Aart Blokhuis Andries E. Brouwer Willem H. Haemers On 3-chromatic distance-regular graphs. 293-305 2007 44 Des. Codes Cryptography 1-3 http://dx.doi.org/10.1007/s10623-007-9100-7 db/journals/dcc/dcc44.html#BlokhuisBH07