In this paper, we give an
optimal regularity result for some class of weakly harmonic maps from a Riemannian
manifold M into a static Lorentzian manifold. Our main result is the following: For
such class of weakly harmonic map w, there exists closed set Σ ⊂ M such that w is
C∞ in M ∖ Σ and the Hausdorff dimension of Σ is less than or equal to
dimM − 3.