[R] iteration scheme

[Troels Ring]

Dear friends.
On p 95 in 3. ed. MASS a zero-truncated Poisson distribution is analyzed. I 
understand the probability distribution and expected mean. The Newton 
iteration scheme is
Lam(m+1)=Lam(m)-[Lam(m)-Ybar(1-exp(-Lam(m))]/[1-Ybar*exp(-Lam(m)], and I 
suppose the latter part should be f(Lam(m))/f ' (Lam(m)) and f(Lam(m)) is 
Lam(m)/(1-exp(-Lam(m)), right ? But then
f ' (Lam(m)) is (exp(Lam(m)*(exp(lam(m)-Lam(m)-1)/(exp(lam(m)-1)^2 and the 
iterations scheme is not right ? So I got this wrong. If possible without 
too much trouble, please tell me how this is done.
I did check that the formula works OK and leads to the right answer.

Best wishes

Troels

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