eng Science and Education Publishing International Journal of Physics 2333-4576 2018-04-13 6 2 47 52 10.12691/ijp-6-2-4 IJP2018624 article Vu B Ho vubho@bigpond.net.au 1 Advanced Study, 9 Adela Court, Mulgrave, Victoria 3170, Australia Investigations into the nature of the principle of least action have shown that there is an intrinsic relationship between geometrical and topological methods and the variational principle in classical mechanics. In this work, we follow and extend this kind of mathematical analysis into the domain of quantum mechanics. First, we show that the identification of the momentum of a quantum particle with the de Broglie wavelength in 2-dimensional space would lead to an interesting feature; namely the action principle S=0 would be satisfied not only by the stationary path, corresponding to the classical motion, but also by any path. Thereupon the Bohr quantum condition possesses a topological character in the sense that the principal quantum number is identified with the winding number, which is used to represent the fundamental group of paths. We extend our discussions into 3-dimensional space and show that the charge of a particle also possesses a topological character and is quantised and classified by the homotopy group of closed surfaces. We then discuss the possibility to extend our discussions into spaces with higher dimensions and show that there exist physical quantities that can be quantised by the higher homotopy groups. Finally we note that if Einstein's field equations of general relativity are derived from Hilbert's action through the principle of least action then for the case of n=2 the field equations are satisfied by any metric if the energy-momentum tensor is identified with the metric tensor, similar to the case when the momentum of a particle is identified with the curvature of the particle's path. http://pubs.sciepub.com/ijp/6/2/4/ijp-6-2-4.pdf principle of least action Feynman path integral method classical mechanics quantum mechanics general relativity