  In this note we give a recursive formula for the derivatives of isotropic
positive definite functions on the Hilbert sphere. We then use it to prove a
conjecture stated by Tr\"ubner and Ziegel, which says that for a positive
definite function on the Hilbert sphere to be in $C^{2\ell}([0,\pi])$, it is
necessary and sufficient for its $\infty$-Schoenberg sequence to satisfy
$\sum\limits_{m=0}^{\infty}a_m m^{\ell}<\infty$.
