# Manifolds

Available geometries and how they are built

## General description

Manifolds in Manopt are represented as structures and are obtained by calling a factory. Such a structure contains a collection of functions that make it possible to interact with the manifold.

## Available manifolds

Manopt comes with a number of implementations for generically useful manifolds. Built-in factories are located in /manopt/manifolds.

Of course, manifolds can also be user-defined. The best way to build your own is probably to read the code of some of the standard factories and to adapt what needs to be changed. If you develop an interesting manifold factory and would like to share it, let us know.

Bear in mind that a manifold can be turned into a Riemannian manifold in many different ways, by choosing one or another metric. Which metric is best for a specific application may vary: experiments will tell.

### Linear and affine subspaces

• Euclidean space (real and complex vectors, matrices, tensors, …)
• $$\mathbb{R}^{n}$$
euclideanfactory(n)
• $$\mathbb{R}^{m \times n}$$
euclideanfactory(m, n)
euclideanlargefactory(m, n) (to handle high dimensions)
• $$\mathbb{C}^{n}$$
euclideancomplexfactory(n)
• $$\mathbb{C}^{m \times n}$$
euclideancomplexfactory(m, n)
• Symmetric matrices
• $$\{ X \in \mathbb{R}^{n \times n} : X = X^\top\}^k$$
symmetricfactory(n, k)
• Skew-symmetric matrices
• $$\{ X \in \mathbb{R}^{n \times n} : X + X^\top = 0\}^k$$
skewsymmetricfactory(n, k)
• Centered matrices
• $$\{ X \in \mathbb{R}^{m \times n} : X\mathbf{1}_n = 0_m \}$$
centeredmatrixfactory(m, n)
• Sparse matrices with fixed sparsity pattern
• $$\{ X \in \mathbb{R}^{m \times n} : X_{ij} = 0 \textrm{ if } A_{ij} = 0 \}$$
euclideansparsefactory(A) (inefficient implementation)
• Linear subspace of a linear space
• $$\{ x \in E : x = \mathrm{proj}(x) \}$$ where $$E$$ is a linear space (from a factory) and $$\mathrm{proj}$$ is an orthogonal projector to the subspace.
euclideansubspacefactory(E, proj, dim)

### Spheres, unit norm and unit modulus

• Sphere
• $$\{x\in\mathbb{R}^{n} : \|x\|_2 = 1\}$$
spherefactory(n) (unit 2-norm)
• $$\{X\in\mathbb{R}^{n\times m} : \|X\|_\mathrm{F} = 1\}$$
spherefactory(n, m) (unit Frobenius norm)
• $$\{x\in\mathbb{C}^{n} : \|x\|_2 = 1\}$$
spherecomplexfactory(n) (unit 2-norm)
• $$\{X\in\mathbb{C}^{n\times m} : \|X\|_\mathrm{F} = 1\}$$
spherecomplexfactory(n, m) (unit Frobenius norm)
• Sphere of symmetric matrices
• $$\{X\in\mathbb{R}^{n\times n} : \|X\|_\mathrm{F} = 1 \textrm{ and } X = X^\top\}$$
spheresymmetricfactory(n)
• Oblique manifold (product of spheres)
• $$\{X\in\mathbb{R}^{n\times m} : \|X_{:1}\| = \cdots = \|X_{:m}\| = 1\}$$
obliquefactory(n, m) ($$m$$ normed columns in $$\mathbb{R}^n$$)
• $$\{X\in\mathbb{R}^{n\times m} : \|X_{1:}\| = \cdots = \|X_{n:}\| = 1\}$$
obliquefactory(n, m, 'rows') ($$n$$ normed rows in $$\mathbb{R}^m$$)
• $$\{X\in\mathbb{C}^{n\times m} : \|X_{:1}\| = \cdots = \|X_{:m}\| = 1\}$$
obliquecomplexfactory(n, m) ($$m$$ normed columns in $$\mathbb{C}^n$$)
• $$\{X\in\mathbb{C}^{n\times m} : \|X_{1:}\| = \cdots = \|X_{n:}\| = 1\}$$
obliquecomplexfactory(n, m, 'rows') ($$n$$ normed rows in $$\mathbb{C}^m$$)
• Complex circle, phases
• $$\{z\in\mathbb{C}^n : |z_1| = \cdots = |z_n| = 1\}$$
complexcirclefactory(n)
• Phases of discrete Fourier transform (DFT) of a real vector
• $$\{z\in\mathbb{C}^n : |z_k| = 1, z_{1+\operatorname{mod}(k, n)} = \bar{z}_{1+\operatorname{mod}(n-k, n)} \ \forall k\}$$
realphasefactory(n)

