1. A. V. Arutyunov, G. G. Magaril-Il’yaev, and V.M. Tikhomirov, Pontryagin’sMaximum Principle: Proof and Applications (Faktorial Press, Moscow, 2006) [in Russian].
2. A. A. Milyutin, A. V. Dmitruk, and N. P. Osmolovskii, Maximum Principle in Optimal Control (Izd. Tsentra Prikl. Issled, Mekh.-Mat., Moskov. Univ., Moscow, 2004).
3. W. Blaschke, Kreis und Kugel (Walter de Gruyter, Berlin, 1956; Nauka, Moscow, 1967).