[R] exponential model with decreasing

[Komine]

Hi everybody, 
I try to do an exponential model with decreasing. In my data, there is no
data missing but there negative values. I adapted this following code whose
the origin code comes from in this forum:
Regress<-read.table("C:\\Users\\Regression.csv",sep=";",dec=",",header=TRUE) 
Regress
f <- function(x,a,b) {a * exp(b * x)} 
x<-Regress$EWT
y<-Regress$MSI7
plot(y ~ x) 
fm <- nls(y ~ f(x,a,b), data =Regress, start = c(a=1, b=1)) 
co <- coef(fm) 
curve(f(x, a=co[1], b=co[2]), add = TRUE)
But in the line 7, it blocks with the following message:
#Error in numericDeriv(form[[3L]], names(ind), env) : 
# Missing value or an infinity produced when evaluating the model
1: In min (x) found no arguments to min; Inf is returned       #it is my
translation
2: In max (x): no arguments to max;-Inf is returned
-	Please where is the problem?
-	What I can do to obtain R^2?
- The way to verify Homoscedasticity and normality is the same for linear
model?
Thank you in advance 
KOMINE 


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