Symmetric positive definite matrices

SymmetricPositiveDefinite{N} <: AbstractDecoratorManifold{ℝ}

The manifold of symmetric positive definite matrices, i.e.

\[\mathcal P(n) = \bigl\{ p ∈ ℝ^{n × n}\ \big|\ a^\mathrm{T}pa > 0 \text{ for all } a ∈ ℝ^{n}\backslash\{0\} \bigr\}\]

The tangent space at $T_p\mathcal P(n)$ reads

\[ T_p\mathcal P(n) = \bigl\{ X \in \mathbb R^{n×n} \big|\ X=X^\mathrm{T} \bigr\},\]

i.e. the set of symmetric matrices,

Constructor

SymmetricPositiveDefinite(n)

generates the manifold $\mathcal P(n) \subset ℝ^{n × n}$

convert(::Type{AbstractMatrix}, p::SPDPoint)

return the point p as a matrix. The matrix is either stored within the SPDPoint or reconstructed from p.eigen.

rand(M::SymmetricPositiveDefinite; σ::Real=1)

Generate a random symmetric positive definite matrix on the SymmetricPositiveDefinite manifold M.

check_point(M::SymmetricPositiveDefinite, p; kwargs...)

checks, whether p is a valid point on the SymmetricPositiveDefinite M, i.e. is a matrix of size (N,N), symmetric and positive definite. The tolerance for the second to last test can be set using the kwargs....

check_vector(M::SymmetricPositiveDefinite, p, X; kwargs... )

Check whether X is a tangent vector to p on the SymmetricPositiveDefinite M, i.e. atfer check_point(M,p), X has to be of same dimension as p and a symmetric matrix, i.e. this stores tangent vetors as elements of the corresponding Lie group. The tolerance for the last test can be set using the kwargs....

injectivity_radius(M::SymmetricPositiveDefinite[, p])
injectivity_radius(M::MetricManifold{SymmetricPositiveDefinite,AffineInvariantMetric}[, p])
injectivity_radius(M::MetricManifold{SymmetricPositiveDefinite,LogCholeskyMetric}[, p])

Return the injectivity radius of the SymmetricPositiveDefinite. Since M is a Hadamard manifold with respect to the AffineInvariantMetric and the LogCholeskyMetric, the injectivity radius is globally $∞$.

is_flat(::SymmetricPositiveDefinite)

Return false. SymmetricPositiveDefinite is not a flat manifold.

manifold_dimension(M::SymmetricPositiveDefinite)

returns the dimension of SymmetricPositiveDefinite M$=\mathcal P(n), n ∈ ℕ$, i.e.

\[\dim \mathcal P(n) = \frac{n(n+1)}{2}.\]

project(M::SymmetricPositiveDefinite, p, X)

project a matrix from the embedding onto the tangent space $T_p\mathcal P(n)$ of the SymmetricPositiveDefinite matrices, i.e. the set of symmetric matrices.

representation_size(M::SymmetricPositiveDefinite)

Return the size of an array representing an element on the SymmetricPositiveDefinite manifold M, i.e. $n × n$, the size of such a symmetric positive definite matrix on $\mathcal M = \mathcal P(n)$.

zero_vector(M::SymmetricPositiveDefinite, p)

returns the zero tangent vector in the tangent space of the symmetric positive definite matrix p on the SymmetricPositiveDefinite manifold M.

