Problem using NonlinearFit/Regress?

*To*: mathgroup at smc.vnet.net*Subject*: [mg33858] Problem using NonlinearFit/Regress?*From*: redrooz at yahoo.com (Red Roo)*Date*: Sat, 20 Apr 2002 02:49:48 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

I am trying to estimate the parameters {a,b} in the function: x / (1 + a (x-1) + a b x (x-1)) using NonlinearFit[...]. The model is known to be a CONCAVE function with 0 < a,b < 1 and b < a (in general). NonlinearFit/Regress always produces negative parameters (i.e., a convex function); even if I try to condition them to be positive e.g., {a, 0, 1} and {b, 0, 1}. Is there something I am missing about the use of NonlinearFit[...] for the above case? --njg