The public debt sustainability condition is absolutely not of the same nature when the interest rate is lower than the growth rate

In the normal configuration, the interest rate is higher than the growth rate. The current public debt is then sustainable if it is lower than the discounted sum of future primary fiscal surpluses. But, in the current configuration, the interest rate is lower than the growth rate. Public debt sustainability is ensured if the future public debt level does not exceed the maximum permissible level. There are two major differences from the situation where the interest rate is higher than the growth rate: This is a limit on the primary fiscal deficit, not a minimum primary fiscal surplus; The current public debt ratio does not matter; only future primary deficits count.