Abstract. A real valued function g(x, t) on R n ×R is called Lorentz invariant if g(x, t) = g(U x, t) for all n×n orthogonal matrices U and all (x, t) in the domain of g. In other words, g is invariant under the linear orthogonal transformations preserving the Lorentz cone:It is easy to see that every Lorentz invariant function can be decomposed as g = f • β, where f : R 2 → R is a symmetric function and β is the root map of the hyperbolic polynomial p(x, t) = t 2 −x 2 1 −· · ·−x 2 n . We investigate variety of important variational and non-smooth properties of g and characterize them in terms of the symmetric function f .