1. Baird, P., Wood, J.C.: Harmonic Morphisms Between Riemannian Manifolds. London Mathematical Society Monographs, vol. 29. Oxford University Press, Oxford (2003)
2. Calvaruso, G.: Harmonicity properties of invariant vector fields on three-dimensional Lorentzian Lie groups. J. Geom. Phys. 61, 498–515 (2011)
3. Djaa, M., Elhendi, H., Ouakkas, S.: On the biharmonic vector field. Turk. J. Math. 36(3), 463–474 (2012)
4. Dombrowski, P.: On the geometry of tangent bundle. J. Reine Angew. Math. 210, 73–88 (1962)
5. Dragomir, S., Perrone, D.: Harmonic Vector Fields: Variational Principles and Differential Geometry. Elsevier, Amsterdam (2011)