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Spectra of graphs

Andries E. Brouwer Willem H. Haemers

2

Contents1 Graph spectrum11

1.1 Matrices associated to a graph . . . . . . . . . . . . . . . . . . . 11

1.2 The spectrum of a graph . . . . . . . . . . . . . . . . . . . . . . 12

1.2.1 Characteristic polynomial . . . . . . . . . . . . . . . . . . 13

1.3 The spectrum of an undirected graph . . . . . . . . . . . . . . . 13

1.3.1 Regular graphs . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3.2 Complements . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.3 Walks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.4 Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.5 Spanning trees . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.6 Bipartite graphs . . . . . . . . . . . . . . . . . . . . . . . 16

1.3.7 Connectedness . . . . . . . . . . . . . . . . . . . . . . . . 16

1.4 Spectrum of some graphs . . . . . . . . . . . . . . . . . . . . . . 17

1.4.1 The complete graph . . . . . . . . . . . . . . . . . . . . . 17

1.4.2 The complete bipartite graph . . . . . . . . . . . . . . . . 17

1.4.3 The cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.4.4 The path . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.4.5 Line graphs . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.4.6 Cartesian products . . . . . . . . . . . . . . . . . . . . . . 19

1.4.7 Kronecker products and bipartite double . . . . . . . . . . 19

1.4.8 Strong products . . . . . . . . . . . . . . . . . . . . . . . 19

1.4.9 Cayley graphs . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.5 Decompositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.5.1 DecomposingK10into Petersen graphs . . . . . . . . . . 20

1.5.2 DecomposingKninto complete bipartite graphs . . . . . 20

1.6 Automorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.7 Algebraic connectivity . . . . . . . . . . . . . . . . . . . . . . . . 22

1.8 Cospectral graphs . . . . . . . . . . . . . . . . . . . . . . . . . . 22

1.8.1 The 4-cube . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.8.2 Seidel switching . . . . . . . . . . . . . . . . . . . . . . . . 23

1.8.3 Godsil-McKay switching . . . . . . . . . . . . . . . . . . . 24

1.8.4 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 24

1.9 Very small graphs . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3

4CONTENTS

2 Linear algebra29

2.1 Simultaneous diagonalization . . . . . . . . . . . . . . . . . . . . 29

2.2 Perron-Frobenius Theory . . . . . . . . . . . . . . . . . . . . . . 29

2.3 Equitable partitions . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3.1 Equitable and almost equitable partitions of graphs .. . 32

2.4 The Rayleigh quotient . . . . . . . . . . . . . . . . . . . . . . . . 33

2.5 Interlacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.6 Schur"s inequality . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.7 Schur complements . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.8 The Courant-Weyl inequalities . . . . . . . . . . . . . . . . . . . 36

2.9 Gram matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.10 Diagonally dominant matrices . . . . . . . . . . . . . . . . . . . 38

2.10.1 Gersgorin circles . . . . . . . . . . . . . . . . . . . . . . . 38

2.11 Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.12 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Eigenvalues and eigenvectors of graphs 41

3.1 The largest eigenvalue . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.1 Graphs with largest eigenvalue at most 2 . . . . . . . . . 42

3.1.2 Subdividing an edge . . . . . . . . . . . . . . . . . . . . . 43

3.1.3 The Kelmans operation . . . . . . . . . . . . . . . . . . . 44

3.1.4 Spectral radius of a graph with a given number of edges .44

3.2 Interlacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.3 Regular graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4 Bipartite graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.5 Cliques and cocliques . . . . . . . . . . . . . . . . . . . . . . . . 47

3.5.1 Using weighted adjacency matrices . . . . . . . . . . . . . 47

3.6 Chromatic number . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.6.1 Using weighted adjacency matrices . . . . . . . . . . . . . 50

3.6.2 Rank and chromatic number . . . . . . . . . . . . . . . . 50

3.7 Shannon capacity . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.7.1 Lov´asz"?-function . . . . . . . . . . . . . . . . . . . . . . 51

3.7.2 The Haemers bound on the Shannon capacity . . . . . . . 53

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