I am trying to write the mathematical ARIMA equation for the following - A seasonal ARIMA(1,0,2)(1,1,1) with quarterly data using two external explanatory variables aside from the explained variable(also of the same frequency) . I have an idea on how to write the ARIMA equation for a standard ARIMA model which does not involve seasonal differences and with just one explanatory variable.
This question stems from the fact that I was trying to understand how the R function arima from the stats package would work under the following command
arima(y, xreg = cbind[x1,x2],order = c(1,0,2),seasonal = c(1,1,1)), where x1 and x2 are the external regressors.