Simply use
log(u) = rho*log(u(-1))-e_u;
In any case, at order=1
the IRFs are linear, i.e. the IRF to a positive shock is just minus the IRF to a negative one.
Simply use
log(u) = rho*log(u(-1))-e_u;
In any case, at order=1
the IRFs are linear, i.e. the IRF to a positive shock is just minus the IRF to a negative one.