R help - May 2011 - arima residuals

Hi

I have fitted an arima model to a monthly series of length 113.

I have use

fit <- arima(x.ts, order = c(1,0,0),seas = list(order = c(1,2,1),12, method
= "CSS")

I'm a little confused because the series of residual has the first 46 values
equal to 0. From there the residuals are approximately while noise, numbers
have a mean close to 0 and a variance value.

What could be causing these results?

-- 
Luis

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