Closed geodesics on surfaces of genus 0
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 1, pp. 19-38. 
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     author = {Klingenberg, Wilhelm},
     title = {Closed geodesics on surfaces of genus 0},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {19--38},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 6},
     number = {1},
     year = {1979},
     mrnumber = {529474},
     zbl = {0426.58008},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1979_4_6_1_19_0/}
}
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Klingenberg, Wilhelm. Closed geodesics on surfaces of genus 0. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 6 (1979) no. 1, pp. 19-38. http://www.numdam.org/item/ASNSP_1979_4_6_1_19_0/

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