One thing that is never mentioned in this article or any other I have read is the fact that they first have to be employed. I would have to assume that in making all these complicated calculations , they have made the assumption that each and every Legal immigrant actually has a job and is paying into the system. It is purely an academic exercise on it's face since at no time in the history of any civilization have that many paying jobs ever been created in any economic system over any length of time.