$4.$ A tridiagonal matrix is an nxn array in which has a$[$i$][$j$]==0$ if $|$i$-$j$|>1.$ What is the maximum number of non zero elements? How can they be stored in memory sequentially? Find a formula k$=$ f$($i$,$j$)$ to store location a$[$i$][$j$]$ in k$,$ when $|$i$-$j$|<=1($you only want to store the nonzero elements$).$ Do not write code. Code would work if you were manually converting the matrix from one form to the other all at once, but you need the formula to convert otherwise.