Using Algebraic Properties of Minimal Idempotents for Exhaustive Computer Generation of Association Schemes

During the past few years we have obtained several new computer classification results on association schemes and in particular distance regular and strongly regular graphs. Central to our success is the use of two algebraic constraints based on properties of the minimal idempotents Ei of these association schemes : the fact that they are positive semidefinite and that they have known rank. Incorporating these constraints into an actual isomorph-free exhaustive generation algorithm turns out to be somewhat complicated in practice. The main problem to be solved is that of numerical inaccuracy: we do not want to discard a potential solution because a value which is close to zero is misinterpreted as being negative (in the first case) or nonzero (in the second). In this paper we give details on how this can be accomplished and also list some new classification results that have been recently obtained using this technique: the uniqueness of the strongly regular (126, 50, 13, 24) graph and some new examples of antipodal distance regular graphs. We give an explicit description of a new antipodal distance regular 3-cover of K14, with vertices that can be represented as ordered triples of collinear points of the Fano plane. the electronic journal of combinatorics 13 (2006), #R00 1