We consider circulant graphs G(r,N) where the vertices are the integers modulo N and the neighbours of 0 are {-r,...,-1,1,...,r}. The energy of G(r,N) is a trigonometric sum of N*r terms. For low values of r we compute this sum explicitly. We also study the asymptotics of the energy of G(r,N) for big N. There is a known integral formula for the linear growth coefficient, we find a new expression of the form of a finite trigonometric sum with r terms. As an application we show that in the family G(r,N) for r less or equal than 4 there is a finite number of hyperenergetic graphs. On the other hand, for each r>4 there is at most a finite number of non-hyperenergetic graphs of the form G(r,N). Finally we show that the graph G(r,2r+1) minimizes the energy among all the regular graphs of degree 2r.
Let $Γ=(V,E)$ be a graph of order $n$. A {\em closed distance magic labeling} of $Γ$ is a bijection $\ell : V \to \{1,2, \ldots, n\}$ for which there exists a positive integer $r$ such that $\sum_{x \in N[u]} \ell(x) = r$ for all vertices $u \in V$, where $N[u]$ is the closed neighborhood of $u$. A graph is said to be {\em closed distance magic} if it admits a closed distance magic labeling. In this paper, we classify all connected closed distance magic circulants with valency at most $5$, that is, Cayley graphs $\operatorname{Cay}(\mathbb{Z}_n;S)$ where $|S| \le 5$ and $S$ generates $\mathbb{…
This note provides formula for determinant and inverse of r-circulant matrices with general sequences of third order. In other words, the study combines many papers in the literature.
In this note, we derive closed formulas for the energy of circulant graphs generated by $1$ and $γ$, where $γ\geqslant2$ is an integer. We also find a formula for the energy of the complete graph without a Hamilton cycle.
We consider the number of spanning trees in circulant graphs of $βn$ vertices with generators depending linearly on $n$. The matrix tree theorem gives a closed formula of $βn$ factors, while we derive a formula of $β-1$ factors. Using the same trick, we also derive a formula for the number of spanning trees in discrete tori. Moreover, the spanning tree entropy of circulant graphs with fixed and non-fixed generators is compared.
An oriented circulant graph is called integral if all eigenvalues of its Hermitian adjacency matrix are integers. The main purpose of this paper is to investigate the existence of perfect state transfer ($\PST$ for short) and multiple state transfer ($\MST$ for short) on integral oriented circulant graphs. Specifically, a characterization of $\PST$ (or $\MST$) on integral oriented circulant graphs is provided. As an application, we also obtain a closed-form expression for the number of integral oriented circulant graphs with fixed order having $\PST$ (or $\MST$).
In the present paper, we investigate the complexity of infinite family of graphs $H_n=H_n(G_1,\,G_2,\ldots,G_m)$ obtained as a circulant foliation over a graph $H$ on $m$ vertices with fibers $G_{1},\,G_{2},\ldots,G_{m}.$ Each fiber $G_{i}=C_{n}(s_{i,1},\,s_{i,2},\ldots,s_{i,k_{i}})$ of this foliation is the circulant graph on $n$ vertices with jumps $s_{i,1},\,s_{i,2},\ldots,s_{i,k_{i}}.$ This family includes the family of generalized Petersen graphs, $I$-graphs, sandwiches of circulant graphs, discrete torus graphs and others. We obtain a closed formula for the number $τ(n)$ of spanning tree…
In this paper we obtain precise asymptotics for certain families of graphs, namely circulant graphs and degenerating discrete tori. The asymptotics contain interesting constants from number theory among which some can be interpreted as corresponding values for continuous limiting objects. We answer one question formulated in a paper from Atajan, Yong and Inaba in [1] and formulate a conjecture in relation to the paper from Zhang, Yong and Golin [21]. A crucial ingredient in the proof is to use the matrix tree theorem and express the combinatorial laplacian determinant in terms of Bessel functi…
A mixed graph is said to be \textit{integral} if all the eigenvalues of its Hermitian adjacency matrix are integer. The \textit{mixed circulant graph} $Circ(\mathbb{Z}_n,\mathcal{C})$ is a mixed graph on the vertex set $\mathbb{Z}_n$ and edge set $\{ (a,b): b-a\in \mathcal{C} \}$, where $0\not\in \mathcal{C}$. If $\mathcal{C}$ is closed under inverse, then $Circ(\mathbb{Z}_n,\mathcal{C})$ is called a \textit{circulant graph}. We express the eigenvalues of $Circ(\mathbb{Z}_n,\mathcal{C})$ in terms of primitive $n$-th roots of unity, and find a sufficient condition for integrality of the eigenva…
