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Manifolds

While the interface ManifoldsBase.jl does not cover concrete manifolds, it provides a few helpers to build or create manifolds based on existing manifolds

A default manifold

DefaultManifold is a simplified version of Euclidean and demonstrates a basic interface implementation. It can be used to perform simple tests. Since when using Manifolds.jl the Euclidean is available, the DefaultManifold itself is not exported.

ManifoldsBase.DefaultManifold — Type
DefaultManifold <: AbstractManifold

This default manifold illustrates the main features of the interface and provides a skeleton to build one's own manifold. It is a simplified/shortened variant of Euclidean from Manifolds.jl.

This manifold further illustrates how to type your manifold points and tangent vectors. Note that the interface does not require this, but it might be handy in debugging and educative situations to verify correctness of involved variables.

Constructor

DefaultManifold(n::Int...; field = ℝ, parameter::Symbol = :field)

Arguments:

  • n: shape of array representing points on the manifold.
  • field: field over which the manifold is defined. Either ℝ, ℂ or ℍ.
  • parameter: whether a type parameter should be used to store n. By default size is stored in a field. Value can either be :field or :type.
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Embedded manifold

The embedded manifold is a manifold $\mathcal M$ which is modelled explicitly specifying its embedding $\mathcal N$ in which the points and tangent vectors are represented. Most prominently is_point and is_vector of an embedded manifold are implemented to check whether the point is a valid point in the embedding. This can of course still be extended by further tests. ManifoldsBase.jl provides two possibilities of easily introducing this in order to dispatch some functions to the embedding.

Implicit case: the IsEmbeddedManifold Trait

For the implicit case, your manifold has to be a subtype of the AbstractDecoratorManifold. Adding a method to the active_traits function for a manifold that returns an AbstractTrait IsEmbeddedManifold, makes that manifold an embedded manifold. You just have to also define get_embedding so that appropriate functions are passed on to that embedding. This is the implicit case, since the manifold type itself does not carry any information about the embedding, just the trait and the function definition do.

Explicit case: the EmbeddedManifold

The EmbeddedManifold itself is an AbstractDecoratorManifold so it is a case of the implicit embedding itself, but internally stores both the original manifold and the embedding. They are also parameters of the type. This way, an additional embedding of one manifold in another can be modelled. That is, if the manifold is implemented using the implicit embedding approach from before but can also be implemented using a different embedding, then this method should be chosen, since you can dispatch functions that you want to implement in this embedding then on the type which explicitly has the manifold and its embedding as parameters.

Hence this case should be used for any further embedding after the first or if the default implementation works without an embedding and the alternative needs one.

ManifoldsBase.EmbeddedManifold — Type
EmbeddedManifold{𝔽, MT <: AbstractManifold, NT <: AbstractManifold} <: AbstractDecoratorManifold{𝔽}

A type to represent an explicit embedding of a AbstractManifold M of type MT embedded into a manifold N of type NT. By default, an embedded manifold is set to be embedded, but neither isometrically embedded nor a submanifold.

Note

This type is not required if a manifold M is to be embedded in one specific manifold N. One can then just implement embed! and project!. You can further pass functions to the embedding, for example, when it is an isometric embedding, by using an AbstractDecoratorManifold. Only for a second –maybe considered non-default– embedding, this type should be considered in order to dispatch on different embed and project methods for different embeddings N.

Fields

  • manifold the manifold that is an embedded manifold
  • embedding a second manifold, the first one is embedded into

Constructor

EmbeddedManifold(M, N)

Generate the EmbeddedManifold of the AbstractManifold M into the AbstractManifold N.

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ManifoldsBase.decorated_manifold — Method
decorated_manifold(M::EmbeddedManifold, d::Val{N} = Val(-1))

Return the manifold of M that is decorated with its embedding. For this specific type the internally stored enhanced manifold M.manifold is returned.

See also base_manifold, where this is used to (potentially) completely undecorate the manifold.

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Metrics

Most metric-related functionality is currently defined in Manifolds.jl but a few basic types are defined here.

