how to forecast seasonal factor?

[hardmann]

Dear Tom:

I want to forecast a quarterly or monthly time series. After seasonal adjustment, we get two adjusted series and seasonal factor. Using special forecast model can get forecast of adjusted series. But how to get forecast of seasonal factor, becourse my origin destination is unadjusted series.

Hardmann

[TomDoan]

The old extrapolation formula was F(t)=1.5*F(t-s)-.5*F(t-2s)