1. Berenstein, C., Zalcman, L.: Pompeiu's Problem in Symmetric Spaces. Comment. Math. Helv.55, 593?621 (1980)
2. Bishop, R., Crittenden, R.: Geometry of Manifolds. New York-London: Academic Press 1964
3. Brooks, R.: Exponential Growth and the Spectrum of the Laplacian. Proc. Amer. Math. Soc.82, 473?477 (1981)
4. Brooks, R.: The Fundamental Group and the Spectrum of the Laplacian. Preprint
5. Cheeger, J.: A Lower Bound for the Smallest Eigenvalue of the Laplacian. In: Problems in analysis (Ed. R.C. Gunning). A Symposium in Honor of Salomon Bochner (Princeton 1969), pp. 195?199. Princeton: Princeton University Press 1970