Contents

Preface

Acknowledgments

About the Authors

About the Cover

1 Introduction

1.1 Complex Networks

1.2 Types of Complex Networks

1.3 Benefits of Studying Complex Networks

1.3.1 Modeling and Characterizing Complex Physical World Systems

1.3.2 Design of New and Efficient Physical World Systems

1.3.3 Development of Solutions to Complex Real-World Problems

1.3.4 Enhancing Biomedical Research through Molecular Network Modeling

1.3.5 Network Medicine

1.3.6 Neutralizing Antisocial Networks

1.3.7 Enhanced Social Science Research through Social Networks

1.4 Challenges in Studying Complex Networks

1.5 What This Book Offers

1.6 Organization of the Book

1.6.1 Suggested Navigation for Contents of the Book

1.7 Support Materials Available for Instructors

1.8 Summary

2 Graph Theory Preliminaries

2.1 Introduction

2.2 Graphs

2.2.1 Subgraphs

2.2.2 Complement of a Graph

2.3 Matrices Related to a Graph

2.3.1 Weight Matrix

2.3.2 Adjacency Matrix

2.3.3 Incidence Matrix

2.3.4 Degree Matrix

2.3.5 Laplacian Matrix

2.4 Basic Graph Metrics

2.4.1 Average Neighbor Degree

2.4.2 Average Clustering Coefficient

2.4.3 Average Path Length

2.4.4 Average Edge Length

2.4.5 Graph Diameter and Volume

2.5 Basic Graph Definitions and Properties

2.5.1 Walk, Path, and Circuits

2.5.2 Connectivity

2.5.3 Acyclicity

2.5.4 Isomorphism

2.5.5 Planarity

2.5.6 Colorability

2.5.7 Traversability

2.5.8 Network Flows

2.5.9 Product Graphs

2.6 Types of Graphs

2.6.1 Regular Graph

2.6.2 Bipartite Graphs

2.6.3 Complete Graphs

2.6.4 Trees

2.6.5 Line Graph

2.6.6 Conflict Graphs

2.7 Other Important Measures for Graphs

2.7.1 Cheeger Constant

2.7.2 Clique Number

2.8 Graph Pathfinding Algorithms

2.8.1 Dijkstra’s Shortest Path Algorithm

2.8.2 All-Pair Shortest Path Algorithm

2.9 Summary

Exercises

3 Introduction to Complex Networks

3.1 Major Types of Complex Networks

3.1.1 Random Networks

3.1.2 Small-World Networks

3.1.3 Scale-Free Networks

3.2 Complex Network Metrics

3.2.1 Average Neighbor Degree

3.2.2 Average Path Length

3.2.3 Network Diameter

3.2.4 Average Clustering Coefficient

3.2.5 Degree Distribution

3.2.6 Centrality Metrics

3.2.7 Degree-Degree Correlation in Complex Networks

3.2.8 Node Criticality

3.2.9 Network Resistance Distance

3.3 Community Detection in Complex Networks

3.3.1 Modularity Maximization

3.3.2 Surprise Maximization

3.3.3 Conflict Graph Transformation-Based Community Detection

3.4 Entropy in Complex Network

3.4.1 Network Entropy

3.4.2 Nodal Degree Entropy

3.4.3 Link Length Variability Entropy

3.4.4 Link Influence Entropy

3.5 Random Networks

3.5.1 Evolution of Random Networks

3.5.2 Erdös-Rényi Random Network Model

3.5.3 Properties of Random Networks

3.6 Open Research Issues

3.7 Summary

Exercises

4 Small-World Networks

4.1 Introduction

4.2 Milgram’s Small-World Experiment

4.3 Characteristics of Small-World Networks

4.4 Real-World Small-World Networks

4.5 Creation and Evolution of Small-World Networks

4.5.1 Rewiring of Existing Links

4.5.2 Pure Random Addition of New LLs

4.5.3 Euclidean Distance-Based Addition of New Links

4.6 Capacity-Based Deterministic Addition of New Links

4.6.1 Max-Flow Min-Cut Theorem

4.6.2 Link Addition Based on Maximum Flow Capacity Strategy

4.7 Creation of Deterministic Small-World Networks

4.7.1 Link Addition Based on Minimum APL

4.7.2 Link Addition Based on Minimum AEL

4.7.3 Link Addition Based on Maximum BC

4.7.4 Link Addition Based on Maximum CC

