1. Energy spectrum of certain harmonic mappings;Adachi, Toshiaki;Compositio Math.,1985
2. Vector fields and Ricci curvature;Bochner, S.;Bull. Amer. Math. Soc.,1946
3. P. Buser and H. Karcher, Gromov’s almost flat manifolds, Astérisque 81 (1981).
4. North-Holland Mathematical Library, Vol. 9;Cheeger, Jeff,1975
5. Textes Math\'{e}matiques [Mathematical Texts];Gromov, Mikhael,1981