  We consider the formulation of "machine unlearning" of Sekhari, Acharya,
Kamath, and Suresh (NeurIPS 2021), which formalizes the so-called "right to be
forgotten" by requiring that a trained model, upon request, should be able to
"unlearn" a number of points from the training data, as if they had never been
included in the first place. Sekhari et al. established some positive and
negative results about the number of data points that can be successfully
unlearnt by a trained model without impacting the model's accuracy (the
"deletion capacity"), showing that machine unlearning could be achieved by
using differentially private (DP) algorithms. However, their results left open
a gap between upper and lower bounds on the deletion capacity of these
algorithms: our work fully closes this gap, obtaining tight bounds on the
deletion capacity achievable by DP-based machine unlearning algorithms.
