9 research outputs found
Spectral characterizations of complex unit gain graphs
While eigenvalues of graphs are well studied, spectral analysis of complex unit gain graphs is still in its infancy. This thesis considers gain graphs whose gain groups are gradually less and less restricted, with the ultimate goal of classifying gain graphs that are characterized by their spectra. In such cases, the eigenvalues of a gain graph contain sufficient structural information that it might be uniquely (up to certain equivalence relations) constructed when only given its spectrum. First, the first infinite family of directed graphs that is β up to isomorphism β determined by its Hermitian spectrum is obtained. Since the entries of the Hermitian adjacency matrix are complex units, these objects may be thought of as gain graphs with a restricted gain group. It is shown that directed graphs with the desired property are extremely rare. Thereafter, the perspective is generalized to include signs on the edges. By encoding the various edge-vertex incidence relations with sixth roots of unity, the above perspective can again be taken. With an interesting mix of algebraic and combinatorial techniques, all signed directed graphs with degree at most 4 or least multiplicity at most 3 are determined. Subsequently, these characterizations are used to obtain signed directed graphs that are determined by their spectra. Finally, an extensive discussion of complex unit gain graphs in their most general form is offered. After exploring their various notions of symmetry and many interesting ties to complex geometries, gain graphs with exactly two distinct eigenvalues are classified
Spectral Fundamentals and Characterizations of Signed Directed Graphs
The spectral properties of signed directed graphs, which may be naturally
obtained by assigning a sign to each edge of a directed graph, have received
substantially less attention than those of their undirected and/or unsigned
counterparts. To represent such signed directed graphs, we use a striking
equivalence to -gain graphs to formulate a Hermitian adjacency
matrix, whose entries are the unit Eisenstein integers Many well-known results, such as (gain) switching and eigenvalue
interlacing, naturally carry over to this paradigm. We show that non-empty
signed directed graphs whose spectra occur uniquely, up to isomorphism, do not
exist, but we provide several infinite families whose spectra occur uniquely up
to switching equivalence. Intermediate results include a classification of all
signed digraphs with rank , and a deep discussion of signed digraphs with
extremely few (1 or 2) non-negative (eq. non-positive) eigenvalues
Symmetry in complex unit gain graphs and their spectra
Complex unit gain graphs may exhibit various kinds of symmetry. In this work,
we explore structural symmetry, spectral symmetry and sign-symmetry in gain
graphs, and their respective relations to one-another. Our main result is a
construction that transforms an arbitrary gain graph into infinitely many
switching-distinct gain graphs whose spectral symmetry does not imply
sign-symmetry. This provides a more general answer to the gain graph analogue
of an existence question that was recently treated in the context of signed
graphs
Spectral characterizations of complex unit gain graphs
While eigenvalues of graphs are well studied, spectral analysis of complex unit gain graphs is still in its infancy. This thesis considers gain graphs whose gain groups are gradually less and less restricted, with the ultimate goal of classifying gain graphs that are characterized by their spectra. In such cases, the eigenvalues of a gain graph contain sufficient structural information that it might be uniquely (up to certain equivalence relations) constructed when only given its spectrum. First, the first infinite family of directed graphs that is β up to isomorphism β determined by its Hermitian spectrum is obtained. Since the entries of the Hermitian adjacency matrix are complex units, these objects may be thought of as gain graphs with a restricted gain group. It is shown that directed graphs with the desired property are extremely rare. Thereafter, the perspective is generalized to include signs on the edges. By encoding the various edge-vertex incidence relations with sixth roots of unity, the above perspective can again be taken. With an interesting mix of algebraic and combinatorial techniques, all signed directed graphs with degree at most 4 or least multiplicity at most 3 are determined. Subsequently, these characterizations are used to obtain signed directed graphs that are determined by their spectra. Finally, an extensive discussion of complex unit gain graphs in their most general form is offered. After exploring their various notions of symmetry and many interesting ties to complex geometries, gain graphs with exactly two distinct eigenvalues are classified
Spectral fundamentals and characterizations of signed directed graphs
