I'm always willing to be told that I'm out of date on something. Getting up to date is, after all, one of the main ways we learn. I'm highly skeptical about David's claim, though. For one thing, you would think that if Von Neumann showed this over half a century ago, my professors at UCLA, especially people like Armen Alchian and Jack Hirshleifer, would have known about it. I don't recall that they contradicted Paul Samuelson on this.