Abstract
We give a proof of the Poincaré inequality in W 1, p (Ω) with a constant that is independent of Ω ∊ , where  is a set of uniformly bounded and uniformly Lipschitz domains in ℝ n . As a byproduct, we obtain the following: The first non vanishing eigenvalues λ2(Ω) of the standard Neumann (variational) boundary value problem on Ω for the Laplace operator are bounded below by a positive constant if the domains Ω vary and remain uniformly bounded and uniformly Lipschitz regular.