Questions tagged [spectral-graph-theory]
Questions related to the spectrum of graphs, defined using one of the possible variants of the discrete Laplace operator or Laplacian matrix. See https://en.wikipedia.org/wiki/Discrete_Laplace_operator
393
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Bound the $\infty$-norm of the eigenvector of the second minimum eigenvalue of normalized Laplacian from below
I meet the above problem while reading a paper. The problem can be stated as below.
Consider an undirected graph $G$. Let $\mathbf{v}$ be a vector such that $\mathbf{D}^{1/2}\mathbf{v}$ is the ...
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1
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Some questions about induced subgraphs of the directed hypercube graph
Let $Q^n$ be the hypercube graph in $n$ dimensions. Hao Huang famously showed that any induced subgraph on more than $2^{n-1}$ must have maximum degree $ \geq \sqrt{n}$. It is also known that this ...
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Given a low-rank symmetric positive semidefinite matrix and a basis of its nullspace, is there a fast way to get the nonzero eigenvalues?
I have a (possibly dense) $k\times k$ real matrix $L = AA^T + B^T B$, a type of combinatorial Laplacian (self-adjoint, symmetric, positive semidefinite) of rank $(k-n)$ and possibly repeated nonzero ...
2
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1
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Invertibility of message passing with invertible parametrization
Consider the message passing framework defined by, $$f(\boldsymbol{x}_i)= \boldsymbol{x}_i + \sum_{j \neq i} (\boldsymbol{x}_i -\boldsymbol{x}_j) g(\|\boldsymbol{x}_i -\boldsymbol{x}_j\|^2),$$ for $i=...
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Bound on the magnitude of the entries of the Laplacian pseudo-inverse
Let $L$ be the laplacian matrix of a connected graph $G$ with real positive weights and $N$ vertices, or that can be assumed to have binary weights for simplicity.My goal is to bound $\Vert L^+\Vert_{\...
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Second eigenvalue of primitive matrix
Let $A$ be a primitive $N\times N$-matrix with positive entries, that is there is $n>0$ such that $(A^n)_{i,j}>0$ for all $i,j$. For brevity, assume the entries consist only of $0$ and $1$.
The ...
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Number of bi-directional (or symmetric edges) [closed]
I am trying to figure out the least number of directed edges that would be bi-directional after constructing a graph with $2k-1$ nodes that are each $k$ in-degree. For example, $2(2)-1=3$ nodes that ...
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Regularize a graph while embedding the spectrum of adjacency matrix
Given an irregular graph $G$ whose maximum degree is $d$, I am interested in producing a new graph $G'$ which is regular and has the spectrum of the adjacency spectrum of $G$ embedded in the spectrum ...
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Diameters of random bipartite graphs
Given two partite sets of vertices $U$ and $V$ of size $n$. Each vertex in $U$ uniformly randomly selects $K$ ($K$ is a constant and $K\ll n$) vertices in $V$ without replacement and connects a ...
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Do balls in expander graphs have small expansion?
Consider a $d$-regular infinite transitive expander graph $G$, and let $B_r$ be a ball of radius $r$ in $G$. Can one place any upper bounds on the expansion of $B_r$?
My intuition is that $B_r$ will ...
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Max-cut from Laplacian
(This question seems like very standard material for those well-versed in the subject. I thought I would get a quick answer from Math stackexchange, but to no avail.)
Given a weighted graph with $n$ ...
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Clique number and spectrum of a graph
In the Wikipedia article on Grassmann graph it is stated that in this graph:
$$\omega=1-\frac{\lambda_{max}}{\lambda_{min}}$$
where $\omega$ is the clique number of the graph, and $\lambda_{max}$ and $...
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1
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107
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Spectral characterization of complete or complete bipartite graphs
The Lemma 6 in this paper mention the following spectral characterization of complete or complete bipartite graphs:
Let $G$ be a connected graph with $\ge 2$ vertices. Then $\lambda_2=...=\lambda_{n-...
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Bounds on the spectral radius of a perturbed directed graph
Suppose $(G_n)$ is a sequence of strongly connected directed graphs (without multiple edges) with $G_n$ having $n$ edges such that the adjacency matrix $A_n$ of $G_n$ is primitive, and let $(G_n’)$ be ...
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Is there a bipartite graph with $\sqrt{2}$ as an eigenvalue with high multiplicity, specifically more than in the Heawood graph?
The Heawood graph is a $3$-regular graph on $14$ vertices. Its (adjacency) spectrum is $\{ (3)^1, (\sqrt{2})^6, (-\sqrt{2})^6, (-3)^1 \}$. So, $3/7 \approx 42.8\%$ of its eigenvalues equal $\sqrt{2}$.
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Controlling quantity related to Laplacian pseudo-inverse of Erdős–Rényi graph
Consider an $n$-node undirected graph $G = (V, E)$ equipped with weights $W$. Let $L$ be the weighted graph Laplacian matrix, i.e. $L_{ij} = -W_{(i,j)}$ for $(i,j)\in E$ and $L_{ii} = \sum_{j:(i,j)\in ...