ManifoldsBase.AbstractMetric — Type
AbstractMetric

Abstract type for the pseudo-Riemannian metric tensor $g$, a family of smoothly varying inner products on the tangent space. See inner.

Functor

(metric::Metric)(M::AbstractManifold)
(metric::Metric)(M::MetricManifold)

Generate the MetricManifold that wraps the manifold M with given metric. This works for both a variable containing the metric as well as a subtype T<:AbstractMetric, where a zero parameter constructor T() is availabe. If M is already a metric manifold, the inner manifold with the new metric is returned.

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ManifoldsBase.EuclideanMetric — Type
EuclideanMetric <: RiemannianMetric

A general type for any manifold that employs the Euclidean Metric, for example the Euclidean manifold itself, or the Sphere, where every tangent space (as a plane in the embedding) uses this metric (in the embedding).

Since the metric is independent of the field type, this metric is also used for the Hermitian metrics, i.e. metrics that are analogous to the EuclideanMetric but where the field type of the manifold is ℂ.

This metric is the default metric for example for the Euclidean manifold.

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ManifoldsBase.RiemannianMetric — Type
RiemannianMetric <: AbstractMetric

Abstract type for Riemannian metrics, a family of positive definite inner products. The positive definite property means that for $X ∈ T_p \mathcal M$, the inner product $g(X, X) > 0$ whenever $X$ is not the zero vector.

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ManifoldsBase.change_metric — Method
change_metric(M::AbstractcManifold, G2::AbstractMetric, p, X)

On the AbstractManifold M with implicitly given metric $g_1$ and a second AbstractMetric $g_2$ this function performs a change of metric in the sense that it returns the tangent vector $Z=BX$ such that the linear map $B$ fulfills

\[g_2(Y_1,Y_2) = g_1(BY_1,BY_2) \quad \text{for all } Y_1, Y_2 ∈ T_p\mathcal M.\]

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ManifoldsBase.change_representer — Method
change_representer(M::AbstractManifold, G2::AbstractMetric, p, X)

Convert the representer X of a linear function (in other words a cotangent vector at p) in the tangent space at p on the AbstractManifold M given with respect to the AbstractMetric G2 into the representer with respect to the (implicit) metric of M.

In order to convert X into the representer with respect to the (implicitly given) metric $g_1$ of M, we have to find the conversion function $c: T_p\mathcal M \to T_p\mathcal M$ such that

\[ g_2(X,Y) = g_1(c(X),Y)\]

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A manifold for validation

ValidationManifold is a simple decorator using the AbstractDecoratorManifold that “decorates” a manifold with tests that all involved points and vectors are valid for the wrapped manifold. For example involved input and output paratemers are checked before and after running a function, repectively. This is done by calling is_point or is_vector whenever applicable.

ManifoldsBase.ValidationFibreVector — Type
ValidationFibreVector{TType<:VectorSpaceType,V,P} <: AbstractFibreVector{TType}

Represent a tangent vector to a point on an ValidationManifold. The original vector of the manifold is stored internally. The corresponding base point of the fibre can be stored as well.

The TType indicates the type of fibre, for example TangentSpaceType or CotangentSpaceType.

Fields

  • value::V: the internally stored vector on the fibre
  • point::P: the point the vector is associated with

Constructor

    ValidationFibreVector{TType}(value, point=nothing)
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ManifoldsBase.ValidationMPoint — Type
ValidationMPoint{P} <: AbstractManifoldPoint

Represent a point on an ValidationManifold. The point is stored internally.

Fields

  • value::P: the internally stored point on a manifold

Constructor

    ValidationMPoint(value)

Create a point on the manifold with the value value.

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ManifoldsBase.ValidationManifold — Type
ValidationManifold{𝔽,M<:AbstractManifold{𝔽}} <: AbstractDecoratorManifold{𝔽}

A manifold to add tests to input and output values of functions defined in the interface.

