Robust nonlinear regression - sin(x)/x?

A small correction to my previous email:
You actually specify the following call to lm:

y <- sin(x)/x
lm(z ~ y - 1)

to make sure that the intercept is not estimated.

Ravi.

----- Original Message -----
From: Ravi Varadhan <rvaradha at jhsph.edu>
Date: Monday, February 2, 2004 2:46 pm
Subject: Re: [R] Robust nonlinear regression - sin(x)/x?

You reall have only one parameter in your model, c = a/b. You 
can't 
identify both a and b from your model, therefore, you should fit 
the 
linear model:  lm(z ~ c* sin(x)/x)

Ravi.

----- Original Message -----
From: cstrato <cstrato at aon.at>
Date: Monday, February 2, 2004 2:28 pm
Subject: [R] Robust nonlinear regression - sin(x)/x?

Dear all

Since I did not receive any answer to my general question (?),
let me ask a concrete question:

How can I fit the simple function y = a*sin(x)/b*x?

This is the code that I tried, but nls gives an error:

x <- seq(1,10,0.1)
y <- sin(x)/x
plot(x,y)
z <- jitter(y,amount=0.1)
plot(x,z)
df <- as.data.frame(cbind(x,z))
nf <- nls(z ~ a*sin(x)/b*x, data=df,
         start=list(a=0.8,b=0.9), trace = TRUE)

I have followed the Puromycin sample which works fine:
Pur.wt <- nls(rate ~ (Vm * conc)/(K + conc), data = Treated,
             start = list(Vm = 200, K = 0.1), trace = TRUE)

Do I make some mistake or is it not possible to fit sin(x)/x?

Thank you in advance
Best regards
Christian
_._._._._._._._._._._._._._._._
C.h.i.s.t.i.a.n S.t.r.a.t.o.w.a
V.i.e.n.n.a       A.u.s.t.r.i.a
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cstrato wrote:

Dear R experts

This is a general question:
Does R have functions for nonlinear robust regression,
analogous to e.g. LTS?

Searching google I have found
1, an abstract to generalize LTS for nonlinear regression
models, see: http://smealsearch.psu.edu/1509.html
2, an AD-model builder, see: http://otter-

rsch.com/admodel/cc1.html> > but no mention of R/S

Thank you in advance
Best regards
Christian
_._._._._._._._._._._._._._._._
C.h.i.s.t.i.a.n S.t.r.a.t.o.w.a
V.i.e.n.n.a       A.u.s.t.r.i.a
_._._._._._._._._._._._._._._._

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