### Abstract

*n*vertices with adjacency matrix A and degree sequence

*(d*. A universal adjacency matrix of Γ is any matrix in Span

_{1},…,d_{n})*{A,D,I,J*} with a nonzero coefficient for

*A*, where

*D*=diag(

*d*) and

_{1},…,d_{n}*I*and

*J*are the

*n × n*identity and all-ones matrix, respectively. Thus a universal adjacency matrix is a common generalization of the adjacency, the Laplacian, the signless Laplacian and the Seidel matrix. We investigate graphs for which some universal adjacency matrix has just two eigenvalues. The regular ones are strongly regular, complete or empty, but several other interesting classes occur.

Original language | English |
---|---|

Pages (from-to) | 2520-2529 |

Journal | Linear Algebra and its Applications |

Volume | 435 |

Issue number | 10 |

DOIs | |

Publication status | Published - 2011 |

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### Cite this

*Linear Algebra and its Applications*,

*435*(10), 2520-2529. https://doi.org/10.1016/j.laa.2011.02.023

}

*Linear Algebra and its Applications*, vol. 435, no. 10, pp. 2520-2529. https://doi.org/10.1016/j.laa.2011.02.023

**Universal adjacency matrices with two eigenvalues.** / Haemers, W.H.; Omidi, G.R.

Research output: Contribution to journal › Article › Scientific › peer-review

TY - JOUR

T1 - Universal adjacency matrices with two eigenvalues

AU - Haemers, W.H.

AU - Omidi, G.R.

N1 - Appeared previously as CentER Discussion Paper 2010-119

PY - 2011

Y1 - 2011

N2 - Consider a graph Γ on n vertices with adjacency matrix A and degree sequence (d1,…,dn). A universal adjacency matrix of Γ is any matrix in Span {A,D,I,J} with a nonzero coefficient for A, where D=diag(d1,…,dn) and I and J are the n × n identity and all-ones matrix, respectively. Thus a universal adjacency matrix is a common generalization of the adjacency, the Laplacian, the signless Laplacian and the Seidel matrix. We investigate graphs for which some universal adjacency matrix has just two eigenvalues. The regular ones are strongly regular, complete or empty, but several other interesting classes occur.

AB - Consider a graph Γ on n vertices with adjacency matrix A and degree sequence (d1,…,dn). A universal adjacency matrix of Γ is any matrix in Span {A,D,I,J} with a nonzero coefficient for A, where D=diag(d1,…,dn) and I and J are the n × n identity and all-ones matrix, respectively. Thus a universal adjacency matrix is a common generalization of the adjacency, the Laplacian, the signless Laplacian and the Seidel matrix. We investigate graphs for which some universal adjacency matrix has just two eigenvalues. The regular ones are strongly regular, complete or empty, but several other interesting classes occur.

U2 - 10.1016/j.laa.2011.02.023

DO - 10.1016/j.laa.2011.02.023

M3 - Article

VL - 435

SP - 2520

EP - 2529

JO - Linear Algebra and its Applications

JF - Linear Algebra and its Applications

SN - 0024-3795

IS - 10

ER -