arXiv subjects

S. Pirzada

Publications and source records attributed to S. Pirzada.

At least 37 records · Page 2Linked to original sources

arXiv2021

On distance Laplacian energy in terms of graph invariants

For a simple connected graph $G$ of order $n$ having distance Laplacian eigenvalues $ ρ^{L}_{1}\geq ρ^{L}_{2}\geq \cdots \geq ρ^{L}_{n}$, the distance Laplacian energy $DLE(G)$ is defined as $DLE(G)=\sum_{i=1}^{n}\left|ρ^{L}_i-\frac{2 W(G)}{n}\right|$, where $W(G)$ is the Wiener index of $G$. We obtain a relationship between the Laplacian energy and distance Laplacian energy for graphs with diameter 2. We obtain lower bounds for the distance Laplacian energy $DLE(G)$ in terms of the order $n$, the Wiener index $W(G)$, independence number, vertex connectivity number and other given parameters. We characterize the extremal graphs attaining these bounds. We show that the complete bipartite graph has the minimum distance Laplacian energy among all connected bipartite graphs and complete split graph has the minimum distance Laplacian energy among all connected graphs with given independence number. Further, we obtain the distance Laplacian spectrum of the join of a graph with the union of two other graphs. We show that the graph $K_{k}\bigtriangledown(K_{t}\cup K_{n-k-t}), 1\leq t \leq \lfloor\frac{n-k}{2}\rfloor $, has the minimum distance Laplacian energy among all connected graphs with vertex connectivity $k$. We conclude this paper with a discussion on trace norm of a matrix and the importance of our results in the theory of trace norm of the matrix $D^L(G)-\frac{2W(G)}{n}I_n$.

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arXiv2021

On the eigenvalues of signed complete bipartite graphs

Let $Γ=(G,σ)$ be a signed graph, where $σ$ is the sign function on the edges of $G$. The adjacency matrix of $Γ=(G, σ)$ is a square matrix $A(Γ)=A(G, σ)=\left(a_{i j}^σ\right)$, where $a_{i j}^σ=σ\left(v_{i} v_{j}\right) a_{i j}$. In this paper, we determine the eigenvalues of the signed complete bipartite graphs. Let $(K_{p, q},σ)$, $p\leq q$, be a signed complete bipartite graph with bipartition $(U_p, V_q)$, where $U_p=\{u_1,u_2,\ldots,u_p\}$ and $V_q=\{v_1,v_2,\ldots,v_q\}$. Let $(K_{p, q},σ)[U_r\cup V_s]$, $r\leq p$ and $s\leq q $, be an induced signed subgraph on minimum vertices $r+s$, which contains all negative edges of the signed graph $(K_{p, q},σ)$. We show that the multiplicity of eigenvalue $0$ in $(K_{p, q},σ)$ is at least $ p+q-2k-2$, where $k=min(r,s)$. We determine the spectrum of signed complete bipartite graph whose negative edges induce disjoint complete bipartite subgraphs and path. We obtain the spectrum of signed complete bipartite graph whose negative edges (positive edges) induce an $r-$ regular subgraph $H$. We find a relation between the eigenvalues of this signed complete bipartite graph and the non-negative eigenvalues of $H$.

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arXiv2021

On $A_α$-spectrum of joined union of graphs and its applications to power graphs of finite groups

For a simple graph $G$, the generalized adjacency matrix $A_α(G)$ is defined as $A_α(G)=αD(G)+(1-α)A(G), α\in [0,1]$, where $A(G)$ is the adjacency matrix and $D(G)$ is the diagonal matrix of vertex degrees of $G$. This matrix generalises the spectral theories of the adjacency matrix and the signless Laplacian matrix of $G$. In this paper, we find $ A_α $-spectrum of the joined union of graphs in terms of spectrum of adjacency matrices of its components and the zeros of the characteristic polynomials of an auxiliary matrix determined by the joined union. We determine the $ A_α$-spectrum of join of two regular graphs, the join of a regular graph with the union of two regular graphs of distinct degrees. As an applications, we investigate the $ A_α $-spectrum of certain power graphs of finite groups.

