numeric vector. Specifies which parameters are fixed and at what value they are fixed.
NAs for parameter that are not fixed.
names
a vector of character strings giving the names of the parameters (should not contain ":").
The order of the parameters is: b1, b2, c, d, e (see under 'Details').
method
character string indicating the self starter function to use.
ssfct
a self starter function to be used.
Value
The value returned is a list containing the nonlinear model function, the self starter function
and the parameter names.
Details
The five-parameter function given by the expression
$$y = c + \frac{d-c}{1+f\exp(b1(\log(x)-\log(e))) + (1-f)\exp(b2(\log(x)-\log(e)))}$$
$$f = 1/( 1 + \exp((2b1b2/|b1+b2|)(\log(x)-\log(e))))$$
If the difference between the parameters b1 and b2 is different from 0 then the function is asymmetric.
References
Ricketts, J. H. and Head, G. A. (1999)
A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies.
Am. J. Physiol. (Regulatory Integrative Comp. Physiol. 46), 277, 441--454.