MathSciNet bibliographic data MR276992 (43 #2730) 57.31 Ku, Hsu-tung; Ku, Mei-chin Characteristic spheres of free differentiable actions of \$S\sp{1}\$$S\sp{1}$ and \$S\sp{3}\$$S\sp{3}$ on homotopy spheres. Trans. Amer. Math. Soc. 156 1971 493–504. Article

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