5.6 Determine whether the following models are stationary and/or invertible and explain your answers.
aX,-1.6X-1+X-2=(1-.9B
a bX,-2.6X,-+1.5X,-2-.38X,-3-.72X,-4=a,-1.9a-1+1.7a-2-.72a-3
cX,=a,-2.9a,-1+2.7a,-2-1.52a-3
dX,-1.9X,-1+2.5X,-2-2.24X,-3+1.36X,-4-.576X,-s=a,-.9a,-1