S == over(c, (1 + exp(-z * A + f)))

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The format is: List of 8 $ name : "Logistic function" $ formula : expression(S == over(c, (1 + exp(-z * A + f)))) $ paramnumber: 3 $ paramnames : "c" "z" "f" $ parLim : "Rplus" "Rplus" "R" $ fun : model function $ rssfun : Residual Sum of Squares function $ init : initial values calculation $ form : a formula object for further calculations

This model is sigmoid asymptotic. Parameter c is the upper asymptote. The function is symmetrical about the inflection point. The curve has a lower asymptote of zero, and the intersection with the y-axis depends on all three parameters. Thus it never goes through the origin. For further details and documentation please see the online documentation at http://mmsar.r-forge.r-project.org/

Ratkowsky (1990), Tjørve (2003).

Ratkowsky, D.A. (1990) Handbook of nonlinear regression models. Marcel Dekker, New York.

Tjørve, E. (2003) Shapes and functions of species–area curves: a review of possible models. Journal of Biogeography, 30, 827–835.

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