To generate a tridiagonal matrix with replicated elements down the diagonals. Sometimes its nice to generate block tridiagonal matrices of the same form.

full(blktridiag(2,-1,-1,5))

ans =

2 -1 0 0 0

-1 2 -1 0 0

0 -1 2 -1 0

0 0 -1 2 -1

0 0 0 -1 2

full(blktridiag(ones(2),2*ones(2),3*ones(2),3))

ans =

1 1 3 3 0 0

1 1 3 3 0 0

2 2 1 1 3 3

2 2 1 1 3 3

0 0 2 2 1 1

0 0 2 2 1 1

This can of course be easily extended to handle general blocks, so since I saw an interest from a user, this release does that too.

Amd = reshape(1:16,[2,2,4]);

Asub = reshape(101:112,[2,2,3]);

Asup = reshape(201:212,[2,2,3]);

A = blktridiag(Amd,Asub,Asup);

full(A)

ans =

1 3 201 203 0 0 0 0

2 4 202 204 0 0 0 0

101 103 5 7 205 207 0 0

102 104 6 8 206 208 0 0

0 0 105 107 9 11 209 211

0 0 106 108 10 12 210 212

0 0 0 0 109 111 13 15

0 0 0 0 110 112 14 16

I've also seen a request to allow non-square blocks. This too is now accommodated in the latest release.

A = blktridiag([1;2],[3;4],[5;6],4);

full(A)

ans =

1 5 0 0

2 6 0 0

3 1 5 0

4 2 6 0

0 3 1 5

0 4 2 6

0 0 3 1

0 0 4 2

http://www.mathworks.com/matlabcentral/fileexchange/10603--block--tri-diagonal-matrices

Additional Links below.

http://www.mathworks.com/matlabcentral/fileexchange/10603--block--tri-diagonal-matrices/content/BLKTRIDIAG/blktridiag.m

http://www.mathworks.com/matlabcentral/fileexchange/10603--block--tri-diagonal-matrices/content/BLKTRIDIAG/demo/html/blktridiag_demo.html

http://math.stackexchange.com/questions/763909/creating-a-tridiagonal-matrix-in-matlab

@kaspars Ozlos Thank you friend