  The present paper implements a complex analytic method to recover the
spectrum of a matrix perturbed by either the addition or the multiplication of
a random matrix noise, under the assumption that the distribution of the noise
is unitarily invariant. This method, introduced by Arizmendi, Tarrago and
Vargas in arXiv:1711.08871, is done in two steps : the first step consists in a
fixed point method to compute the Stieltjes transform of the desired
distribution in a certain domain, and the second step is a classical
deconvolution by a Cauchy distribution, whose parameter depends on the
intensity of the noise. We also provide explicit bounds for the mean squared
error of the first step.
