The spdiags function extracts all nonzero diagonals from the m-by-n matrix A.
So if A=[1 2 3;4 5 6;7 8 9],
spdiags(A)=[7 4 1 0 0;0 8 5 2 0;0 0 9 6 3].
But if I give you spdiags(A), can you give me A ?
% this problem is not guaranteed solvable
isequal(spdiags([eye(5) zeros(5)]),spdiags(eye(5)))