[R] Difference between arima(1, 1, 1) for y and arima(1, 0, 1) for diff(y)

[george]

Dear all,
When I run an arima(1,1,1) on an I(1) variable, e.g. y, I get different
estimates to when I first difference the variable myself, e.g y2<-diff(y),
and then run arima(1,0,1) on y2. Shouldn't these two approaches give the
same output?
Any help will be much appreciated.
george



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