# Exponential and Logarithm Mapping on Stiefel Manifold

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Feb 11, 2021, 3:59:18 PMFeb 11
to Manopt
Hello,

I've trying to use manopt to solve a cost function of the form: with x, the vectorization of X, an X matrix in the Stiefel Manifold. The mapping between the manifold and the tangent space is exponential. for this function, the gradient in the Stiefel Manifold is a 3x1 matrix, and the hessian is 3x3.

I tried using the next lines, but I get an error due to the size of the gradient

M = stiefelfactory(3, 3);
problem.M = M;
problem.hess = @(X,u)Hessf(X,u,store);

[x_opt cost_opt info] = trustregions(problem, X0, options);

Is this the correct way to use the toolbox?
Do you have any suggestions to overcome this problem?

Ana Ramirez

### Nicolas Boumal

Feb 12, 2021, 2:04:38 AMFeb 12
to Manopt
Hello Ana,

I'm not clear on the setup. Is $r$ in $f(r)$ supposed to be $x$? Even so, I do not see what $\eta$ is. Looking at the expression for $\eta$ I'm wondering if what you actually want to optimize over is the orthogonal group?

In any case, M = stiefelmanifold(3, 3) will give you a structure M that describes the following manifold:

${ X \in R^{3 x 3} : X^T X = I_3 }$

where $I_3$ is the identity matrix of size 3 x 3.

So, points and vectors on the manifold are matrices of size 3 x 3. They are not column vectors. You can use Matlab instructions to vectorize them of course, but that is just a Matlab language issue.

Best,
Nicolas