AffineInvariantMetric <: AbstractMetric

The linear affine metric is the metric for symmetric positive definite matrices, that employs matrix logarithms and exponentials, which yields a linear and affine metric.

exp(M::SymmetricPositiveDefinite, p, X)
exp(M::MetricManifold{SymmetricPositiveDefinite{N},AffineInvariantMetric}, p, X)

Compute the exponential map from p with tangent vector X on the SymmetricPositiveDefinite M with its default MetricManifold having the AffineInvariantMetric. The formula reads

\[\exp_p X = p^{\frac{1}{2}}\operatorname{Exp}(p^{-\frac{1}{2}} X p^{-\frac{1}{2}})p^{\frac{1}{2}},\]

where $\operatorname{Exp}$ denotes to the matrix exponential.

log(M::SymmetricPositiveDefinite, p, q)
log(M::MetricManifold{SymmetricPositiveDefinite,AffineInvariantMetric}, p, q)

Compute the logarithmic map from p to q on the SymmetricPositiveDefinite as a MetricManifold with AffineInvariantMetric. The formula reads

\[\log_p q = p^{\frac{1}{2}}\operatorname{Log}(p^{-\frac{1}{2}}qp^{-\frac{1}{2}})p^{\frac{1}{2}},\]

where $\operatorname{Log}$ denotes to the matrix logarithm.

riemannian_Hessian(M::SymmetricPositiveDefinite, p, G, H, X)

The Riemannian Hessian can be computed as stated in Eq. (7.3) [Ngu23]. Let $\nabla f(p)$ denote the Euclidean gradient G, $\nabla^2 f(p)[X]$ the Euclidean Hessian H, and $\operatorname{sym}(X) = \frac{1}{2}\bigl(X^{\mathrm{T}}+X\bigr)$ the symmetrization operator. Then the formula reads

\[ \operatorname{Hess}f(p)[X] = p\operatorname{sym}(∇^2 f(p)[X])p + \operatorname{sym}\bigl( X\operatorname{sym}\bigl(∇ f(p)\bigr)p)\]

manifold_volume(::SymmetricPositiveDefinite)

Return volume of the SymmetricPositiveDefinite manifold, i.e. infinity.

volume_density(::SymmetricPositiveDefinite{n}, p, X) where {n}

Compute the volume density of the SymmetricPositiveDefinite manifold at p in direction X. See [CKA17], Section 6.2 for details. Note that metric in Manifolds.jl has a different scaling factor than the reference.

change_metric(M::SymmetricPositiveDefinite{n}, E::EuclideanMetric, p, X)

Given a tangent vector $X ∈ T_p\mathcal P(n)$ with respect to the EuclideanMetric g_E, this function changes into the AffineInvariantMetric (default) metric on the SymmetricPositiveDefinite M.

To be precise we are looking for $c\colon T_p\mathcal P(n) \to T_p\mathcal P(n)$ such that for all $Y,Z ∈ T_p\mathcal P(n)$` it holds

\[⟨Y,Z⟩ = \operatorname{tr}(YZ) = \operatorname{tr}(p^{-1}c(Y)p^{-1}c(Z)) = g_p(c(Z),c(Y))\]

and hence $c(X) = pX$ is computed.

change_representer(M::SymmetricPositiveDefinite, E::EuclideanMetric, p, X)

Given a tangent vector $X ∈ T_p\mathcal M$ representing a linear function on the tangent space at p with respect to the EuclideanMetric g_E, this is turned into the representer with respect to the (default) metric, the AffineInvariantMetric on the SymmetricPositiveDefinite M.

To be precise we are looking for $Z∈T_p\mathcal P(n)$ such that for all $Y∈T_p\mathcal P(n)$` it holds

\[⟨X,Y⟩ = \operatorname{tr}(XY) = \operatorname{tr}(p^{-1}Zp^{-1}Y) = g_p(Z,Y)\]

and hence $Z = pXp$.

distance(M::SymmetricPositiveDefinite, p, q)
distance(M::MetricManifold{SymmetricPositiveDefinite,AffineInvariantMetric}, p, q)

Compute the distance on the SymmetricPositiveDefinite manifold between p and q, as a MetricManifold with AffineInvariantMetric. The formula reads

\[d_{\mathcal P(n)}(p,q) = \lVert \operatorname{Log}(p^{-\frac{1}{2}}qp^{-\frac{1}{2}})\rVert_{\mathrm{F}}.,\]

where $\operatorname{Log}$ denotes the matrix logarithm and $\lVert\cdot\rVert_{\mathrm{F}}$ denotes the matrix Frobenius norm.