4.8 Anchor Points in a String Topology Small-World Network

4.8.1 Significance of Anchor Points

4.8.2 Locations of Anchor Points

4.9 Heuristic Approach-Based Deterministic Link Addition

4.9.1 Maximum Closeness Centrality Disparity

4.9.2 Sequential Deterministic LL Addition

4.9.3 Average Flow Capacity Enhancement Using Small-World Characteristics

4.10 Routing in Small-World Networks

4.10.1 The Decentralized Routing Algorithm

4.10.2 The Adaptive Decentralized Routing Algorithm

4.10.3 The Lookahead Routing Algorithm

4.11 Capacity of Small-World Networks

4.11.1 Capacity of Small-World Networks with Rewiring of Existing NLs

4.11.2 Capacity of Small-World Networks with LL Addition

4.12 Open Research Issues

4.13 Summary

Exercises

5 Scale-Free Networks

5.1 Introduction

5.1.1 What Does Scale-Free Mean?

5.2 Characteristics of Scale-Free Networks

5.3 Real-World Scale-Free Networks

5.3.1 The Author Citation Networks

5.3.2 The Autonomous Systems in the Internet

5.3.3 The Air Traffic Networks

5.3.4 Identification of Scale-Free Networks

5.4 Scale-Free Network Formation

5.4.1 Scale-Free Network Creation by Preferential Attachment

5.4.2 Scale-Free Network Creation by Fitness-Based Modeling

5.4.3 Scale-Free Network Creation by Varying Intrinsic Fitness

5.4.4 Scale-Free Network Creation by Optimization

5.4.5 Scale-Free Network Creation with Exponent 1

5.4.6 Scale-Free Network Creation with Greedy Global Decision Making

5.5 Preferential Attachment–Based Scale-Free Network Creation

5.5.1 Barabási-Albert (BA) Network Model

5.5.2 Observations and Discussion

5.6 Fitness-Based Scale-Free Network Creation

5.6.1 Fitness-Based Network Model

5.6.2 Observations and Discussion

5.7 Varying Intrinsic Fitness-Based Scale-Free Network Creation

5.7.1 Varying Intrinsic Fitness-Based Network Model

5.7.2 Observations and Discussion

5.8 Optimization-Based Scale-Free Network Creation

5.8.1 Observations and Discussion

5.9 Scale-Free Network Creation with Exponent 1

5.9.1 Scale-Free Network Creation with Rewiring

5.9.2 Observations and Discussion

5.10 Greedy Global Decision–Based Scale-Free Network Creation

5.10.1 Greedy Global LL Addition

5.10.2 Observations on Greedy Global Decision–Based Scale-Free Network

5.11 Deterministic Scale-Free Network Creation

5.11.1 Deterministic Scale-Free Network Model

5.11.2 Observations on Deterministic Scale-Free Network Creation

5.12 Open Research Issues

5.13 Summary

Exercises

6 Small-World Wireless Mesh Networks

6.1 Introduction

6.1.1 The Small-World Characteristics

6.1.2 Small-World Wireless Mesh Networks

6.2 Classification of Small-World Wireless Mesh Networks

6.3 Creation of Random Long-Ranged Links

6.3.1 Random LL Creation by Rewiring the Normal Links

6.3.2 Random LL Creation by Addition of New Links

6.4 Small-World Based on Pure Random Link Addition

6.5 Small-World Based on Euclidean Distance

6.6 Realization of Small-World Networks Based on Antenna Metrics

6.6.1 LL Addition Based on Transmission Power

6.6.2 LL Addition Based on Randomized Beamforming

6.6.3 LL Addition Based on Transmission Power and Beamforming

6.7 Algorithmic Approaches to Create Small-World Wireless Mesh Networks

6.7.1 Contact-Based LL Creation

6.7.2 Genetic Algorithm–Based LL Addition

6.7.3 Small-World Cooperative Routing–Based LL Addition

6.8 Gateway-Router-Centric Small-World Network Formation

6.8.1 Single Gateway-Router-Based LL Addition

6.8.2 Multi-Gateway-Router-Based LL Addition

