[1] B.-Y. Chen, L. Vanhecke: 
Total curvatures of geodesic spheres. Archiv d. Math. 32 (1979), 404-411. 
MR 0545165 | 
Zbl 0416.53025[2] B.-Y. Chen, L. Vanhecke: 
Differential geometry of geodesic spheres. J. f. d. reine u. angew. Math. 325 (1981), 28-67. 
MR 0618545 | 
Zbl 0503.53013[3] R. Courant, D. Hilbert: Methods of Mathematical Physics. Vol. 2, Interscience, New York, 1977.
[4] F. Gackstatter, B. Gackstatter: 
Über Volumendefekte und Krümmung in Riemannschen Mannigfaltigkeiten mit Anwendungen in der Relativitätstheorie. Annalen d. Physik (7) 41 (1984), 35-44. 
MR 0780988 | 
Zbl 0551.53037[5] A. Gray: 
The volume of a small geodesic ball of a Riemannian manifold. Michigan Math. J. 20 (1973), 329-344. 
MR 0339002 | 
Zbl 0279.58003[6] A. Gray: 
Geodesic balls in Riemannian product manifolds. in: M. Cahen and M. Flato (eds.), Differential Geometry and Relativity, Reidel, Dordrecht, 1976. 
MR 0438252 | 
Zbl 0345.53007[7] A. Gray, L. Vanhecke: 
Riemannian geometry as determined by the volumes of small geodesic balls. Acta math. 142 (1979), 157-198. 
MR 0521460 | 
Zbl 0428.53017[8] P. Günther: 
Einige Sätze über das Volumenelement eines Riemannschen Raumes. Publ. Math. Debrecen 7 (1960), 78-93. 
MR 0141058[9] O. Kowalski: 
Additive volume invariants of Riemannian manifolds. Acta math. 145 (1980), 205-225. 
MR 0590290 | 
Zbl 0454.53031[10] O. Kowalski: 
The volume conjecture and four-dimensional hypersurfaces. Comment. Math. Univers. Carol. 23 (1982), 81-87. 
MR 0653352 | 
Zbl 0489.53050[11] O. Kowalski, L. Vanhecke: 
Ball-homogeneous and disk-homogeneous Riemannian manifolds. Math. Z. 180 (1982), 429-444. 
MR 0666999 | 
Zbl 0476.53023[12] O. Kowalski, L. Vanhecke: 
On disk-homogeneous symmetric spaces. Ann. Glob. Analysis and Geom. 1 (1983), 91-104. 
MR 0739899 | 
Zbl 0533.53047[13] O. Kowalski, L. Vanhecke: 
The volume of geodesic disks in a Riemannian manifold. Czech. Math. J. 35 (1985), 66-77. 
MR 0779337 | 
Zbl 0586.53006[14] O. Kowalski, L.  Vanhecke: 
Two-point functions on Riemannian manifolds. Ann. Glob. Analysis and Geom. 3 (1985), 95-119. 
MR 0812315 | 
Zbl 0571.53031[15] A. Lichnerowicz, A. G. Walker: 
Sur les espaces riemanniens harmoniques de type hyperbolique normal. C. R. Acad. Sc. Paris 221 (1945), 394-396. 
MR 0015269 | 
Zbl 0060.38507[16] V. Miquel: 
The volumes of small geodesic balls for a metric connection. Compositio Math. 46 (1982), 121-132. 
MR 0660156 | 
Zbl 0489.53043[17] V. Miquel: 
Volumes of certain small geodesic balls and almost Hermitean geometry. Geometriae Dedicata 15 (1984), 261 -267. 
MR 0739929[18] H. S. Ruse A. G. Walker, and T. J. Willmore: 
Harmonic Spaces. Edizioni Cremonese, Roma, 1961. 
MR 0142062[19] R. Schimming: 
Riemannian manifolds for which a power of the radius is k-harmonic. Z. f. Analysis u. ihre Anw. 4 (1985), 235-249. 
MR 0807133 | 
Zbl 0571.53012