F(t)           slowly changing
n   ?A(t)           < 1   small in
?    A (t)                    relation to one

It+?=aIt+b?t+F(t)                        yt:
yt+?=ayt+b?yt+F(t)                   particular
                                                   solution
The particular 
solution may 
be exponential.
"Nearly exponential"-elegant solution which has a price:
Exogenous solution,  The dependence of F(t) on the

i.e. we must ?             endogenous variables is not explicitly