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arXiv2021

On a conjecture of Laplacian energy of trees

Let $G$ be a simple graph with $n$ vertices, $m$ edges having Laplacian eigenvalues $μ_1, μ_2, \dots, μ_{n-1},μ_n=0$. The Laplacian energy $LE(G)$ is defined as $LE(G)=\sum_{i=1}^{n}|μ_i-\overline{d}|$, where $\overline{d}=\frac{2m}{n}$ is the average degree of $G$. Radenković and Gutman conjectured that among all trees of order $n$, the path graph $P_n$ has the smallest Laplacian energy. Let $ \mathcal{T}_{n}(d) $ be the family of trees of order $n$ having diameter $ d $. In this paper, we show that Laplacian energy of any tree $T\in \mathcal{T}_{n}(4)$ is greater than the Laplacian energy of $P_n$, thereby proving the conjecture for all trees of diameter $4$. We also show the truth of conjecture for all trees with number of non-pendent vertices at most $\frac{9n}{25}-2$. Further, we give some sufficient conditions for the conjecture to hold for a tree of order $n$.

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arXiv2021

On normalized Laplacian eigenvalues of power graphs associated to finite cyclic groups

For a simple connected graph $ G $ of order $ n $, the normalized Laplacian is a square matrix of order $ n $, defined as $\mathcal{L}(G)= D(G)^{-\frac{1}{2}}L(G)D(G)^{-\frac{1}{2}}$, where $ D(G)^{-\frac{1}{2}} $ is the diagonal matrix whose $ i$-th diagonal entry is $ \frac{1}{\sqrt{d_{i}}} $. In this article, we find the normalized Laplacian eigenvalues of the joined union of regular graphs in terms of the adjacency eigenvalues and the eigenvalues of quotient matrix associated with graph $ G $. For a finite group $\mathcal{G}$, the power graph $\mathcal{P}(\mathcal{G})$ of a group $ \mathcal{G} $ is defined as the simple graph in which two distinct vertices are joined by an edge if and only if one is the power of other. As a consequence of the joined union of graphs, we investigate the normalized Laplacian eigenvalues of power graphs of finite cyclic group $ \mathbb{Z}_{n}. $

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arXiv2020

Sphere of Influence Dimension Conjecture 'Almost Proved'

The sphere-of-influence graph (SIG) on a finite set of points in a metric space, each with an open ball centred about it of radius equal to the distance between that point and its nearest neighbor, is defined to be the intersection graph of these balls. Let $G$ be a graph of order $n,$ having no isolated vertices. The SIG-dimension of $G,$ denoted by $SIG(G),$ is defined to be the least possible $d$ such that $G$ can be realized as a sphere of influence graph in $\mathbb{R}^d,$ equipped with sup-norm. In 2000, Boyer [E. Boyer, L. Lister and B. Shader, Sphere of influence graphs using the sup-norm, Mathematical and Computer Modelling 32 (2000) 1071-1082] put forward the SIG dimension conjecture, which states that $$SIG(G)\leq \bigg\lceil \frac{2n}{3}\bigg\rceil.$$ In this paper, we 'almost' establish this conjecture by proving that $$SIG(G)\leq \bigg{ \lfloor}\frac{2n}{3}\bigg{ \rfloor}+2.$$

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arXiv2020

On $α$-adjacency energy of graphs and Zagreb index

Let $A(G)$ be the adjacency matrix and $D(G)$ be the diagonal matrix of the vertex degrees of a simple connected graph $G$. Nikiforov defined the matrix $A_α(G)$ of the convex combinations of $D(G)$ and $A(G)$ as $A_α(G)=αD(G)+(1-α)A(G)$, for $0\leq α\leq 1$. If $ ρ_{1}\geq ρ_{2}\geq \dots \geq ρ_{n}$ are the eigenvalues of $A_α(G)$ (which we call $α$-adjacency eigenvalues of $G$), the $ α$-adjacency energy of $G$ is defined as $E^{A_α}(G)=\sum_{i=1}^{n}\left|ρ_i-\frac{2αm}{n}\right|$, where $n$ is the order and $m$ is the size of $G$. We obtain the upper and lower bounds for $E^{A_α}(G) $ in terms of order $n$, size $m$ and Zagreb index $Zg(G)$ associated to the structure of $G$. Further, we characterize the extremal graphs attaining these bounds.

