Before considering phase diagrams, I must show that a competitive equilibrium exists for some class of models. Standard results concerning the existence of solutions of differential equations can be used to prove that the equations for λ and ḱ determine a unique trajectory through any point (k, λ) in the phase plane. The difficulty arises in showing that for any given value of k_0 there exists some value of λ_0 such that the transversality condition at infinity is satisfied along the trajectory through (k_0,λ_0). As opposed to the case in which these equations are generated by a concave maximization problem known to have a solution, there is no assurance that such a λ_0 xists.