The spectral properties of signed directed graphs, which may be naturally obtained by assigning a sign to each edge of a directed graph, have received substantially less attention than those of their undirected and/or unsigned counterparts. To represent such signed directed graphs, we use a striking equivalence to -gain graphs to formulate a Hermitian adjacency matrix, whose entries are the unit Eisenstein integers , . Many well-known results, such as (gain) switching and eigenvalue interlacing, naturally carry over to this paradigm. We show that non-empty signed directed graphs whose spectra occur uniquely, up to isomorphism, do not exist, but we provide several infinite families whose spectra occur uniquely up to switching equivalence. Intermediate results include a classification of all signed digraphs with rank , and a deep discussion of signed digraphs with extremely few (1 or 2) non-negative (eq. non-positive) eigenvalues
The negative tetrahedron and the first infinite family of connected digraphs that are strongly determined by the Hermitian spectrum
Thus far, digraphs that are uniquely determined by their Hermitian spectra have proven elusive. Instead, researchers have turned to spectral determination of classes of switching equivalent digraphs, rather than individual digraphs. In the present paper, we consider the traditional notion: a digraph (or mixed graph) is said to be strongly determined by its Hermitian spectrum (abbreviated SHDS) if it is isomorphic to each digraph to which it is cospectral. Convincing numerical evidence to support the claim that this property is extremely rare is provided. Nonetheless, the first infinite family of connected digraphs that is SHDS is constructed. This family is obtained via the introduction of twin vertices into a structure that is named negative tetrahedron. This special digraph, that exhibits extreme spectral behavior, is contained in the surprisingly small collection of all digraphs with exactly one negative eigenvalue, which is determined as an intermediate result
The negative tetrahedron and the first infinite family of connected digraphs that are strongly determined by the Hermitian spectrum
Thus far, digraphs that are uniquely determined by their Hermitian spectra
have proven elusive. Instead, researchers have turned to spectral determination
of classes of switching equivalent digraphs, rather than individual digraphs.
In the present paper, we consider the traditional notion: a digraph (or mixed
graph) is said to be strongly determined by its Hermitian spectrum (abbreviated
SHDS) if it is isomorphic to each digraph to which it is cospectral. Convincing
numerical evidence to support the claim that this property is extremely rare is
provided. Nonetheless, the first infinite family of connected digraphs that is
SHDS is constructed. This family is obtained via the introduction of twin
vertices into a structure that is named negative tetrahedron. This special
digraph, that exhibits extreme spectral behavior, is contained in the
surprisingly small collection of all digraphs with exactly one negative
eigenvalue, which is determined as an intermediate result.Comment: To appear in Journal of Combinatorial Theory, Series
Unit gain graphs with two distinct eigenvalues and systems of lines in complex space
Since the introduction of the Hermitian adjacency matrix for digraphs, interest in so-called complex unit gain graphs has surged. In this work, we consider gain graphs whose spectra contain the minimum number of two distinct eigenvalues. Analogously to graphs with few distinct eigenvalues, a great deal of structural symmetry is required for a gain graph to attain this minimum. This allows us to draw a surprising parallel to well-studied systems of lines in complex space, through a natural correspondence to unit-norm tight frames. We offer a full classification of two-eigenvalue gain graphs with degree at most 4, or with multiplicity at most 3. Intermediate results include an extensive review of various relevant concepts related to lines in complex space, including SIC-POVMs, MUBs and geometries such as the Coxeter-Todd lattice, and many examples obtained as induced subgraphs by employing a technique parallel to the dismantling of association schemes. Finally, we touch on an innovative application of simulated annealing to find examples by computer. (c) 2022 The Author(s). Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)
Unit gain graphs with two distinct eigenvalues and systems of lines in complex space
Since the introduction of the Hermitian adjacency matrix for digraphs, interest in so-called complex unit gain graphs has surged. In this work, we consider gain graphs whose spectra contain the minimum number of two distinct eigenvalues. Analogously to graphs with few distinct eigenvalues, a great deal of structural symmetry is required for a gain graph to attain this minimum. This allows us to draw a surprising parallel to well-studied systems of lines in complex space, through a natural correspondence to unit-norm tight frames. We offer a full classification of two-eigenvalue gain graphs with degree at most 4, or with multiplicity at most 3. Intermediate results include an extensive review of various relevant concepts related to lines in complex space, including SIC-POVMs, MUBs and geometries such as the Coxeter-Todd lattice, and many examples obtained as induced subgraphs by employing a technique parallel to the dismantling of association schemes. Finally, we touch on an innovative application of simulated annealing to find examples by computer. (c) 2022 The Author(s). Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)