1. Gellerstedt and Laplace-Beltrami operators relative to a mixed signature metric,;J. Barros-Neto;Ann. Mat. Pura Appl. 188 (2009),2009
2. Saggio di interpretazione della geometria non-euclidea,;E. Beltrami;Giornale di Matematiche 6 (1868),1868
3. Symmetric positive linear differential equations,;K. O. Friedrichs;Commun. Pure Appl. Math. 11 (1958),1958
4. Relativistic Cosmology, An Introduction.;J. Heidmann;Springer-Verlag,1980
5. On certain classes of elliptic equations with singularity on the boundary of the domain (Russian]),;M. V. Keldysh;Dokl. Akad. Nauk SSSR 77 (1951),1951