Uniqueness of Positive Fixed Points for Increasing Concave Functions on Rn: An Elementary Result , Review of Economic Dynamics 4, October 2001, 893-899 (doi:10.1006/redy.2001.0133). [PDF]
John Kennan
Example: y1 = y2 = sin(x1) + sqrt(x1) + sin(x2) + sqrt(x2) has a unique fixed point.
