A tridiagonal matrix is a matrix that has non-zero elements only at the main diagonal, diagonal below and above it. All other elements are zero. For this reason tridiagonal matrices of dimension smaller than or equal to 3 seem meaningless.
Example 1:
[a11, a22, 0 , 0 , 0 , 0 ]
[a21, a22, a23, 0 , 0 , 0 ]
[0 , a32, a33, a34, 0 , 0 ]
[0 , 0 , a43, a44, a55, 0 ]
[0 , 0 , 0 , a54, a55, a56]
[0 , 0 , 0 , 0 , a65, a66]
Example 2:
[1, 1, 0, 0, 0, 0]
[1, 1, 1, 0, 0, 0]
[0, 1, 1, 1, 0, 0]
[0, 0, 1, 1, 1, 0]
[0, 0, 0, 1, 1, 1]
[0, 0, 0, 0, 1, 1]