type=="band"
which denotes the band structure, with precision matrix
$$
C_{i,j}=\left\{\begin{array}{ll}
0.5,&\textrm{if $\left| j-i \right|=1, i=2,...,(p-1),$}\\
0.25,&\textrm{if $\left| j-i \right|=2, i=3,...,(p-2),$}\\
1,&\textrm{if $i=j, i=1,...,p,$}\\
0,&\textrm{otherwise.}
\end{array}\right. $$