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arXiv2019

On spectral spread of generalized distance matrix of a graph

For a simple connected graph $G$, let $D(G)$, $Tr(G)$, $D^{L}(G)$ and $D^{Q}(G)$, respectively be the distance matrix, the diagonal matrix of the vertex transmissions, distance Laplacian matrix and the distance signless Laplacian matrix of a graph $G$. The convex linear combinations $D_α(G)$ of $Tr(G)$ and $D(G)$ is defined as $D_α(G)=αTr(G)+(1-α)D(G)$, $0\leq α\leq 1$. As $D_{0}(G)=D(G), ~~~ 2D_{\frac{1}{2}}(G)=D^{Q}(G), ~~~ D_{1}(G)=Tr(G)$ and $D_α(G)-D_β(G)=(α-β)D^{L}(G)$, this matrix reduces to merging the distance spectral, distance Laplacian spectral and distance signless Laplacian spectral theories. Let $\partial_{1}(G)\geq \partial_{2}(G)\geq \dots \geq \partial_{n}(G)$ be the eigenvalues of $D_α(G)$ and let $D_αS(G)=\partial_{1}(G)-\partial_{n}(G)$ be the generalized distance spectral spread of the graph $G$. In this paper, we obtain some bounds for the generalized distance spectral spread $D_α(G)$. We also obtain relation between the generalized distance spectral spread $D_α(G)$ and the distance spectral spread $S_{D}(G)$. Further, we obtain the lower bounds for $D_αS(G)$ of bipartite graphs involving different graph parameters and we characterize the extremal graphs for some cases. We also obtain lower bounds for $D_αS(G)$ in terms of clique number and independence number of the graph $G$ and characterize the extremal graphs for some cases.

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arXiv2018

Mixing Time of Markov chain of the Knapsack Problem

To find the number of assignments of zeros and ones satisfying a specific Knapsack Problem is $\#P$ hard, so only approximations are envisageable. A Markov chain allowing uniform sampling of all possible solutions is given by Luby, Randall and Sinclair. In 2005, Morris and Sinclair, by using a flow argument, have shown that the mixing time of this Markov chain is $\mathcal{O}(n^{9/2+ε})$, for any $ε> 0$. By using a canonical path argument on the distributive lattice structure of the set of solutions, we obtain an improved bound, the mixing time is given as $τ_{_{x}}(ε) \leq n^{3} \ln (16 ε^{-1})$.

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arXiv2015

Spectra and energy of bipartite signed digraphs

The set of distinct eigenvalues of a signed digraph $S$ together with their multiplicities is called its spectrum. The energy of a signed digraph $S$ with eigenvalues $z_1,z_2,\cdots,z_n$ is defined as $E(S)=\sum_{j=1}^{n}|\Re z_j|$, where $\Re z_j $ denotes real part of complex number $z_j$. In this paper, we show that the characteristic polynomial of a bipartite signed digraph of order $n$ with each cycle of length $\equiv 0\pmod 4$ negative and each cycle of length $\equiv 2\pmod 4$ positive is of the form \\ $$ϕ_S(z)=z^n+\sum\limits_{j=1}^{\lfloor{\frac{n}{2}}\rfloor}(-1)^j c_{2j}(S)z^{n-2j},$$\\ where $c_{2j}(S)$ are nonnegative integers. We define a quasi-order relation in this case and show energy is increasing. It is shown that the characteristic polynomial of a bipartite signed digraph of order $n$ with each cycle negative has the form $$ϕ_S(z)=z^n+\sum\limits_{j=1}^{\lfloor{\frac{n}{2}}\rfloor}c_{2j}(S)z^{n-2j},$$ where $c_{2j}(S)$ are nonnegative integers. We study integral, real, Gaussian signed digraphs and quasi-cospectral digraphs and show for each positive integer $n\ge 4$ there exists a family of $n$ cospectral, non symmetric, strongly connected, integral, real, Gaussian signed digraphs (non cycle balanced) and quasi-cospectral digraphs of order $4^n$. We obtain a new family of pairs of equienergetic strongly connected signed digraphs and answer to open problem $(2)$ posed in Pirzada and Mushtaq, Energy of signed digraphs, Discrete Applied Mathematics 169 (2014) 195-205.

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arXiv2013

On the construction of l-equienergetic graphs

For a graph with $n$ vertices and $m$ edges, having Laplacian spectrum $μ_1, μ_2, \cdots,μ_n$ and signless Laplacian spectrum $μ^+_1,μ^+_2, \cdots,μ^+_n$, the Laplacian energy and signless Laplacian energy of $G$ are respectively, defined as $LE(G)=\sum_{i=1}^{n}|μ_i-\frac{2m}{n}|$ and $LE^+(G)=\sum_{i=1}^{n}|μ^+_i-\frac{2m}{n}|$. Two graphs $G_1$ and $G_2$ of same order are said to be $L$-equienergetic if $LE(G_1)=LE(G_2)$ and $Q$-equienergetic if $LE^{+}(G_1)=LE^{+}(G_2)$. The problem of constructing graphs having same Laplacian energy has been considered by Stevanovic for threshold graphs and by Liu and Liu for those graphs whose order is $n\equiv 0$ (mod 7). In general the problem of constructing $L$-equienergetic graphs from any pair of given graphs is still not solved, and this work is an attempt in that direction. We construct sequences of non-cospectral (Laplacian, signless Laplacian) $L$-equienergetic and $Q$-equienergetic graphs from any pair of graphs having same number of vertices and edges.

