On Siegel results about the zeros of the auxiliary function of Riemann

We state and give complete proof of the results of Siegel about the zeros of the auxiliary function of Riemann $(s)$. We point out the importance of the determination of the limit to the left of the zeros of $(s)$ with positive imaginary part, obtaining the term $-\sqrt{T\mathrm{/}2\pi }P(\sqrt{T\mathrm{/}2\pi })$ that would explain the periodic behaviour observed with the statistical study of the zeros of $(s)$. We precise also the connection of the position on the zeros of $(s)$ with the zeros of $\zeta (s)$ in the critical line.