[Ξ,κ] = get_basis(M::SymmetricPositiveDefinite, p, B::DefaultOrthonormalBasis)
[Ξ,κ] = get_basis(M::MetricManifold{SymmetricPositiveDefinite{N},AffineInvariantMetric}, p, B::DefaultOrthonormalBasis)

Return a default ONB for the tangent space $T_p\mathcal P(n)$ of the SymmetricPositiveDefinite with respect to the AffineInvariantMetric.

\[ g_p(X,Y) = \operatorname{tr}(p^{-1} X p^{-1} Y),\]

The basis constructed here is based on the ONB for symmetric matrices constructed as follows. Let

\[\Delta_{i,j} = (a_{k,l})_{k,l=1}^n \quad \text{ with } a_{k,l} = \begin{cases} 1 & \mbox{ for } k=l \text{ if } i=j\\ \frac{1}{\sqrt{2}} & \mbox{ for } k=i, l=j \text{ or } k=j, l=i\\ 0 & \text{ else.} \end{cases}\]

which forms an ONB for the space of symmetric matrices.

We then form the ONB by

\[ \Xi_{i,j} = p^{\frac{1}{2}}\Delta_{i,j}p^{\frac{1}{2}},\qquad i=1,\ldots,n, j=i,\ldots,n.\]

[Ξ,κ] = get_basis_diagonalizing(M::SymmetricPositiveDefinite, p, B::DiagonalizingOrthonormalBasis)
[Ξ,κ] = get_basis_diagonalizing(M::MetricManifold{SymmetricPositiveDefinite{N},AffineInvariantMetric}, p, B::DiagonalizingOrthonormalBasis)

Return a orthonormal basis Ξ as a vector of tangent vectors (of length manifold_dimension of M) in the tangent space of p on the MetricManifold of SymmetricPositiveDefinite manifold M with AffineInvariantMetric that diagonalizes the curvature tensor $R(u,v)w$ with eigenvalues κ and where the direction B.frame_direction $V$ has curvature 0.

The construction is based on an ONB for the symmetric matrices similar to get_basis(::SymmetricPositiveDefinite, p, ::DefaultOrthonormalBasis just that the ONB here is build from the eigen vectors of $p^{\frac{1}{2}}Vp^{\frac{1}{2}}$.

get_coordinates(::SymmetricPositiveDefinite, p, X, ::DefaultOrthonormalBasis)

Using the basis from get_basis the coordinates with respect to this ONB can be simplified to

\[ c_k = \mathrm{tr}(p^{-\frac{1}{2}}\Delta_{i,j} X)\]

where $k$ is trhe linearized index of the $i=1,\ldots,n, j=i,\ldots,n$.

get_vector(::SymmetricPositiveDefinite, p, c, ::DefaultOrthonormalBasis)

Using the basis from get_basis the vector reconstruction with respect to this ONB can be simplified to

\[ X = p^{\frac{1}{2}} \Biggl( \sum_{i=1,j=i}^n c_k \Delta_{i,j} \Biggr) p^{\frac{1}{2}}\]

where $k$ is the linearized index of the $i=1,\ldots,n, j=i,\ldots,n$.

inner(M::SymmetricPositiveDefinite, p, X, Y)
inner(M::MetricManifold{SymmetricPositiveDefinite,AffineInvariantMetric}, p, X, Y)

Compute the inner product of X, Y in the tangent space of p on the SymmetricPositiveDefinite manifold M, as a MetricManifold with AffineInvariantMetric. The formula reads

\[g_p(X,Y) = \operatorname{tr}(p^{-1} X p^{-1} Y),\]

is_flat(::MetricManifold{ℝ,<:SymmetricPositiveDefinite,AffineInvariantMetric})

