# How to derive compact convex set K and its diameter D to program Accelegrad algorithm in practice?

Given the original paper (https://arxiv.org/pdf/1809.02864.pdf), I would like to implement the Accelegrad algorithm for which I report the pseudocode of the paper:

In the pseudocode, the authors refer to a compact convex set $$K$$ of diameter $$D$$. The question is whether I can know such elements. I think that they are theoretical conditions to satisfy some theorems. The problem is that the diameter $$D$$ is used in the learning rate and also the convex set $$K$$ is used to perform the projection of the gradient descent. How can I proceed?