R Under development (unstable) (2023-12-07 r85661 ucrt) -- "Unsuffered Consequences"
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> library(bbmle)
Loading required package: stats4
> d <- data.frame(x=0:10,
+                 y=c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8))
> 
> maxit <- 1000
> mle2(y~dpois(lambda=exp(lymax)/(1+x/exp(lhalf))),
+    start=list(lymax=0,lhalf=0),
+    data=d,
+      control=list(maxit=maxit),
+    parameters=list(lymax~1,lhalf~1))

Call:
mle2(minuslogl = y ~ dpois(lambda = exp(lymax)/(1 + x/exp(lhalf))), 
    start = list(lymax = 0, lhalf = 0), data = d, parameters = list(lymax ~ 
        1, lhalf ~ 1), control = list(maxit = maxit))

Coefficients:
   lymax    lhalf 
3.218853 1.117035 

Log-likelihood: -28.6 
> 
> proc.time()
   user  system elapsed 
   1.28    0.14    1.40