A typical transfer function model with m inputs can be represented as follows:
Yt denotes the output series
X1 to Xm denote m input series
et represents the noise series
X1, td1 indicates the series X1 is indexed by t with a d1-step lag
μ represents the mean level of the model
ϕ(B) and θ(B) represent autoregressive and moving average polynomials from an ARIMA model
ωk(B) and δk(B) represent numerator and denominator factors (or polynomials) for individual transfer functions, with k representing an index for the 1 to m individual inputs.

Help created on 9/19/2017