1. Chen, B.Y., Nagano, T.: Harmonic metrics, harmonic tensors and Gauss maps. J. Math. Soc. Jpn. 36, 295–313 (1981)
2. Chen, B.Y.: Differential geometry of identity maps: A survey. Mathematics 8(8), 1–33 (2020)
3. Eells, J., Lemaire, L.: Selected topics in harmonic maps. Conf. Board Math. Sci. A.M.S 50, 8 (1983)
4. Kantowski, R., Sachs, R.K.: Some spatially homogenous anisotropic cosmological models. J. Math. Phys. 7(3), 443–446 (1966)
5. Oniciuc, C.: Tangent bundles and harmonicity. Ann. Sti. Ale. Uni. “Al. I. Cuza’’ Iasi 18(1), 151–172 (1997)