  Hoffman's ratio bound is an upper bound for the independence number of a
regular graph in terms of the eigenvalues of the adjacency matrix. The bound
has proved to be very useful and has been applied many times. Hoffman did not
publish his result, and for a great number of users the emergence of Hoffman's
bound is a black hole. With his note I hope to clarify the history of this
bound and some of its generalizations.
