x<-c(0, 2, 4, 8, 12)
y<-c(1, 0.7, 0.4, 0.3, 0.04)
(dat<-data.frame(x, y))

(n1<-nls(y~exp(-a*x), dat, start=list(a=1)))
a<-0.1939
f1<-function(x)(exp(-a*x)) 

resid<-function(w){ 
yhat<-exp(-(w[1]*x)^w[2])
sum((y-yhat)^2)
}

(bb<-optim(c(1,1),resid))
(n2<-nls(y~exp(-(b*x)^c), dat, start=list(b=0.1939223, c=0.9872986)))

b<-0.1939223
c<-0.9872986
f2<-function(x)(exp(-(b*x)^c)) 

##plot(dat,type="l",col="blue")
##curve(f1,add=T,col="red")
##curve(f2,add=T,col="green")

AIC(n1,n2)