We determine all graphs whose adjacency matrices have at most two eigenvalues (multiplicities included) different from +/- 1 and decide which of these graphs are determined by their spectrum. This includes the so-called friendship graphs, which consist of a number of edge-disjoint triangles meeting in one vertex. It turns out that the friendship graph is determined by its spectrum, except when the number of triangles equals sixteen.
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Coll William & Mary, Dept Math, POB 8795, Williamsburg, VA 23187 USAColl William & Mary, Dept Math, POB 8795, Williamsburg, VA 23187 USA
Johnson, Charles R.
Duarte, Antonio Leal
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Univ Coimbra, Dept Matemat, P-300145 Coimbra, PortugalColl William & Mary, Dept Math, POB 8795, Williamsburg, VA 23187 USA
Duarte, Antonio Leal
Saiago, Carlos M.
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Univ Nova Lisboa, Fac Ciencias & Tecnol, Dept Matemat, P- 2829516 Quinta Da Torre, PortugalColl William & Mary, Dept Math, POB 8795, Williamsburg, VA 23187 USA
Saiago, Carlos M.
Sher, David
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Johns Hopkins Univ, Dept Math, Baltimore, MD 21218 USAColl William & Mary, Dept Math, POB 8795, Williamsburg, VA 23187 USA
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Shandong Univ Sci & Technol, Coll Math & Syst Sci, Qingdao 266590, Shandong, Peoples R ChinaShandong Univ Sci & Technol, Coll Math & Syst Sci, Qingdao 266590, Shandong, Peoples R China
Liu, Shuting
Das, Kinkar Chandra
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Sungkyunkwan Univ, Dept Math, Suwon 16419, South KoreaShandong Univ Sci & Technol, Coll Math & Syst Sci, Qingdao 266590, Shandong, Peoples R China
Das, Kinkar Chandra
Shu, Jinlong
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East China Normal Univ, Dept Comp Sci & Technol, Shanghai 200062, Peoples R ChinaShandong Univ Sci & Technol, Coll Math & Syst Sci, Qingdao 266590, Shandong, Peoples R China