The Spectrum of Q-Isotropic Binary Quadratic Forms

We give a complete list of the points in the spectrum Z=inf(x,y)∈Λ,xy≠0 xy,\,Λ$ is a unimodular rational lattice of $R^2\ above 1/3. We further show that the set of limit points of Z with values larger than 1/3, is equal to the set \2m√9m²-4+3m, wheremis a Markoff number\.