  Federated learning (FL) is a trending distributed learning framework that
enables collaborative model training without data sharing. Machine learning
models trained on datasets can potentially expose the private information of
the training data, revealing details about individual data records. In this
study, we focus on the FL paradigm that grants clients the ``right to be
forgotten''. The forgettable FL framework should bleach its global model
weights as it has never seen that client and hence does not reveal any
information about the client. To this end, we propose the Forgettable Federated
Linear Learning (2F2L) framework featured with novel training and data removal
strategies. The training pipeline, named Federated linear training, employs
linear approximation on the model parameter space to enable our 2F2L framework
work for deep neural networks while achieving comparable results with canonical
neural network training. We also introduce FedRemoval, an efficient and
effective removal strategy that tackles the computational challenges in FL by
approximating the Hessian matrix using public server data from the pretrained
model. Unlike the previous uncertified and heuristic machine unlearning methods
in FL, we provide theoretical guarantees by bounding the differences of model
weights by our FedRemoval and that from retraining from scratch. Experimental
results on MNIST and Fashion-MNIST datasets demonstrate the effectiveness of
our method in achieving a balance between model accuracy and information
removal, outperforming baseline strategies and approaching retraining from
scratch.
