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[GW] A. Gray T. J. Willmore: Mean-value theorems for Riemannian manifolds. To appear in Proc. R. Soc. Edinburgh. MR 0677493

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[RWW] H. S. Ruse A. G. Walker, T. J. Willmore: Harmonic spaces. Edizioni Cremonese, Roma, 1961. MR 0142062