ARC: Update AMPL interest rate curve to account for over-approximation in compounded interest

[Naguib]

For posters asking about or reporting inability to withdraw please read the most recent updates here:

AMPL problem on Aave v2 Ethereum

Dear AAVE and Ampleforth Community, I’m posting to provide an outline of proposed steps to address the inability to withdraw aAMPL balances. To recap the source of the issue: More interest was credited to depositors than was charged from borrowers. This discrepancy has led to situations where some depositors were able to withdraw more than what should have been possible under normal circumstances. As a direct consequence, we’re now facing a liquidity shortfall, preventing current depositors fr…