Geodesic
Quasi-extremality for controllable systems
Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya, Itogi Nauki i Tekhniki. Seriya "Sovremennye Problemy Matematiki. Noveishie Dostizheniya", Tome 35 (1989), pp. 109-134 
A. A. Agrachev; R. V. Gamkrelidze. Quasi-extremality for controllable systems. Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya, Itogi Nauki i Tekhniki. Seriya "Sovremennye Problemy Matematiki. Noveishie Dostizheniya", Tome 35 (1989), pp. 109-134. http://geodesic.mathdoc.fr/item/INTD_1989_35_a1/
@article{INTD_1989_35_a1,
     author = {A. A. Agrachev and R. V. Gamkrelidze},
     title = {Quasi-extremality for controllable systems},
     journal = {Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya},
     pages = {109--134},
     year = {1989},
     volume = {35},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/INTD_1989_35_a1/}
}
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

A full exposition of the authors' previously announced results about the extremality index of controls in smooth control systems and a generalization of these results to systems with constraints on the controls.