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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Zhangzhou Normal Univ, Dept Math & Informat Sci, Zhangzhou, Fujian, Peoples R ChinaZhangzhou Normal Univ, Dept Math & Informat Sci, Zhangzhou, Fujian, Peoples R China
Li, Jianxi
Guo, Ji-Ming
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China Univ Petr, Dept Appl Math, Dongying, Shandong, Peoples R ChinaZhangzhou Normal Univ, Dept Math & Informat Sci, Zhangzhou, Fujian, Peoples R China
Guo, Ji-Ming
Shiu, Wai Chee
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Hong Kong Baptist Univ, Dept Math, Kowloon Tong, Hong Kong, Peoples R ChinaZhangzhou Normal Univ, Dept Math & Informat Sci, Zhangzhou, Fujian, Peoples R China
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Sungkyunkwan Univ, Dept Math, Suwon 440746, South KoreaSungkyunkwan Univ, Dept Math, Suwon 440746, South Korea
Das, Kinkar Ch.
Xu, Kexiang
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Nanjing Univ Aeronaut & Astronaut, Coll Sci, Nanjing, Jiangsu, Peoples R ChinaSungkyunkwan Univ, Dept Math, Suwon 440746, South Korea
Xu, Kexiang
Liu, Muhuo
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South China Agr Univ, Dept Appl Math, Guangzhou 510642, Guangdong, Peoples R China
Nanjing Normal Univ, Sch Math Sci, Inst Math, Nanjing 210046, Jiangsu, Peoples R ChinaSungkyunkwan Univ, Dept Math, Suwon 440746, South Korea