SSbiexp
Biexponential Model: The Sum of Two Exponentials
Description
Evaluates the biexponential function and its gradient, and computes the
initial parameters for fitting data to a biexponential model. This is a selfStart function.
Usage
SSbiexp(input, A1, lrc1, A2, lrc2)
Arguments
| input |
a numeric vector of values at which to evaluate the model.
|
| A1 |
a numeric value representing the multiplier of the first exponential.
|
| lrc1 |
a numeric value representing the natural logarithm of the rate constant of the first exponential.
|
| A2 |
a numeric value representing the multiplier of the second exponential.
|
| lrc2 |
a numeric value representing the natural logarithm of the rate constant of the second exponential.
|
Details
Because it is a selfStart function, SSbiexp has an attribute called "initial", which
is a function that nls can call to compute reasonable starting values for fitting a biexponential
function to the input data.
Value
returns a numeric vector of the same length as the input. It is the value of the
expression A1*exp(-exp(lrc1)*input)+A2*exp(-exp(lrc2)*input).
If the arguments A1, lrc1, A2, and lrc2 are the names of objects,
then the gradient (Jacobian) matrix with respect to these names, evaluated at the values of those
names, is attached as an attribute named gradient.
Note
This function is intended for use in formulae given to the nls function or similar functions.
See Also
Examples
SSbiexp(seq(0,1,by=.1), A1=3, lrc1=3, A2=6, lrc2=.5)
with(list(beta1=3, rate1=3, beta2=6, rate2=.5),
SSbiexp(seq(0,1,by=.1), A1=beta1, lrc1=rate1, A2=beta2, lrc2=rate2))
tDat <- data.frame(
time = c(0, .1, .2, .3, .4, .5, .6, .7, .8, .9, 1.0),
excessTemp = c(9, 5.5, 4.4, 3.7, 3.1, 2.6, 2.2, 1.9, 1.6, 1.4, 1.1))
nls(excessTemp ~ SSbiexp(time, beta1, rate1, beta2, rate2), data = tDat)