[R] Robust nonlinear regression - sin(x)/x?

[Ravi Varadhan]

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
> _._._._._._._._._._._._._._._._
> 
> 
> 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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