6.9 Creation of Deterministic Small-World Wireless Mesh Networks

6.9.1 Exhaustive Search–Based Deterministic LL Addition

6.9.2 Heuristic Approach-Based Deterministic LL Addition

6.10 Creation of Non-Persistent Small-World Wireless Mesh Networks

6.10.1 Data-Mule-Based LL Creation

6.10.2 Load-Aware Long-Ranged Link Creation

6.11 Non-Persistent Routing in Small-World Wireless Mesh Networks

6.11.1 Load-Aware Non-Persistent Small-World Routing

6.11.2 Performance Evaluation of LNPR Algorithm

6.12 Qualitative Comparison of Existing Solutions

6.13 Open Research Issues

6.14 Summary

Exercises

7 Small-World Wireless Sensor Networks

7.1 Introduction

7.2 Small-World Wireless Mesh Networks vs. Small-World Wireless Sensor Networks

7.3 Why Small-World Wireless Sensor Networks?

7.4 Challenges in Transforming WSNs to SWWSNs

7.5 Types of Long-Ranged Links for SWWSNs

7.6 Approaches for Transforming WSNs to SWWSNs

7.6.1 Classification of Existing Approaches

7.6.2 Metrics for Performance Estimation

7.6.3 Transforming Regular Topology WSNs to SWWSNs

7.6.4 Random Model Heterogeneous SWWSNs

7.6.5 Newman-Watts Model–Based SWWSNs

7.6.6 Kleinberg Model–Based SWWSNs

7.6.7 Directed Random Model–Based SWWSNs

7.6.8 Variable Rate Adaptive Modulation–Based SWWSNs

7.6.9 Degree-Based LL Addition for Creating SWWSNs

7.6.10 Inhibition Distance–Based LL Addition for Creating SWWSNs

7.6.11 Homogeneous SWWSNs

7.7 SWWSNs with Wired LLs

7.8 Open Research Issues

7.9 Summary

Exercises

8 Spectra of Complex Networks

8.1 Introduction

8.2 Spectrum of a Graph

8.3 Adjacency Matrix Spectrum of a Graph

8.3.1 Bounds on the Eigenvalues

8.3.2 Adjacency Matrix Spectra of Special Graphs

8.4 Adjacency Matrix Spectra of Complex Networks

8.4.1 Random Networks

8.4.2 Random Regular Networks

8.4.3 Small-World Networks

8.4.4 Scale-Free Networks

8.5 Laplacian Spectrum of a Graph

8.5.1 Bounds on the Eigenvalues of the Laplacian

8.5.2 Bounds on the Eigenvalues of the Normalized Laplacian

8.5.3 Matrix Tree Theorem

8.5.4 Laplacian Spectrum and Graph Connectivity

8.5.5 Spectral Graph Clustering

8.5.6 Laplacian Spectra of Special Graphs

8.6 Laplacian Spectra of Complex Networks

8.6.1 Random Networks

8.6.2 Random Regular Networks

8.6.3 Small-World Networks

8.6.4 Scale-Free Networks

8.7 Network Classification Using Spectral Densities

8.8 Open Research Issues

8.9 Summary

Exercises

9 Signal Processing on Complex Networks

9.1 Introduction to Graph Signal Processing

9.1.1 Mathematical Representation of Graph Signals

9.2 Comparison between Classical and Graph Signal Processing

9.2.1 Relationship between GFT and Classical DFT

9.3 The Graph Laplacian as an Operator

9.3.1 Properties of the Graph Laplacian

9.3.2 Graph Spectrum

9.4 Quantifying Variations in Graph Signals

9.5 Graph Fourier Transform

9.5.1 Notion of Frequency and Frequency Ordering

9.5.2 Bandlimited Graph Signals

9.5.3 Effect of Vertex Indexing

9.6 Generalized Operators for Graph Signals

9.6.1 Filtering

9.6.2 Convolution

9.6.3 Translation

9.6.4 Modulation

9.7 Applications

9.7.1 Spectral Analysis of Node Centralities

9.7.2 Graph Fourier Transform Centrality

9.7.3 Malfunction Detection in Sensor Networks

9.8 Windowed Graph Fourier Transform

9.8.1 Example of WGFT

9.9 Open Research Issues

9.10 Summary

Exercises

10 Graph Signal Processing Approaches

10.1 Introduction

10.2 Graph Signal Processing Based on Laplacian Matrix

10.3 DSPG Framework

10.3.1 Linear Graph Filters and Shift Invariance