In this paper, we develop a new method to produce explicit formulas for the number $f_{G}(n)$ of rooted spanning forests in the circulant graphs $ G=C_{n}(s_1,s_2,\ldots,s_k)$ and $ G=C_{2n}(s_1,s_2,\ldots,s_k,n).$ These formulas are expressed through Chebyshev polynomials. We prove that in both cases the number of rooted spanning forests can be represented in the form $f_{G}(n)=p\,a(n)^2,$ where $a(n)$ is an integer sequence and $p$ is a prescribed natural number depending on the parity of $n$. Finally, we find an asymptotic formula for $f_{G}(n)$ through the Mahler measure of the associated …
In this paper, we develop a new method to produce explicit formulas for the number $τ(n)$ of spanning trees in the undirected circulant graphs $C_{n}(s_1,s_2,\ldots,s_k)$ and $C_{2n}(s_1,s_2,\ldots,s_k,n).$ Also, we prove that in both cases the number of spanning trees can be represented in the form $τ(n)=p \,n \,a(n)^2,$ where $a(n)$ is an integer sequence and $p$ is a prescribed natural number depending on the parity of $n.$ Finally, we find an asymptotic formula for $τ(n)$ through the Mahler measure of the associated Laurent polynomial $L(z)=2k-\sum\limits_{i=1}^k(z^{s_i}+z^{-s_i}).$
The boxicity of a graph $G$, denoted by $box(G)$, is the least positive integer $\ell$ such that $G$ can be isomorphic to the intersection graph of a family of boxes in Euclidean $\ell$-space, where box in an Euclidean $\ell$-space is the Cartesian product of $\ell$ closed intervals on the real line. Let $k$ and $d$ be two positive integers with $k\geq 2d$. The circulant graph $G_k^d$ is the graph with vertices set $V(G_k^d)=\{a_0, a_1,\ldots, a_{k-1}\}$ and edge set $E(G_k^d)=\{a_i a_j | \ d\leq |i-j|\leq k-d\}$. Denote $χ(G)$ the chromatic number of a graph $G$. In \cite{Aki} Akira Kamibeppu…
New results on the energy of integral circulant graphs
循环图是并行分布计算中一类重要的互连网络。积分循环图在量子自旋网络的建模中扮演着重要的角色，它支持完美的态转移。整数循环图$\ICG_n (D)$具有顶点集$Z_n = \{0, 1, 2, ..., n - 1\}$，并且如果$\gcd(a-b,n)\in D$，则顶点$a$和$b$相邻，其中$D \subseteq \{d : d \mid n,\ 1\leq d<n\}$。这些图是高度对称的，具有积分谱和一些连接化学图论和数论的显著性质。图的能量最早由Gutman定义为图的邻接矩阵的特征值的绝对值之和。近年来，对具有相等能量的非共谱图对和族的研究有了很大的发展。在[R。B. Bapat，S. Pati，\textit{Energy of a graph is never an odd integer}，Bull. Kerala Math.Assoc.1（2004）129- 132.]，我们刻画了模4整循环图的能量。此外，我们还建立了整循环图能量的一般封闭表达式，推广了[A. Ili ' c，\textit{The energy of unitary Cayley graphs}，Linear Algebra Appl.431（2009），1881- 1889.]。最后，我们提出了一些开放的问题，并在整循环图的$n$顶点，如果$n$是偶数的图中，具有最小能量的极图的特征。
In the present paper, we investigate a family of circulant graphs with non-fixed jumps $$G_n=C_{βn}(s_1, \ldots,s_k,α_1n,\ldots,α_\ell n),\, 1\le s_1<\ldots<s_k\le[\frac{βn}{2}],\, 1\le α_1< \ldots<α_\ell\le[\fracβ{2}].$$ Here $n$ is an arbitrary large natural number and integers $s_1, \ldots,s_k,α_1, \ldots,α_\ell$ are supposed to be fixed. First, we present an explicit formula for the number of spanning trees in the graph $G_n.$ This formula is a product of $βs_k-1$ factors, each given by the $n$-th Chebyshev polynomial of the first kind evaluated at the roots of some prescribed polynomial o…
Two methods, structural (constructive) and multiplier (analytical), of exact enumeration of undirected and directed circulant graphs of orders 27 and 125 are elaborated and represented in detail here together with intermediate and final numerical data. The first method is based on the known useful classification of circulant graphs in terms of $S$-rings and results in exhaustive listing (with the use of COCO and GAP) of all corresponding $S$-rings of the indicated orders. The latter method is conducted in the framework of a general approach developed earlier for counting circulant graphs of pr…
The diameter of a graph is the maximum distance among all pairs of vertices. Thus a graph $G$ has diameter $d$ if any two vertices are at distance at most $d$ and there are two vertices at distance $d$. We are interested in studying the diameter of circulant graphs $C_n(1,s)$, i.e., graphs with the set $\{0,1,\ldots, n-1\}$ of integers as vertex set and in which two distinct vertices $i,j \in \{0,1,\ldots, n-1\}$ are adjacent if and only if $|i-j|_n\in \{1,s\}$, where $2\leq s\leq \lfloor \frac{n-1}{2} \rfloor$ and $|x|_n=\min(|x|, n-|x|)$. Despite the regularity of circulant graphs, it is dif…
