The solution to this is what is called padding! We simply define a new vector \( x \) with two added elements set to zero before \( x(0) \) and two new elements after \( x(3) \) set to zero. That is, we augment the length of \( \boldsymbol{x} \) from \( n=4 \) to \( n+2P=8 \), where \( P=2 \) is the padding constant (a new hyperparameter), see discussions below as well.