Sario, L., Nakai, M., Wang, C., & Chung, L. O. (1977). Classification theory of Riemann manifolds: Harmonic, quasiharmonic and biharmonic functions. Springer-Verlag.

Sario, Leo, Mitsuru Nakai, Cecilia Wang, and Lung Ock Chung. Classification Theory of Riemann Manifolds: Harmonic, Quasiharmonic and Biharmonic Functions. Berlín: Springer-Verlag, 1977.

Sario, Leo, et al. Classification Theory of Riemann Manifolds: Harmonic, Quasiharmonic and Biharmonic Functions. Springer-Verlag, 1977.