Return false. SymmetricPositiveDefinite with AffineInvariantMetric is not a flat manifold.

parallel_transport_to(M::SymmetricPositiveDefinite, p, X, q)
parallel_transport_to(M::MetricManifold{SymmetricPositiveDefinite,AffineInvariantMetric}, p, X, y)

Compute the parallel transport of X from the tangent space at p to the tangent space at q on the SymmetricPositiveDefinite as a MetricManifold with the AffineInvariantMetric. The formula reads

\[\mathcal P_{q←p}X = p^{\frac{1}{2}} \operatorname{Exp}\bigl( \frac{1}{2}p^{-\frac{1}{2}}\log_p(q)p^{-\frac{1}{2}} \bigr) p^{-\frac{1}{2}}X p^{-\frac{1}{2}} \operatorname{Exp}\bigl( \frac{1}{2}p^{-\frac{1}{2}}\log_p(q)p^{-\frac{1}{2}} \bigr) p^{\frac{1}{2}},\]

where $\operatorname{Exp}$ denotes the matrix exponential and log the logarithmic map on SymmetricPositiveDefinite (again with respect to the AffineInvariantMetric).

riemann_tensor(::SymmetricPositiveDefinite, p, X, Y, Z)

Compute the value of Riemann tensor on the SymmetricPositiveDefinite manifold. The formula reads [Ren11] $R(X,Y)Z=p^{1/2}R(X_I, Y_I)Z_Ip^{1/2}$, where $R_I(X_I, Y_I)Z_I=\frac{1}{4}[Z_I, [X_I, Y_I]]$, $X_I=p^{-1/2}Xp^{-1/2}$, $Y_I=p^{-1/2}Yp^{-1/2}$ and $Z_I=p^{-1/2}Zp^{-1/2}$.

BurresWassertseinMetric <: AbstractMetric

The Bures Wasserstein metric for symmetric positive definite matrices [MMP18]

exp(::MatricManifold{ℝ,SymmetricPositiveDefinite,BuresWassersteinMetric}, p, X)

Compute the exponential map on SymmetricPositiveDefinite with respect to the BuresWassersteinMetric given by

\[ \exp_p(X) = p+X+L_p(X)pL_p(X)\]

where $q=L_p(X)$ denotes the Lyapunov operator, i.e. it solves $pq + qp = X$.

log(::MatricManifold{SymmetricPositiveDefinite,BuresWassersteinMetric}, p, q)

Compute the logarithmic map on SymmetricPositiveDefinite with respect to the BuresWassersteinMetric given by

\[ \log_p(q) = (pq)^{\frac{1}{2}} + (qp)^{\frac{1}{2}} - 2 p\]

where $q=L_p(X)$ denotes the Lyapunov operator, i.e. it solves $pq + qp = X$.

change_representer(M::MetricManifold{ℝ,SymmetricPositiveDefinite,BuresWassersteinMetric}, E::EuclideanMetric, p, X)

Given a tangent vector $X ∈ T_p\mathcal M$ representing a linear function on the tangent space at p with respect to the EuclideanMetric g_E, this is turned into the representer with respect to the (default) metric, the BuresWassersteinMetric on the SymmetricPositiveDefinite M.

To be precise we are looking for $Z∈T_p\mathcal P(n)$ such that for all $Y∈T_p\mathcal P(n)$` it holds

\[⟨X,Y⟩ = \operatorname{tr}(XY) = ⟨Z,Y⟩_{\mathrm{BW}}\]

for all $Y$ and hence we get $Z$= 2(A+A^{\mathrm{T}})$with$A=Xp``.

distance(::MatricManifold{SymmetricPositiveDefinite,BuresWassersteinMetric}, p, q)

Compute the distance with respect to the BuresWassersteinMetric on SymmetricPositiveDefinite matrices, i.e.

