per-block interest rate calculation

Hi! I’m going through Compound.Whitepaper. In the Market Dynamics section, the formulas for totalBorrowBalance and reserves contain ‘r’, which is per-block interest rate (if I understand it correctly). But it’s not clear to me how it is calculated. Is it described anywhere? I couldn’t find. I would appreciate any hints and explanations. Thanks!

This is not the best medium for asking questions like this. Discord is more suitable.

Be advised that the Whitepaper applies to the v2 protocol, which is currently going through a community led deprecation process. There is an answer to this here in Discord.