[R] error in function: nls (urgent)

[tedzzx]

Hi,all:
I am running a nonlinear regression and there is a problem.
There is a data frame: data
     p       s     x          t
1  875.0 12392.5 11600 0.06967213
2  615.0 12332.5 12000 0.06967213
3  595.0 12332.5 12000 0.06967213
4  592.5 12337.0 12000 0.06967213
5  650.0 12430.0 12000 0.06967213
6  715.0 12477.5 12000 0.06967213
.
.
.
.
str(data):
'data.frame':	234 obs. of  4 variables:
 $ p: num  875 615 595 592 650 ...
 $ s: num  12392 12332 12332 12337 12430 ...
 $ x: num  11600 12000 12000 12000 12000 12000 12200 12200 12200 12200 ...
 $ t: num  0.0697 0.0697 0.0697 0.0697 0.0697 ...

My code to estimate a0,a1,a2:

bs<-function(s,x,t,a0,a1,a2){
	v=exp(a0+a1*(x/s)+a2*(x/s)^2)
	d1=(log(s/x)+(v^2)*t/2)/(v*sqrt(t))
	d2=(log(s/x)-(v^2)*t/2)/(v*sqrt(t))
	s*pnorm(d1)-x*pnorm(d2)
}
res<-nls(p~bs(s,x,t,a0,a1,a2),data=data,start=list(a0=-100,a1=100,a2=-100))

It returns the error:
Error in nlsModel(formula, mf, start, wts) : 
  singular gradient matrix at initial parameter estimates

By the way, bs function actually based on the Black-sholes option pricing
model. There is a specific function in Rmetrics, can I call the function in
Rmetrics package? Something like this:

bs<-function(s,x,t,a0,a1,a2,...){
	v=exp(a0+a1*(x/s)+a2*(x/s)^2)
	GBSOption(TypeFlag=c("c"),S=s,X=x,Time=t,r=0,b=0,sigma=v)@price
}

When I use this one, it returns the error similar to the upper one.
Please help me, many thanks.

Ted

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