R help - Apr 2010 - Restricting optimisation algorithm's parameter space in GNLM

Hello, 

I have a problem. I am  using the NLME library to fit a non-linear model. There
is a linear  component to the model that has a couple parameter values that can
only  be positive (the coefficients are embedded in a sqrt). When I try and  fit
the model to data the search algorithm tries to see if a negative  value for one
of these parameter values will produce an optimal fit. When  it does so, it
crashes because the equation can not have a negative  value because its in a
sqrt function.

QUESTION: How do I  restrict the optimisation algorithm's  parameter space
so it does not  search negative values when using GNLM? Are there other
Libraries that Fit Non-linear models and allow for one to control the parameter
space the search algorithm is restricted by?

Any help would be  appreciated.
Thanks 
Dom 


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