Hello, I do not know if I am completely out of it but . . . if x,y,z is a point in a sphere and [u,v,w]'A[u,v,w] = 1 is the equation of an ellipsoid and A = T'T (cholesky) then T.[x,y,z] should be a point in the ellipsoid ? isn't it? Heberto Ghezzo McGill University Montreal Canada