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Graph Laplacians, Riemannian manifolds, and object collisions
To preface this question, I am a part-time game developer and full-time optimization fiend.
I am working on object collisions at the moment and many resources I have found online are more-or-less just ...
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Tightness of the bounding the operator norm of graph by average degree from below
Let $G = (V,E)$ be a simple graph with adjacency matrix $A$. It is well known that the largest eigenvalue
$\lambda_{1}$ of $A$ is contained within the interval $[d, D]$ where $d$ is the average degree ...
3
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answer
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Spectrum of the adjacency matrix of certain directed graphs
For an undirected graph $G$, its adjacency matrix $A_G$ is symmetric, and by the (consequence of) spectral theorem, each of its Jordan blocks has size $1$. This is not true for a general directed ...
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Diameter bound for graphs: spectral and random walk versions
This question can be phrased in different settings. I will discuss a spectral formulation and the equivalent random walk version. The question came up naturally in recent work with Devriendt and ...
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The Smith decomposition of the graph Laplacian and Locality
Let $X$ be a graph. Let $V(X)$ and $E(X)$ be the sets of vertices and edges of the graph respectively. If $f:V(X) \rightarrow G$ where $G$ is an abelian group, then one can define a graph Laplacian as ...
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Resolvent estimates for Laplacians on directed graphs
Let us consider a directed and weighted graph $G$ with $N = |G|$ nodes. Denote the corresponding (weighted) adjacency matrix by $W \in \mathbb{R}^{N\times N}_{\geq 0}$ and let $D$ be the diagonal in-...
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On eigenvalues of signed complete graphs
From Akbari et al. -1 as well as Akbari et al. -2, we have that the eigenvalues of a signed complete graph $K_n$ with the negative edges forming an induced graph $H$ with order $k$ has the ...
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When is the sum of matrices (circulant + [super upper triangular]) not diagonalizable?
By the circulant matrix $C \in M_n(\mathbb{R})$, we mean that
$$ C = \left[\begin{array}{c|c|c|c} e_n & e_1 & \cdots & e_{n-1} \end{array}\right] $$
where $e_1,\dots,e_n$ are the standard ...
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Is the sum of the circulant matrix with a super upper triangular matrix diagonalizable?
By the circulant matrix $C$ in $M_n(\mathbb{R})$, we mean that
$$C=[e_n|e_1|\cdots|e_{n-1}]$$ where $e_1,\cdots,e_n$ are the standard basis vectors in $\mathbb{R}^n$. It is well-known that
$$C=\...
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1
answer
118
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Directed graph whose adjacency matrix admits only 0 as eigenvalue
Let $G$ be a directed graph and let $P_i$
be its vertices. Let $A$
be the corresponding adjacency matrix of $G$, i.e. $a_{i,j}=1$
if and only if there is a directed edge from $P_i$
to $P_j$, ($a_{i,...
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Non-diagonalizability of the adjacency matrix of a directed graph
Let $G$ be a directed graph with no multiple edges or loops and let $P_i$ be its vertices. Let $A$ be the corresponding adjacency matrix of $G$, i.e. $a_{i,j}=1$ if and only if there is a directed ...
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1
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Two fractionally isomorphic graphs but only one having eigenvalue zero
I am looking for two undirected graphs $G$ and $H$ of the same order (i.e., they have the same number of vertices) with adjacency matrices $A_G$ and $A_H$, respectively, such that
$G$ and $H$ are ...
4
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1
answer
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Strongly/distance regular graphs over $\mathbb{Z}_2^n$ with the same parameters
I am wondering if there is a known example of a pair of non-isomorphic graphs $G$ and $H$ that are both Cayley graphs for $\mathbb{Z}_2^n$ (for some $n$) and are both distance regular and have the ...
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Relation between the left-dominant eigenvectors (eigenvector corresponding to 0 eigenvalue) of two Laplacian matrices
Let $G_1=(v,\epsilon_1)$,$G_2=(v,\epsilon_2)$ be two graphs with the same set of vertices and $\epsilon_1 \subset \epsilon_2$. $L_1$ and $L_2$ be the Laplacian matrices associated with graph $G_1$ and ...
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On the absolute difference of the Laplacian eigenvalues of an unbalanced signed graph and its underlying graph
Let $\Sigma=(G,\sigma)$ be an unbalanced signed graph with the underlying connected graph $G=(V,E)$ and $\sigma:E\rightarrow \{-1,1\}$, the signing function. Let the Laplacian eigenvalues of $\Sigma$ ...
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Can we calculate the spectral radius of the universal cover for specific graphs?
Background
For a finite graph $G$, let $\tilde{G}$ denote the universal cover of $G$. For a vertex $v$, let $p_{2n}(v)$ denote the number of paths of length $2n$ that start and end at $v$. The ...
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answers
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What do you call this class of matrices with a unique positive eigenvalue associated to a graph?
I am looking for the name of a class of symmetric matrices $M\in\Bbb R^{n\times n}$ that I can associate to a (finite simple) graph $G=(V,E)$ with $V=\{1,...,n\}$ and that have the following ...
5
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0
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(How) does the spectral gap of the $n\times n$ Rubik's cube close with $n$?
