[R] Error messages of nlme

[Grathwohl,Dominik,LAUSANNE,NRC/NT]

Hello, everyone,

I would like to know the meaning of the error messages of nlme
in order to get converging fits. The application is a simple 
non-linear model to a dataset of increasing Apo B. 
I use:

platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    1              
minor    5.1            
year     2002           
month    06             
day      17             
language R

The non-linear function:

mu2 <- function(k, A, B0, t) {
	c0 <- exp(B0)
	i  <- exp(k)
	c1 <- exp(A)
	c0 * exp(-i*t) + c1/i * (1-exp(-i*t))
}

For example this error message:

> m2e.1 <- nlme(y ~ mu2(k, A, B0, t), data=thelp, 
+ fixed = list(k + A + B0 ~ 1),
+ random = list(id = B0 ~ 1, f = B0 ~ 1),
+ start=c(k=-1.7, A=0.86, B0=1.44))
Error: Singularity in backsolve at level 0, block 1

or:

> m2e.15 <- update(m2e.13, corr = corExp())
Error in as.vector(dist(1:length(x))) : couldn't find function "dist"

What have I to do to get proper fits? Nevertheless, I get some nice fits 
with other models. From my experience with this models, they are relatively 
robust against the choose of starting values. 
Can somebody explain me the meaning of the error messages?

Kind regards,

Dominik
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