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I have some fitted ARIMA model:

> fit
Series: mydata[1] 
ARIMA(1,0,1) with non-zero mean 
Box Cox transformation: lambda= 0.1088793 

Coefficients:
         ar1      ma1     mean
      0.4536  -0.1702  -0.1868
s.e.  0.0094   0.0104   0.0033

sigma^2 estimated as 0.461:  log likelihood=-98549.78
AIC=197107.5   AICc=197107.5   BIC=197145.4

Now I want to check the prediction of this model. To do so, I want just to elongate some primer sequence

primer<-c(0.500,0.610,1.275,2.057,0.361,0.480,1.133,0.240,0.612,1.712,0.686)

Thus, I use forecast() function:

fcast <- forecast(object = primer, model = fit)

My problems are:

1) I need only one value per step, but not mean

2) I am not sure, that this syntactic construction does what I need: use prior information only from primer and produces lambda adjusted results

UPDATE:

I found, that I should use simulate function, and it will generate the sample paths conditional on the previous data. Thus I still have a question how to replace the data used for fitting by primer data?

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You can apply a previously fitted ARIMA model to new data by using the Arima() function, feeding the new data into the y parameter and the fitted model into model. (Note the capitalization; this is a different function than arima()!)

Arima(y=primer,model=fit)

Thus, you can simulate like this:

simulate(Arima(y=primer,model=fit),100)
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