10.4 DSPG Framework Based on Weight Matrix

10.4.1 Shift Operator

10.4.2 Linear Shift Invariant Graph Filters

10.4.3 Total Variation

10.4.4 Graph Fourier Transform

10.4.5 Frequency Response of LSI Graph Filters

10.5 DSPG Framework Based on Directed Laplacian

10.5.1 Directed Laplacian

10.5.2 Shift Operator

10.5.3 Linear Shift Invariant Graph Filters

10.5.4 Total Variation

10.5.5 Graph Fourier Transform Based on Directed Laplacian

10.5.6 Frequency Response of LSI Graph Filters

10.6 Comparison of Graph Signal Processing Approaches

10.7 Open Research Issues

10.8 Summary

Exercises

11 Multiscale Analysis of Complex Networks

11.1 Introduction

11.2 Multiscale Transforms for Complex Network Data

11.2.1 Vertex Domain Designs

11.2.2 Spectral Domain Designs

11.3 Crovella and Kolaczyk Wavelet Transform

11.3.1 CK Wavelets

11.3.2 Wavelet Transform

11.3.3 Wavelet Properties

11.3.4 Examples

11.3.5 Advantages and Disadvantages

11.4 Random Transform

11.4.1 Advantages and Disadvantages

11.5 Lifting-Based Wavelets

11.5.1 Splitting of a Graph into Even and Odd Nodes

11.5.2 Lifting-Based Transform

11.6 Two-Channel Graph Wavelet Filter Banks

11.6.1 Downsampling and Upsampling in Graphs

11.6.2 Two-Channel Graph Wavelet Filter Banks

11.6.3 Graph Quadrature-Mirror Filterbanks

11.6.4 Multidimensional Separable Wavelet Filter Banks for Arbitrary Graphs

11.7 Spectral Graph Wavelet Transform

11.7.1 Matrix Form of SGWT

11.7.2 Wavelet Generating Kernels

11.7.3 An Example of SGWT

11.7.4 Advantages and Disadvantages

11.8 Spectral Graph Wavelet Transform Based on Directed Laplacian

11.8.1 Wavelets

11.8.2 Wavelet Generating Kernel

11.8.3 Examples

11.9 Diffusion Wavelets

11.9.1 Advantages and Disadvantages

11.10 Open Research Issues

11.11 Summary

Exercises

A Vectors and Matrices

A.1 Vectors and Norms

A.1.1 Orthogonal and Orthonormal Vectors

A.1.2 Linear Span of a Set of Vectors

A.2 Matrices

A.2.1 Trace of a Matrix

A.2.2 Matrix Vector Mutiplication

A.2.3 Column Space, Null Space, and Rank of a Matrix

A.2.4 Special Matrices

A.3 Eigenvalues and Eigenvectors

A.3.1 Characteristic Equation

A.3.2 Eigenspaces

A.3.3 Eigenvalue Multiplicity

A.4 Matrix Diagonalization

A.5 Jordan Decomposition

A.6 Spectral Density

A.7 Wigner’s Semicircle Law

A.8 Gershgorin’s Theorem

B Classical Signal Processing

B.1 Linear Time Invariant Filters

B.1.1 Impulse Response and Convolution

B.1.2 Eigenfunctions

B.2 Fourier Transform

B.2.1 Continuous-Time Fourier Transform

B.2.2 Discrete-Time Fourier Transform

B.2.3 Discrete Fourier Transform

B.3 Digital Filter Banks

B.3.1 Downsampler and Upsampler

B.4 Two-Channel Filter Bank

B.5 Windowed Fourier Transform

B.5.1 Spectrogram

B.6 Continuous-Time Wavelet Transform

C Analysis of Locations of Anchor Points

D Asymptotic Behavior of Functions

D.1 Big Oh (O(·)) Notation

D.2 Big Omega (Ω(·)) Notation

D.3 Big Theta (Θ(·)) Notation

E Relevant Academic Courses and Programs

E.1 Academic Courses on Complex Networks

E.2 Online Courses on Complex Networks

E.3 Selective Academic Programs on Complex Networks

F Relevant Journals and Conferences

F.1 List of Top Journals in Complex Networks

F.2 List of Top Conferences in Complex Networks

G Relevant Datasets and Visualization Tools

G.1 Relevant Dataset Repositories

G.2 Relevant Graph Visualization and Analysis Tools

H Relevant Research Groups

H.1 Other Important Resources

Notation

Acronyms

Bibliography

Index

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