How can i describe this matrix of the size M by M by using ''spdiags command'' ?

[John D'Errico]

What have you tried? If nothing, why not?

You clearly know what function you need to use. The diagonals are trivial to generate, since there are only two of them, and the matrix is constant down those diagonals. So why not take a shot, and then show what you tried?

[Walter Roberson]

Four diagonals, actually, considering the top right and bottom left.

Answer 1

[Walter Roberson]

Hint:

init = repmat((1:6).',1,2)+[0 10]

init = 6×2

1 11 2 12 3 13 4 14 5 15 6 16

full(spdiags(init, [-4 4], spalloc(6,6,0)))

ans = 6×6

0 0 0 0 15 0 0 0 0 0 0 16 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 2 0 0 0 0

So negative diagonals take elements from the top of the initialization matrix, and positive diagonals take elements from the bottom of the initialization matrix.