Segre, Beniamino (1968), «Alcune proprietà differenziali in grande delle curve chiuse sghembe», Rendiconti di Matematica1: 237-297, MR0243466.
Weiner, Joel L. (1977), «Global properties of spherical curves», Journal of Differential Geometry12 (3): 425-434, MR0514446.. For the tennis ball theorem (applicable more generally to curves that are not contained in a single hemisphere), see Theorem 2, p. 427
Thorbergsson, Gudlaugur; Umehara, Masaaki (1999), «A unified approach to the four vertex theorems II», en Tabachnikov, Serge, ed., Differential and Symplectic Topology of Knots and Curves, Amer. Math. Soc. Transl. Ser. 2 190, Amer. Math. Soc., Providence, RI, pp. 229-252, MR1738398, doi:10.1090/trans2/190/12.. See in particular pp. 242–243.
Thorbergsson, Gudlaugur; Umehara, Masaaki (1999), «A unified approach to the four vertex theorems II», en Tabachnikov, Serge, ed., Differential and Symplectic Topology of Knots and Curves, Amer. Math. Soc. Transl. Ser. 2 190, Amer. Math. Soc., Providence, RI, pp. 229-252, MR1738398, doi:10.1090/trans2/190/12.. See in particular pp. 242–243.
Thorbergsson, Gudlaugur; Umehara, Masaaki (1999), «A unified approach to the four vertex theorems II», en Tabachnikov, Serge, ed., Differential and Symplectic Topology of Knots and Curves, Amer. Math. Soc. Transl. Ser. 2 190, Amer. Math. Soc., Providence, RI, pp. 229-252, MR1738398, doi:10.1090/trans2/190/12.. See in particular pp. 242–243.
projecteuclid.org
Weiner, Joel L. (1977), «Global properties of spherical curves», Journal of Differential Geometry12 (3): 425-434, MR0514446.. For the tennis ball theorem (applicable more generally to curves that are not contained in a single hemisphere), see Theorem 2, p. 427