SOME HARMONIC ANALYSIS ON THE QUATERNIONIC SPHERE VALENTINA CASARINO ` DEGLI STUDI DI PADOVA UNIVERSITA In this talk, we present some results, obtained in collaboration with V´eronique Fischer (Imperial College, Londra) and Paolo Ciatti (Universit`a di Padova), concerning the harmonic analysis on the unit sphere S 4n−1 in Hn . We shall discuss, in particular, some elliptic and sub elliptic operators, acting on S 4n−1 , providing optimal bounds for the associated spectral projectors. These estimates turn out to be quite different from those known for the real and the complex sphere. Our work is based on the analysis carried out by K. D. Johnson and N. R. Wallach [JW]. References [JW] K. D. Johnson and N. R. Wallach, Composition series and intertwining operators for the spherical principal series. I, Trans. Amer. Math. Soc., 229 (1977), 137-173. E-mail address: valentina.casarino@unipd.it 1