@article{IM_1981__65_142845,
     author = {Karsten Grove and Detlef Gromoll},
     title = {On {Metrics} on {S2} {All} of {Whose} {Geodesics} {Are} {Closed.}},
     journal = {Inventiones mathematicae},
     pages = {175--178},
     publisher = {mathdoc},
     volume = {65},
     year = {1981},
     zbl = {0477.53044},
     url = {http://geodesic.mathdoc.fr/item/IM_1981__65_142845/}
}