Additionally the points and tangent vectors can also be encapsulated, cf. ValidationMPoint, ValidationTVector, and ValidationCoTVector. These types can be used to see where some data is assumed to be from, when working on manifolds where both points and tangent vectors are represented as (plain) arrays.

Using the ignore_contexts keyword allows to specify a single Symbol or a vector of Symbols Of which contexts to ignore.

Current contexts are

  • :All: disable all checks
  • :Point: checks for points
  • :Vector: checks for vectors
  • :Output: checks for output
  • :Input: checks for input variables

Using the ignore_functions keyword (dictionary) allows to disable/ignore certain checks within single functions for this manifold. The key of the dictionary has to be the Function to exclude something in. The value is either a single symbol or a vector of symbols with the same meaning as the ignore_contexts keyword, but limited to this function

Examples

  • exp => :All disables all checks in the exp function
  • exp => :Point excludes point checks in the exp function
  • exp => [:Point, :Vector] excludes point and vector checks in the exp function

This manifold is a decorator for a manifold, i.e. it decorates a AbstractManifold M with types points, vectors, and covectors.

Fields

  • manifold::M: The manifold to be decorated
  • mode::Symbol: The mode to be used for error handling, either :error or :warn
  • ignore_contexts::AbstractVector{Symbol}: store contexts to be ignored of validation.
  • ignore_functions::Dict{<:Function,<:Union{Symbol,<:AbstractVector{Symbol}}: store contexts to be ignored with in a function or its mutating variant.

Constructors

ValidationManifold(M::AbstractManifold; kwargs...)

Generate the Validation manifold

ValidationManifold(M::AbstractManifold, V::ValidationManifold; kwargs...)

Generate the Validation manifold for M with the default values of V.

Keyword arguments

  • error::Symbol=:error: specify how errors in the validation should be reported. this is passed to is_point and is_vector as the error keyword argument. Available values are :error, :warn, :info, and :none. Every other value is treated as :none.
  • store_base_point::Bool=false: specify whether or not to store the point p a tangent or cotangent vector is associated with. This can be useful for debugging purposes.
  • ignore_contexts = Vector{Symbol}() a vector to indicate which validation contexts should not be performed.
  • ignore_functions=Dict{Function,Union{Symbol,Vector{Symbol}}}() a dictionary to disable certain contexts within functions. The key here is the non-mutating function variant (if it exists). The contexts are thre same as in ignore_contexts.
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ManifoldsBase._msg — Method
_msg(str; error=:None, within::Union{Nothing,<:Function} = nothing,
context::Union{NTuple{N,Symbol} where N} = NTuple{0,Symbol}())

issue a message str according to the mode mode (as @error, @warn, @info).

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ManifoldsBase._vMc — Function
_vMc(M::ValidationManifold, f::Function, context::Symbol)
_vMc(M::ValidationManifold, f::Function, context::NTuple{N,Symbol}) where {N}

Return whether a check should be performed within f and the context(s) provided.

This function returns false and hence indicates not to check, when

  • (one of the) context(s) is in the ignore list for f within ignore_functions
  • (one of the) context(s) is in the ignore_contexts list

Otherwise the test is active.

!!! Note This function is internal and used very often, co it has a very short name; _vMc stands for "ValidationManifold check".

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ManifoldsBase.is_point — Method
is_point(M::ValidationManifold, p; kwargs...)

Perform is_point on a ValidationManifold, where two additional keywords can be used

  • within=nothing to specify a function from within which this call was issued
  • context::NTuple{N,Symbol} where N=() to specify one or more contexts, this call was issued in. The context :Point is added before checking whether the test should be performed

all other keywords are passed on.

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ManifoldsBase.is_vector — Function
is_vector(M::ValidationManifold, p, X, cbp=true; kwargs...)

perform is_vector on a ValidationManifold, where two additional keywords can be used

  • within=nothing to specify a function from within which this call was issued
  • context::NTuple{N,Symbol} where N=() to specify one or more contexts, this call was issued in. The context :Point is added before checking whether the test should be performed

all other keywords are passed on.

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