Orbital measures on SU(2)/SO(2)

Abstract

We let U=SU(2) and K=SO(2) and denote N_{U}(K) the normalizer of K in U. For a an element of U\ N_{U} (K), we let μ_{a} be the normalized singular measure supported in KaK. For p a positive integer, it was proved that μ_{a}^{( p)}, the convolution of p copies of μ_{a}, is absolutely continuous with respect to the Haar measure of the group U as soon as p>=2. The aim of this paper is to go a step further by proving the following two results : (i) for every a in U\ N_{U} (K) and every integer p >=3, the Radon-Nikodym derivative of μ_{a}^{(p)} with respect to the Haar measure m_{U} on U, namely dμ_{a}^{(p)}/d m_{U}, is in L^{2}(U), and (ii) there exist a in U\ N_{U} (K) for which dμ_{a}^{(2)}/ dm_{U} is not in L^{2}(U), hence a counter example to the dichotomy conjecture. Since L^{2} (G) \subseteq L^{1} (G), our result gives in particular a new proof of the result when p>2.