On the moments of roots of Laguerre-polynomials and the Marchenko-Pastur law

In this paper we compute the leading terms in the sum of the $k^{th}$ power of the roots of $L_{p}^{(\alpha)}$, the Laguerre-polynomial of degree $p$ with parameter $\alpha$. The connection between the Laguerre-polynomials and the Marchenko-Pastur distribution is expressed by the fact, among others, that the limiting distribution of the empirical distribution of the normalized roots of the Laguerre-polynomials is given by the Marchenko-Pastur distribution. We give a direct proof of this statement based on the recursion satisfied by the Laguerre-polynomials. At the same time, our main result gives that the leading term in $p$ and $(\alpha+p)$ of the sum of the $k^{th}$ power of the roots of $L_{p}^{(\alpha)}$ coincides with the $k^{th}$ moment of the Marchenko-Pastur law. We also mention the fact that the expectation of the characteristic polynomial of a $XX^T$ type random covariance matrix, where $X$ is a $p\times n$ random matrix with iid elements, is $\ell^{(n-p)}_p$, i.e. the monic version of the $p^{th}$ Laguerre polynomial with parameter $n-p$.