### Stiefel, Grassmann and rotations

• Stiefel manifold
• $$\{X \in \mathbb{R}^{n \times p} : X^\top X = I_p\}^k$$
stiefelfactory(n, p, k)
• $$\{X \in \mathbb{C}^{n \times p} : X^*X = I_p\}^k$$
stiefelcomplexfactory(n, p, k)
• Generalized Stiefel manifold
• $$\{X \in \mathbb{R}^{n \times p} : X^\top BX = I_p\}$$ for some $$B \succ 0$$
stiefelgeneralizedfactory(n, p, B)
• Stiefel manifold, stacked
• $$\{X \in \mathbb{R}^{md \times k} : (XX^\top)_{ii} = I_d\}$$
stiefelstackedfactory(m, d, k)
• Rotation group
• $$\{R \in \mathbb{R}^{n \times n} : R^\top R = I_n, \det(R) = 1\}^k$$
rotationsfactory(n, k)
See the dedicated documentation page.
• Special Euclidean group (rigid motion)
• $$\{ (R, t) \in \mathbb{R}^{n \times n} \times \mathbb{R}^n : R^\top R = I_n, \det(R) = 1 \}^k$$
specialeuclideanfactory(n, k)
• Unitary matrices
• $$\{ U \in \mathbb{C}^{n \times n} : U^*U = I_n \}^k$$
unitaryfactory(n, k)
• Grassmann manifold
• $$\{\operatorname{span}(X) : X \in \mathbb{R}^{n \times p}, X^\top X = I_p\}^k$$
grassmannfactory(n, p, k)
• $$\{\operatorname{span}(X) : X \in \mathbb{C}^{n \times p}, X^*X = I_p\}^k$$
grassmanncomplexfactory(n, p, k)
• Generalized Grassmann manifold
• $$\{\operatorname{span}(X) : X \in \mathbb{R}^{n \times p}, X^\top BX = I_p\}$$ for some $$B \succ 0$$
grassmannfactory(n, p, B)

### Rank constraints

• Fixed-rank matrices
• $$\{X \in \mathbb{R}^{m \times n} : \operatorname{rank}(X) = r\}$$
fixedrankembeddedfactory(m, n, r)
fixedrankfactory_2factors(m, n, r)
fixedrankfactory_2factors_preconditioned(m, n, r)
fixedrankfactory_2factors_subspace_projection(m, n, r)
fixedrankfactory_3factors(m, n, r)
fixedrankMNquotientfactory(m, n, r)
• Bounded-rank matrices
• $$\{X \in \mathbb{R}^{m \times n} : \operatorname{rank}(X) \leq r\}$$
desingularizationfactory(m, n, r) (see also lifts)
• Fixed-rank tensors, Tucker
• Tensors of fixed multilinear rank in Tucker format
fixedranktensorembeddedfactory
fixedrankfactory_tucker_preconditioned
• Fixed-rank tensors, tensor train
• Tensors of fixed tensor train rank in TT format
fixedTTrankfactory

See more rank-constrained manifolds in the category Positivity constraints.

### Positivity constraints

The positivity constraints here are strict ($$x > 0$$). See also lifts for nonnegativity constraints ($$x \geq 0$$).

