... 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 3 In the rest of this introductory ... as the diameter and the radius[16] . Here, after introducing these polynomials and recalling its main properties,we survey some of the main known results related to them. For example, a regular graph ... Then, following Godsil [24] , we say that the graphΓiis the i-th class of the scheme, and so we indistinctly use the words “graph” or“class” to mean the same thing.2 The Proper and Adjacency...