G)}$ $Ψ(d_{u},d_{v})$, where $uv$ is the edge connecting the vertices $u,v$ of the graph $G$, $E(G)$ is the edge set of $G$, $d_{u}$ is the degree of the vertex $u$ and $Ψ$ is a non-negative real valued (symmetric) function of $d_{u}$ and $d_{v}$. Here, it has been proven that if the extension of $Ψ$ to the interval $[0,\infty)$ satisfies certain conditions then the extremal $(n,m)$-graph with respect to the BID index (corresponding to $Ψ$) must contain at least one vertex of degree $n-1$. It has been shown that these conditions are satisfied for the general sum-connectivity index (whose special cases are: the first Zagreb index and the Hyper Zagreb index), for the general Platt index (whose special cases are: the first reformulated Zagreb index and the Platt index) and for the variable sum exdeg index. Applying aforementioned result, graphs with maximum values of the aforementioned BID indices among tree, unicyclic, bicyclic, tricyclic and tetracyclic graphs were characterized. Some of these results are new and the already existing results are proven in a shorter and more unified way.

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arXiv2017

A Novel/Old Modification of the First Zagreb Index

In the paper [I. Gutman, N. Trinajstić, Chem. Phys. Lett. 17 (1972), 535], it was shown that total $π$-electron energy ($E$) of a molecule $M$ depends on the quantity $\sum_{v\in V(G)}d_{v}^{2}$ (nowadays known as the "first Zagreb index"), where $G$ is the graph corresponding to $M$, $V(G)$ is the vertex set of $G$ and $d_{v}$ is degree of the vertex $v$. In the same paper, the graph invariant $\sum_{v\in V(G)}d_{v}τ_{v}$ (where $τ_{v}$ is the connection number of $v$, that is the number of vertices at distance 2 from $v$) was also proved to influence $E$, but this invariant was never restudied explicitly. We call it "modified first Zagreb connection index" and denote it by $ZC_{1}^{*}$. In this paper, we characterize the extremal elements with respect to the graph invariant $ZC_{1}^{*}$ among the collection of all $n$-vertex chemical trees.

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arXiv2016

Extremal Triangular Chain Graphs for Bond Incident Degree (BID) Indices

A general expression for calculating the bond incident degree (BID) indices of certain triangular chain graphs is derived. The extremal triangular chain graphs with respect to several well known BID indices are also characterized over a particular collection of triangular chain graphs.

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arXiv2016

A note on the augmented Zagreb index of cacti with fixed number of vertices and cycles

Let $\mathcal{C}_{n,k}$ be the family of all cacti with $k$ cycles and $n\geq4$ vertices. In the present note, the element of the class $\mathcal{C}_{n,k}$ having minimum augmented Zagreb index ($AZI$) is characterized. Moreover, some structural properties of the graph(s) having maximum $AZI$ value over the collection $\mathcal{C}_{n,0}$, are also reported.

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arXiv2015

On the Augmented Zagreb Index

Topological indices play an important role in mathematical chemistry especially in the quantitative structure-property relationship (QSPR) and quantitative structure-activity relationship (QSAR). Recent research indicates that the augmented Zagreb index (AZI) possess the best correlating ability among several topological indices. The main purpose of the current study is to establish some mathematical properties of this index, or more precisely, to report tight bounds for the AZI of chemical bicyclic and chemical unicyclic graphs. A Nordhaus-Gaddum-type result for the AZI (of connected graph whose complement is connected) is also derived.

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arXiv2015

More on Comparison Between First Geometric-Arithmetic Index and Atom-Bond Connectivity Index

The first geometric-arithmetic (GA) index and atom-bond connectivity (ABC) index are molecular structure descriptors which play a significant role in quantitative structure-property relationship (QSPR) and quantitative structure-activity relationship (QSAR) studies. Das and Trinajstić [\textit{Chem. Phys. Lett.} \textbf{497} (2010) 149-151] showed that $GA$ index is greater than $ABC$ index for all those graphs (except $K_{1,4}$ and $T^{*}$, see Figure 1) in which the difference between maximum and minimum degree is less than or equal to 3. In this note, it is proved that $GA$ index is greater than $ABC$ index for line graphs of molecular graphs, for general graphs in which the difference between maximum and minimum degree is less than or equal to $(2δ-1)^{2}$ (where $δ$ is the minimum degree and $δ\geq2$) and for some families of trees. Thereby, a partial solution to an open problem proposed by Das and Trinajstić is given.

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arXiv2014

Further Inequalities Between Vertex-Degree-Based Topological Indices

Continuing the recent work of L. Zhong and K. Xu [MATCH Commun. Math. Comput. Chem.71(2014) 627-642], we determine inequalities among several vertex-degree-based topological indices; first geometric-arithmetic index(GA), augmented Zagreb index (AZI), Randi$\acute{c}$ index (R), atom-bond connectivity index (ABC), sum-connectivity index (X)and harmonic index (H).

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