• Matrices with strictly positive entries
• $$\{ X \in \mathbb{R}^{m\times n} : X_{ij} > 0 \ \forall i, j\}$$
positivefactory(m, n)
• Symmetric, positive definite matrices
• $$\{ X \in \mathbb{R}^{n\times n} : X = X^\top \textrm{ and } X \succ 0\}$$
sympositivedefinitefactory(n)
• Symmetric positive semidefinite, fixed-rank
• $$\{X \in \mathbb{R}^{n \times n} : X = X^\top \succeq 0 \textrm{ and } \operatorname{rank}(X) = k\}$$
symfixedrankYYfactory(n, k)
• $$\{X \in \mathbb{C}^{n \times n} : X = X^* \succeq 0 \textrm{ and } \operatorname{rank}(X) = k\}$$
symfixedrankYYcomplexfactory(n, k)
• Symmetric positive semidefinite, fixed-rank with unit diagonal
• $$\{X \in \mathbb{R}^{n \times n} : X = X^\top \succeq 0, \operatorname{rank}(X) = k, \operatorname{diag}(X) = 1\}$$
elliptopefactory(n, k)
• Symmetric positive semidefinite, fixed-rank with unit trace
• $$\{X \in \mathbb{R}^{n \times n} : X = X^\top \succeq 0, \operatorname{rank}(X) = k, \operatorname{trace}(X) = 1\}$$
spectrahedronfactory(n, k)
• Multinomial manifold (strict simplex elements)
• $$\{ X \in \mathbb{R}^{n\times m} : X_{ij} > 0 \ \forall i,j \textrm{ and } X^\top \mathbf{1}_m = \mathbf{1}_n \}$$
multinomialfactory(n, m)
• Multinomial doubly stochastic manifold
• $$\{ X \in \mathbb{R}^{n\times n} : X_{ij} > 0 \ \forall i,j \textrm{ and } X \mathbf{1}_n = \mathbf{1}_n, X^\top \mathbf{1}_n = \mathbf{1}_n \}$$
multinomialdoublystochasticfactory(n)
• Multinomial symmetric and stochastic manifold
• $$\{ X \in \mathbb{R}^{n\times n} : X_{ij} > 0 \ \forall i,j \textrm{ and } X \mathbf{1}_n = \mathbf{1}_n, X = X^\top \}$$
multinomialsymmetricfactory(n)
• Positive definite simplex
• $$\{ (X_1, \ldots, X_k) \in (\mathbb{R}^{n \times n})^k : X_i \succ 0 \ \forall i \textrm{ and } X_1 + \cdots + X_k = I_n \}$$
sympositivedefinitesimplexfactory(n, k)
• $$\{ (X_1, \ldots, X_k) \in (\mathbb{C}^{n \times n})^k : X_i \succ 0 \ \forall i \textrm{ and } X_1 + \cdots + X_k = I_n \}$$
sympositivedefinitesimplexcomplexfactory(n, k)

### Miscellaneous

• Constant manifold (singleton)
• $$\{ A \}$$
constantfactory(A)
This is convenient with productmanifold, to fix some variables.
• Hyperbolic manifold
• $$\{ x \in \mathbb{R}^{n+1} : x_0^2 = x_1^2 + \cdots + x_n^2 + 1 \}^m$$ with Minkowski metric
hyperbolicfactory(n, m)
• Poincaré ball
• $$\{ x \in \mathbb{R}^{k} : x^\top x < 1 \}^n$$ with Poincaré metric
poincareballfactory(k, n)
• Essential manifold
• Epipolar constraint between projected points in two perspective views
essentialfactory(k, 'signed') (or unsigned)

## Product manifolds

The tools productmanifold and powermanifold make it easy to work on products of existing manifolds.

For example, if you are minimizing a function $$f(X, Y)$$ where $$X$$ is a $$3 \times 3$$ matrix with unit Frobenius norm and $$Y$$ is $$4 \times 2$$ with orthonormal columns, then use productmanifold as follows:

elements.X = spherefactory(3, 3);
elements.Y = stiefelfactory(4, 2);
M = productmanifold(elements);

Points on M are encoded as structures with fields X and Y. Likewise for tangent vectors and vectors in the embedding space.

If you are minimizing a function $$f(X_1, \ldots, X_7)$$ where each $$X_i$$ is a $$3 \times 4$$ matrix of rank $$2$$ (the same manifold for each), then use powermanifold as follows:

N = fixedrankembeddedfactory(3, 4, 2);
M = powermanifold(N, 7)

Points on M are encoded as cells with $$7$$ entries. Likewise for tangent vectors and vectors in the embedding space.

Of course, calls to productmanifold and powermanifold can be composed.

However, bear in mind that these tools are provided mostly for convenience. They allow fast prototyping, but it may be possible to write a more efficient factory for your specific product manifold. For example, obliquefactory(n, m) encodes the same geometry as powermanifold(spherefactory(n), m), but the former is more efficient than the latter.

If a factory offers built-in support for products, it is highly recommend to prefer that over the tools here. For example, stiefelfactory(n, p, k) encodes the same geometry as powermanifold(stiefelfactory(n, p), k), but the former is more efficient than the latter.

