... Haemers and Van Dam, [12] , and Garriga,Yebra, and the author [17] ,[18] ,[20] ,[21] , and also the recent theses of Van Dam [10], Garriga [23] and Rodr´ıguez [37] . In their study, Garriga and the ... are trivial.) Then, by the converseof Theorem 2.5, it must be Qku≥ 1. On the other hand, in the proof of Theorem the electronic journal of combinatorics 4 (1997), #R21 3In the rest of ... As usual, J denotes the n × n matrix with all entries equal to 1, and similarlyj ∈Rnis the all-1 vector. The spectrum of Γ is the set of eigenvalues of A togetherwith their multiplicitiesS(Γ)...