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What is a good way to create a tridiagonal matrix in matlab?
How does that work?
With spdiags, you can input the diagonals of the matrix as vectors, and specify their positions relative to the main diagonal. For example, if you want to create a tridiagonal matrix with 1's on the main diagonal and -1's on the upper and lower diagonals, you can use the following code: 
>> n = 5;
>> maindiag = ones(n,1);
>> offdiag = -1*ones(n-1,1);
>> A = spdiags([offdiag maindiag offdiag], [-1 0 1], n, n)
This will create a 5x5 tridiagonal matrix A with 1's on the main diagonal and -1's on the upper and lower diagonals. Does that help?