A Frobenius group is a transitive permutation group which is not regular but only the identity element can fix two points. Such a group can be expressed as the semi-direct product $G = K \rtimes H$ of a nilpotent normal subgroup $K$ and another group $H$ fixing a point. A first-kind $G$-Frobenius graph is a connected Cayley graph on $K$ with connection set an $H$-orbit $a^H$ on $K$ that generates $K$, where $H$ has an even order or $a$ is an involution. It is known that the first-kind Frobenius graphs admit attractive routing and gossiping algorithms. A complete rotation in a Cayley graph on a…
Chromatic numbers of circulants with indispensable generators
University of California, Irvine（加州大学尔湾分校）；University of California, San Diego（加州大学圣地亚哥分校）；California State University, Los Angeles（加州州立大学洛杉矶分校）；Rice University（莱斯大学）
Let $G=(V,E)$ be a graph of order $n$. A closed distance magic labeling of $G$ is a bijection $\ell \colon V(G)\rightarrow \{1,\ldots ,n\}$ for which there exists a positive integer $k$ such that $\sum_{x\in N[v]}\ell (x)=k$ for all $v\in V $, where $N[v]$ is the closed neighborhood of $v$. We consider the closed distance magic graphs in the algebraic context. In particular we analyze the relations between the closed distance magic labelings and the spectra of graphs. These results are then applied to the strong product of graphs with complete graph or cycle and to the circulant graphs. We end…
Let $G=(V,E)$ be a loopless graph and $\mathcal{T}(G)$ be the set of all spanning trees of $G$. Let $L(G)$ be the line graph of the graph $G$ and $t(L(G))$ be the number of spanning trees of $L(G)$. Then, by using techniques from electrical networks, we obtain the following formula: $$ t(L(G)) = \frac{1}{\prod_{v\in V}d^2(v)}\sum_{T\subseteq \mathcal{T}(G)}\big[\prod_{e = xy\in T}d(x)d(y)\big]\big[\prod_{e = uv\in E\backslash T}[d(u)+d(v)]\big]. $$ As a result, we provide a very simple and different proof of the formula on the number of spanning trees of some irregular line graphs, and give a …
Chromatic polynomials have been studied extensively, giving us results such as the Fundamental Reduction Theorem and closed formulas for the chromatic polynomials of common classes of graphs. Though, none of those extend to the context of fractional colorings. We thus present closed formulas for the "fractional" chromatic polynomial - a function that counts the number of distinct $b$-fold $λ$-colorings on a given graph - of complete graphs, trees and forests, as well as a generalized form of the Fundamental Reduction Theorem.
Normal forms for wide classes of closed IL formulas were given in [4]. Here we quantify asymptotically, in exact numbers, how wide those classes are. As a consequence, we show that the "majority" of closed IL formulas have GL-equivalents, and by that, they have the same normal forms as GL formulas. Our approach is entirely syntactical, except for applying the results of [4]. As a byproduct we devise a convenient way of computing asymptotic behaviors of somewhat general classes of formulas given by their grammar rules. Its applications do not require any knowledge of the recurrence relations, g…
In these notes we present a closed-formula solution to the problem of decomposing traces of Lie algebra generators into symmetrized traces and structure constants. The solution is written in terms of Solomon idempotents and exploits a projection derived by Solomon in his work on the Poincare-Birkhoff-Witt theorem.
Fondazione Bruno Kessler（布鲁诺·凯斯勒基金会）；University of Trento（特伦托大学）
Weighted First-Order Model Counting (WFOMC) computes the weighted sum of the models of a first-order logic theory on a given finite domain. First-Order Logic theories that admit polynomial-time WFOMC w.r.t domain cardinality are called domain liftable. We introduce the concept of lifted interpretations as a tool for formulating closed-forms for WFOMC. Using lifted interpretations, we reconstruct the closed-form formula for polynomial-time FOMC in the universally quantified fragment of FO2, earlier proposed by Beame et al. We then expand this closed-form to incorporate cardinality constraints, …