Here is a complete example showing how to solve $$\min_{x \in \Rm, y \in \Rn} x\transpose Ay$$ subject to $$\|x\|=1$$ and $$\|y\|=1$$ for some given $$A \in \Rmn$$. Note that the optimal value is the negative of the largest singular value of $$A$$. The Euclidean gradient of $$f(x, y) = x\transpose A y$$ is $$\grad f(x, y) = (Ay, A\transpose x)$$. The Euclidean Hessian is $$\Hess f(x, y)[\dot x, \dot y] = (A\dot y, A\transpose \dot x)$$.

m = 5;
n = 10;
A = randn(m, n);

% Create the manifold M as a product of two spheres (in R^m and R^n).
elems.x = spherefactory(m);
elems.y = spherefactory(n);
manifold = productmanifold(elems);

% The cost function is defined on the product manifold M.
% Thus, a point xy is a structure with two fields: x and y.
% Likewise, tangent vectors are structures with fields x and y.
problem.M = manifold;
problem.cost = @(xy) xy.x'*A*xy.y;
problem.egrad = @(xy) struct('x', A*xy.y, 'y', A'*xy.x);
problem.ehess = @(xy, xydot) struct('x', A*xydot.y, 'y', A'*xydot.x);

xy = trustregions(problem);

fprintf('Compare these values: %g, %g.\n', ...
getCost(problem, xy), -max(svd(A)));

## Manifold structures: what’s in them?

The documentation for a manifold factory should specify

• How points on the manifold are represented numerically,
• How tangent vectors to the manifold are represented numerically,
• And likewise for the embedding space (submanifolds) or total space (quotient manifolds).

Typically, points and tangent vector are represented by matrices, but they could be represented by structures, cells, etc.: there are no limitations. They can even be represented by data on a GPU.

Based on these, the implementation of a Riemannian manifold is a collection of functions. These functions are collected into a structure: that is what a factory returns as output.

These functions are listed below. Not all factories populate all of these fields, and that’s okay: for many purposes, only a subset of these functions are necessary.

Notice that it is possible to add or replace fields in a manifold structure after it was returned by a factory. Thus, one can easily experiment with various retractions, transporters, etc. If you find ways to improve the built-in geometries, let us know.

