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
nls, selfStart.
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)

Package stats version 6.1.5-18
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