Consider the spectrum of the adjacency matrix $A$ of the Cayley graph of the standard, 3x3x3 Rubik's cube generated with the usual quarter-turn and half-turn twists of each face (the Singmaster ...
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Comparing spectral radius of two graphs using the entry of Perron vector
Suppose we have a graph $G$.
Let $A$ be the adjacency matrix of $G$ and $x$ be the corresponding Perron vector.
Let $x = (x_1,x_2,\cdots,x_n)^t$, where $x_i$ corresponds to the vertex $i \in V(G)$.
We ...
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answers
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Explicit constructions of Ramanujan graphs
I am trying to find a list of all the explicit constructions of Ramanujan graphs. By a Ramanujan graph I mean a $k$-regular multi-graph $G$ such that all the non-trivial eigenvalues $\lambda$ of the ...
1
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1
answer
381
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Vertex degree on random graphs
Let $p = d/n$ with $d$ constant. How do I prove that, with high probability, $G_{n,p}$ contains a vertex of degree at least $(\log n)^{1/2}$,
where $G_{n,p}$ is a graph with $n$ vertices and the ...
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1
answer
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Eigenvalues of directed graph with one outward edge for each vertex
I am concerned with unweighted directed graphs where each node contains exactly one edge pointing to another node, which could be itself. In other words, each row of the adjacency matrix contains one ...
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1
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122
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Entropy of eigenvectors of (traceless) laplacian of a bipartite graph
This problem is motivated by the edge states that sometimes appear (and sometimes not) at the level of Huckel hamiltonians for $\pi$-conjugated benzenoid hydrocarbons. If this sentence is cryptic, ...
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1
answer
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Series analyzed in Lubotzky–Phillips–Sarnak "Ramanujan Graphs"
In the LPS paper "Ramanujan graphs" the adjacency matrix of $X^{p,q}$, for simplicity say that $p,q\equiv1\mod{4}$ and $\left(\frac{p}{q}\right)=1$ (so, nonbipartite) and $n=\lvert X^{p,q}\...
3
votes
1
answer
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Relation between spectra of a Cayley graph of a group and irreducible characters of that group
I know the following fact:
If $G$ is an abelian group and $S\subset G$ be a subset of G such that $1\notin G$ and $S=S^{-1}$ and we draw an edge between $g$ and $h$ if and only if $hg^{-1}\in S$,then ...
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0
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171
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Non-backtracking operator and spectra
Let $A$ be the adjacency operator of a symmetric graph $\Gamma$. (It may be weighted and/or non-regular, but, to keep it simple, let us say it is unweighted and regular of degree $d$.) We want to ...
7
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Surprising symmetry in the Ramanujan bound
The condition for a connected $(q+1)$-regular graph to be Ramanujan is that every nonzero eigenvalue $\lambda$ of the graph Laplacian satisfy
$$q+1-2\sqrt{q}\le \lambda\le q+1+2\sqrt{q}.$$
With a ...
8
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answers
122
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Conceptual explanation for the gap in the spectrum of biregular trees
Ramanujan graphs are defined as $(q+1)$-regular graphs for which all nontrivial eigenvalues of the adjacency operator are contained in the interval
$$[-2\sqrt{q}, 2\sqrt{q}].$$
The reason for this ...
1
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0
answers
81
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Graph energy and spectral radius
Suppose $G$ is a simple graph of order $n$ with eigenvalues $\lambda_1\geq \cdots\geq \lambda_n$. I've encountered the quantity $L=\big\vert |\lambda_1|-|\lambda_2|-\cdots-|\lambda_n|\big\vert$. Note ...
2
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1
answer
80
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Counting Euler circuits through labelled trees where $v_1$ and $v_2$ have distance two
Let $T_n$ be the set of all labelled trees with $n$ vertices. For any $T \in T_n$ let $D(T)$ be the 'doubled tree', where each edge of $T$ is replaced by one directed edge in each direction. $D(T)$ is ...
4
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0
answers
132
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Derivative of characteristic polynomial of a graph and derivative of characteristic polynomial of a vertex-deleted subgraph have a common root
Let $G$ be a simple graph and $G-i$ be one of its vertex-deleted subgraphs. Let $\phi(G,x)$ and $\phi(G-i,x)$ be the characteristic polynomials of $G$ and of $G-i$ respectively, with respect to their ...
1
vote
0
answers
60
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Angles between edges of a geometric graph and graph invariants
Are there any clever ways in which the angles between edges in a geometric graph are encoded in the graph spectrum, or another object associated with the graph?
I'm interested to see what else is ...
0
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0
answers
111
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Does an extension of the B.E.S.T. theorem for multiple Eulerian circuits exist?
Given a directed multigraph $G=(V,E)$ (multiple edges and loops are permitted) the number of distinct Eulerian circuits for $G$ can be calculated with the B.E.S.T. theorem. Does a similar theory for ...
0
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0
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52
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Variation in eigenvalues of adjacency matrices of regular graphs
What is known about the range of spectra of regular graphs? That is, I wish to know the largest intervals in which the minimum and maximum eigenvalues of a graph lie. For example, it is known that the ...