Abstract. Various classes of functions with vanishing integrals over all balls of a fixed radius on the sphere S n are studied. For such functions, uniqueness theorems are proved, and representations in the form of series in special functions are obtained. These results made it possible to completely resolve the problem concerning the existence of a nonzero function with vanishing integrals over all balls on S n the radii of which belong to a given two-element set. §1. Introduction Let X be a Riemannian two-point homogeneous manifold (see [1, Chapter 1]), let G be the isometry group of X, and let dx be Riemannian measure on X. We consider a collectionIf U = X, we call P F ;U the global Pompeiu transformation and denote it by P F .For given F and U, the following questions arise.
Problem (see [2]). Is the transformation P F ;U injective? If not, what is its kernel?A set E ⊂ X for which P E is injective is called a Pompeiu set. The injectivity of the Pompeiu transformation and related questions for some X, F, and U were studied in many papers (see the surveys [2]-[5]). It turned out that there is a qualitative difference between the results for noncompact spaces X and their analogs for compact spaces. Here, we consider some of them.Suppose X is a noncompact two-point homogeneous space, i.e., X is isometric either to the Euclidean space R n , or to one of the hyperbolic spaces H n (R), H n (C), H n (H), or H 16 (Cay) (see [1, Chapter 1,§4, Subsection 3]. Then every ball in X is not a Pompeiu set. This fact follows easily from the mean-value theorem for eigenfunctions of the LaplaceBeltrami operator on X (see [1, Chapter 4, Proposition 2.4]) (for Euclidean spaces, this fact was first mentioned in the paper [6]). Some necessary and sufficient conditions for the injectivity of P F for the family F = B r 1 , B r 2 (where the symbol B r i , i = 1, 2, stands for the open ball of radius r i and centered at the origin of X, and B r i is the closure of B r i ) were obtained in [7]. The results of this type are called "two-radii theorems". Very general conditions ensuring the injectivity of P E were obtained by Williams [8] for X = R n , and by Berenstein and Shahshahani [9] for hyperbolic spaces. In these papers, it was proved that if a bounded open set E with connected complement and with Lipschitz 2000 Mathematics Subject Classification. Primary 26B15, 44A15, 49Q15.