SSbiexp                 package:nls                 R Documentation

Biexponential model

Description:

     This `selfStart' model evaluates the biexponential model function
     and its gradient.  It has an `initial' attribute that  creates
     initial estimates of the parameters `A1', `lrc1', `A2', and
     `lrc2'.

Usage:

     SSbiexp(input, A1, lrc1, A2, lrc2)

Arguments:

   input: a numeric vector of values at which to evaluate the model.

      A1: a numeric parameter representing the multiplier of the first
          exponential.

    lrc1: a numeric parameter representing the natural logarithm of the
          rate constant of the first exponential.

      A2: a numeric parameter representing the multiplier of the second
          exponential.

    lrc2: a numeric parameter representing the natural logarithm of the
          rate constant of the second exponential.

Value:

     a numeric vector of the same length as `input'.  It is the value
     of the expression
     `A1*exp(-exp(lrc1)*input)+A2*exp(-exp(lrc2)*input)'. If all of the
     arguments `A1', `lrc1', `A2', and `lrc2' are names of objects, the
     gradient matrix with respect to these names is attached as an
     attribute named `gradient'.

Author(s):

     Jose Pinheiro and Douglas Bates

See Also:

     `nls', `selfStart'

Examples:

     data( Indometh )
     Indo.1 <- Indometh[Indometh$Subject == 1, ]
     SSbiexp( Indo.1$time, 3, 1, 0.6, -1.3 )  # response only
     A1 <- 3; lrc1 <- 1; A2 <- 0.6; lrc2 <- -1.3
     SSbiexp( Indo.1$time, A1, lrc1, A2, lrc2 ) # response and gradient
     getInitial(conc ~ SSbiexp(time, A1, lrc1, A2, lrc2), data = Indo.1)
     ## Initial values are in fact the converged values
     fm1 <- nls(conc ~ SSbiexp(time, A1, lrc1, A2, lrc2), data = Indo.1)
     summary(fm1)

