Geodesic
Extremal paths in the nilpotent sub-Riemannian problem on the Engel group (subcritical case of pendulum oscillations)
Contemporary Mathematics. Fundamental Directions, Proceedings of the International Conference on Mathematical Control Theory and Mechanics (Suzdal, July 3–7, 2009), Tome 42 (2011), pp. 30-35 
A. A. Ardentov. Extremal paths in the nilpotent sub-Riemannian problem on the Engel group (subcritical case of pendulum oscillations). Contemporary Mathematics. Fundamental Directions, Proceedings of the International Conference on Mathematical Control Theory and Mechanics (Suzdal, July 3–7, 2009), Tome 42 (2011), pp. 30-35. http://geodesic.mathdoc.fr/item/CMFD_2011_42_a3/
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     author = {A. A. Ardentov},
     title = {Extremal paths in the nilpotent {sub-Riemannian} problem on the {Engel} group (subcritical case of pendulum oscillations)},
     journal = {Contemporary Mathematics. Fundamental Directions},
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     year = {2011},
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     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/CMFD_2011_42_a3/}
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

We consider a left-invariant sub-Riemannian problem on an Engel group. This problem arises as a nilpotent approximation of nonholonomic systems in the four-dimensional space with two-dimensional control (e.g., the system describing the motion of a mobile robot with a trailer). For the sub-Riemannian problem on the Engel group, abnormal extremal paths are calculated. The subsystem for conjugate variables of normal Hamiltonian system of Pontryagin's maximum principle is reduced to the pendulum equation. Normal extremal paths corresponding to subcritical pendulum oscillations were calculated.

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