DeLong and Krugman vs Mankiw
This has become a total mess and I moved my algebra over to my personal blog. The point is that the whole discussion depends on the difference in the long term effect of a cut in a tax on capital income on revenue collected from old capital and the effect ofa cut in a tax on capital on revenue collected from old capital.
This is an extremely uninteresting issue.
Either pole owes more critical thought about their assumptions.
Time for Barry Ritholtz to post his visual on the top logic traps.