R for arma mdel with constraints on parameters

Hi,

i am learning R recently and find it very helpful in time series model.



In ARMA model, given (p,q) it can get the estimation of a[i] and 
b[j] easily with arima() function.

X[t] = a[1]X[t-1] + ... + a[p]X[t-p] + e[t] + b[1]e[t-1] + ... + b[q]e[t-q]

but in my recent data model, i met a problem. In the ARMA model, p 
and q are fixed, but there are some constraints in the parameters 
a[i] and b[j], such as for some i (i<p), a[i]=0. my problem is how 
to get these parameters' estimation for these constrants with 
arima() function? or i need to write functions to realize it.

Best regards,

Tongen
from Beijing