[math-fun] A real, live, practical use for Lambert's W-function

FYI -- For those who 1) have a mortgage; that 2) isn't underwater; and 3) still have a job & can qualify for refinancing: "Optimal Mortgage Refinancing: A Closed Form Solution" http://www.economics.harvard.edu/faculty/laibson/files/Mortgage%20refinancin... http://www.nber.org/papers/w13487 An actual mortgage refinancing calculator based on the formulae from this paper: http://zwicke.nber.org/refinance/index.py