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arXiv2013

Energy, Laplacian energy of double graphs and new families of equienergetic graphs

For a graph $G$ with vertex set $V(G)=\{v_1, v_2, \cdots, v_n\}$, the extended double cover $G^*$ is a bipartite graph with bipartition (X, Y), $X=\{x_1, x_2, \cdots, x_n\}$ and $Y=\{y_1, y_2, \cdots, y_n\}$, where two vertices $x_i$ and $y_j$ are adjacent if and only if $i=j$ or $v_i$ adjacent to $v_j$ in $G$. The double graph $D[G]$ of $G$ is a graph obtained by taking two copies of $G$ and joining each vertex in one copy with the neighbours of corresponding vertex in another copy. In this paper we study energy and Laplacian energy of the graphs $G^*$ and $D[G]$, $L$-spectra of $G^{k*}$ the $k$-th iterated extended double cover of $G$. We obtain a formula for the number of spanning trees of $G^*$. We also obtain some new families of equienergetic and $L$-equienergetic graphs.

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arXiv2013

Energy of signed digraphs

In this paper we extend the concept of energy to signed digraphs. We obtain Coulson's integral formula for energy of signed digraphs. Formulae for energies of signed directed cycles are computed and it is shown that energy of non cycle balanced signed directed cycles increases monotonically with respect to number of vertices. Characterization of signed digraphs having energy equal to zero is given. We extend the concept of non complete extended $p$ sum (or briefly, NEPS) to signed digraphs. An infinite family of equienergetic signed digraphs is constructed. Moreover, we extend McClelland's inequality to signed digraphs and also obtain sharp upper bound for energy of signed digraph in terms the number of arcs. Some open problems are also given at the end.

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arXiv2010

Imbalances in directed multigraphs

In a directed multigraph, the imbalance of a vertex $v_{i}$ is defined as $b_{v_{i}}=d_{v_{i}}^{+}-d_{v_{i}}^{-}$, where $d_{v_{i}}^{+}$ and $d_{v_{i}}^{-}$ denote the outdegree and indegree respectively of $v_{i}$. We characterize imbalances in directed multigraphs and obtain lower and upper bounds on imbalances in such digraphs. Also, we show the existence of a directed multigraph with a given imbalance set.

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arXiv2006

Signed degree sets in signed bipartite graphs

A signed bipartite graph G(U, V) is a bipartite graph in which each edge is assigned a positive or a negative sign. The signed degree of a vertex x in G(U, V) is the number of positive edges incident with x less the number of negative edges incident with x. The set S of distinct signed degrees of the vertices of G(U, V) is called its signed degree set. In this paper, we prove that every set of integers is the signed degree set of some connected signed bipartite graph. \medskip \medskip

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arXiv2006

Kings and serfs in oriented graphs

In this paper, we extend the concept of kings and serfs in tournaments to that of weak kings and weak serfs in oriented graphs. We obtain various results on the existence of weak kings(weak serfs) in oriented graphs, and show the existence of n-oriented graphs containing exactly k weak kings(weak serfs). Also, we give the existence of n-oriented graphs containing exactly k weak kings and exactly s weak serfs such that b weak kings from k are also weak serfs.

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arXiv2006

Signed degree sets in signed graphs

The set D of distinct signed degrees of the vertices in a signed graph G is called its signed degree set. In this paper, we prove that every non-empty set of positive (negative) integers is the signed degree set of some connected signed graph and determine the smallest possible order for such a signed graph. We also prove that every non-empty set of integers is the signed degree set of some connected signed graph.

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arXiv2006

Mark sequences in digraphs

A k-digraph is an orientation of a multi-graph that is without loops and contains at most k edges between any pair of distinct vertices. We obtain necessary and sufficient conditions for a sequence of non-negative integers in non-decreasing order to be a sequence of numbers, called marks (k-scores), attached to vertices of a k-digraph. We characterize irreducible mark sequences in k-digraphs and uniquely realizable mark sequences in 2-digraphs.

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