Yesterday's post was a bit long winded (by my standards) but the fuss over that paper has helped me refine my critique.
Never mind the economics or the maths, the problem I see in Reinhart and Rogoff is an error of logic. The idea of a 90% threshold is an assumption. It cannot therefore be a conclusion.
I can expose the logic using an old fashioned syllogism:
High debt correlates to low growth
The threshold for high debt is 90%
Therefore 90% debt is the threshold for low growth