1. K. Yano and S. Bochner, Curvature and Betti Numbers, Princeton Univ. Press, Princeton (1953).
2. S. Kobayashi and K. Nomizu, Foundations of Differential Geometry [Russian translation], Vol. 1, Nauka, Moscow (1981).
3. A. L. Besse, Géometrie Riemannienne en Dimension 4, Cedic-Fernand Nathan, Paris (1981).
4. S. E. Stepanov, Differentsial'naya Geometriya Mnogoobrazii Figur, No. 22, 101 (1991).
5. Dzh. Bim and P. Erlikh, Global Lorentz Geometry [Russian translation], Mir, Moscow (1985).