Manifolds are encoded as structures which contain fields. Most of these fields are function handles we use to interact with the manifold.
Name Field usage Functionality
Name M.name() Name of the manifold as a string.
Dimension M.dim() Dimension of the manifold.
Metric M.inner(x, u, v) Riemannian metric $$\inner{u}{v}_x.$$
Norm M.norm(x, u) Riemannian norm $$\|u\|_x = \sqrt{\inner{u}{u}_x}.$$
Distance M.dist(x, y) Riemannian distance $$\operatorname{dist}(x, y)$$.
Typical distance M.typicaldist() “Scale” of the manifold. This is used by the trust-regions solver for example, to determine default initial and maximal trust-region radii.
Tangent space projector M.proj(x, u) For manifolds embedded in a Euclidean space, computes the orthogonal projection $$\operatorname{Proj}_x u$$ of the vector $$u$$ from the embedding space to the tangent space at $$x$$. If M is a quotient manifold, then the projection is from the embedding space of the total space to the horizontal space at $$x.$$
Euclidean to Riemannian gradient M.egrad2rgrad(x, egrad) For manifolds embedded in a Euclidean space, converts the gradient of $$f$$ at $$x$$ seen as a function in that Euclidean space to the Riemannian gradient of $$f$$ on the manifold. Allowed to take (storedb, key) as input for caching, but must also allow to be called without it.
Euclidean to Riemannian Hessian M.ehess2rhess(x, egrad, ehess, u) Similarly to egrad2rgrad, converts the Euclidean gradient and Hessian of $$f$$ at $$x$$ along a tangent vector $$u$$ to the Riemannian Hessian of $$f$$ at $$x$$ along $$u$$ on the manifold. Allowed to take (storedb, key) as input for caching, but must also allow to be called without it. Mind the warning below.
Tangentialize M.tangent(x, u) Re-tangentializes a vector. The input is a vector in the tangent vector representation, which possibly (for example because of error accumulations) is no longer tangent. The output is the “closest” tangent vector to the input. If tangent vectors are represented in the ambient space, this is equivalent to proj.
Tangent to ambient representation M.tangent2ambient(x, u) Tangent vectors are sometimes represented differently from their counterpart in the ambient space. This function returns the ambient space representation of a tangent vector $$u$$. Useful when defining the Euclidean Hessian ehess for example. If missing, assumed to be identity. Should have M.tangent2ambient_is_identity = false if it is not, to enable diagnostics and proper AD. Read more here.
Exponential map M.exp(x, u, t) Computes $$\operatorname{Exp}_x(tu)$$, the point you reach by following the geodesic with initial velocity $$u$$ scaled by $$t$$ starting at $$x$$. This field should only exist if the manifold provides a genuine exponential map. If missing, manually fall back to M.retr2 or M.retr.
Retraction M.retr(x, u, t) Computes $$\Retr_x(tu)$$, where $$\Retr$$ is a retraction: a cheaper proxy for the exponential map.
⤷ second order M.retr2(x, u, t) A second-order retraction. May be the same as M.retr.
Logarithmic map M.log(x, y) Computes $$\operatorname{Log}_x(y)$$, a tangent vector at $$x$$ pointing toward $$y$$. This is meant to be an inverse of $$\operatorname{Exp}$$.
Inverse retraction M.invretr(x, y) Computes an inverse of the retraction: a tangent vector at $$x$$ pointing toward $$y$$.
Random point M.rand() Computes a random point on the manifold.
Random vector M.randvec(x) Computes a random, unit-norm tangent vector in the tangent space at $$x$$.
Zero vector M.zerovec(x) Returns the zero tangent vector at $$x$$.
Linear combination M.lincomb(x, a1, u1, a2, u2) Computes $$v = a_1 u_1 + a_2 u_2$$, where $$a_1, a_2$$ are scalars and $$u_1, u_2$$ are tangent vectors at $$x$$. The inputs $$a_2, u_2$$ are optional.
Transporter M.transp(x, y, u) Computes a tangent vector at $$y$$ that “looks like” the tangent vector $$u$$ at $$x$$. This is closely related to parallel transports and vector transports.
Isometric transporter M.isotransp(x, y, u) An isometric transporter. May be the same as M.transp. This is not necessarily parallel transport.
Parallel transport M.paralleltransp(x, y, u) The parallel transport. May be the same as M.transp or M.isotransp. It is always isometric.
Pair mean M.pairmean(x, y) Computes the intrinsic mean of $$x$$ and $$y$$, that is, a point that lies mid-way between $$x$$ and $$y$$ on the geodesic arc joining them.
Hashing function M.hash(x) Computes a string that (almost) uniquely identifies the point $$x$$ and that can serve as a field name for a structure. (No longer used as of Manopt 2.0.)
Vector representation M.vec(x, u) Returns a real column-vector representation of the tangent vector $$u$$. The length of the output is always the same and at least M.dim(). This function is linear and invertible in $$u$$ on the tangent space at $$x$$.
Normal representation M.mat(x, u_vec) The inverse of the vec function: returns a tangent vector representation from a column vector such that M.mat(x, M.vec(x, u)) = u.
Isometry check for vec and mat M.vecmatareisometries() Returns true if M.vec is a linear isometry, i.e., if for all tangent vectors $$u,v$$, M.inner(x, u, v) == M.vec(x, u).'*M.vec(x, v). Then, M.mat is both the adjoint and the inverse of M.vec (on the tangent space).
Lie group identity M.lie_identity() If the manifold is a Lie group, this function returns the identity element (e.g., for the rotation group in $$\mathbb{R}^d$$, that would be the identity matrix of size $$d$$).
Point check M.offmanifold(x) Outputs a positive double that is machine-precision zero if x is a valid point on M, further away from zero if x lacks accuracy, or Inf if it is not even in the right format. See tool offmanifold.
Tangent check M.offtangent(x, v) Outputs a positive double that is machine-precision zero if v is a valid tangent vector to M at x, further away from zero if v lacks accuracy, or Inf if x, v are not even in the right format. See tool offtangent.
Mind the details for ehess2rhess

Consider a Riemannian manifold M embedded in a Euclidean space E. In the call pattern

rhess = M.ehess2rhess(x, egrad, ehess, u)

the vectors egrad and ehess are vectors in the embedding space E, whereas the vectors u and rhess are tangent vectors to M at x. It is important to use the corresponding numerical representations, as documented by the manifold’s factory.

For factories which use the same numerical representation for tangent vectors and vectors in the embedding space (e.g., spherefactory), this causes no difficulties. However, for factories such as rotationsfactory, the distinction matters: see this page. The functions M.proj (from embedding space to tangent space) and M.tangent2ambient (from tangent space to embedding space) help convert between representations.

If M is a quotient manifold of a manifold N (called the total space) embedded in E, then egrad and ehess are still vectors in the embedding space E but u and rhess are horizontal vectors at x. For example, grassmannfactory(n, p) is a quotient of stiefelfactory(n, p) which is embedded in $$\mathbb{R}^{n \times p}$$. Since stiefelfactory(n, p) uses the same representation for tangent vectors and vectors in the embedding space, there is no friction for that one either.