\[d(p,q) = \operatorname{tr}(p) + \operatorname{tr}(q) - 2\operatorname{tr}\Bigl( (p^{\frac{1}{2}}qp^{\frac{1}{2}} \bigr)^\frac{1}{2} \Bigr),\]

where the last trace can be simplified (by rotating the matrix products in the trace) to $\operatorname{tr}(pq)$.

inner(::MetricManifold{ℝ,SymmetricPositiveDefinite,BuresWassersteinMetric}, p, X, Y)

Compute the inner product SymmetricPositiveDefinite with respect to the BuresWassersteinMetric given by

\[ ⟨X,Y⟩ = \frac{1}{2}\operatorname{tr}(L_p(X)Y)\]

where $q=L_p(X)$ denotes the Lyapunov operator, i.e. it solves $pq + qp = X$.

is_flat(::MetricManifold{ℝ,<:SymmetricPositiveDefinite,BuresWassersteinMetric})

Return false. SymmetricPositiveDefinite with BuresWassersteinMetric is not a flat manifold.

GeneralizedBurresWassertseinMetric{T<:AbstractMatrix} <: AbstractMetric

The generalized Bures Wasserstein metric for symmetric positive definite matrices, see [HMJG21].

This metric internally stores the symmetric positive definite matrix $M$ to generalise the metric, where the name also follows the mentioned preprint.

exp(::MatricManifold{ℝ,SymmetricPositiveDefinite,GeneralizedBuresWassersteinMetric}, p, X)

Compute the exponential map on SymmetricPositiveDefinite with respect to the GeneralizedBuresWassersteinMetric given by

\[ \exp_p(X) = p+X+\mathcal ML_{p,M}(X)pML_{p,M}(X)\]

where $q=L_{M,p}(X)$ denotes the generalized Lyapunov operator, i.e. it solves $pqM + Mqp = X$.

log(::MatricManifold{SymmetricPositiveDefinite,GeneralizedBuresWassersteinMetric}, p, q)

Compute the logarithmic map on SymmetricPositiveDefinite with respect to the BuresWassersteinMetric given by

\[ \log_p(q) = M(M^{-1}pM^{-1}q)^{\frac{1}{2}} + (qM^{-1}pM^{-1})^{\frac{1}{2}}M - 2 p.\]

change_representer(M::MetricManifold{ℝ,SymmetricPositiveDefinite,GeneralizedBuresWassersteinMetric}, E::EuclideanMetric, p, X)

Given a tangent vector $X ∈ T_p\mathcal M$ representing a linear function on the tangent space at p with respect to the EuclideanMetric g_E, this is turned into the representer with respect to the (default) metric, the GeneralizedBuresWassersteinMetric on the SymmetricPositiveDefinite M.

To be precise we are looking for $Z∈T_p\mathcal P(n)$ such that for all $Y∈T_p\mathcal P(n)$ it holds

\[⟨X,Y⟩ = \operatorname{tr}(XY) = ⟨Z,Y⟩_{\mathrm{BW}}\]

for all $Y$ and hence we get $Z = 2pXM + 2MXp$.

distance(::MatricManifold{SymmetricPositiveDefinite,GeneralizedBuresWassersteinMetric}, p, q)

Compute the distance with respect to the BuresWassersteinMetric on SymmetricPositiveDefinite matrices, i.e.

\[d(p,q) = \operatorname{tr}(M^{-1}p) + \operatorname{tr}(M^{-1}q) - 2\operatorname{tr}\bigl( (p^{\frac{1}{2}}M^{-1}qM^{-1}p^{\frac{1}{2}} \bigr)^{\frac{1}{2}},\]

inner(::MetricManifold{ℝ,SymmetricPositiveDefinite,GeneralizedBuresWassersteinMetric}, p, X, Y)

Compute the inner product SymmetricPositiveDefinite with respect to the GeneralizedBuresWassersteinMetric given by

\[ ⟨X,Y⟩ = \frac{1}{2}\operatorname{tr}(L_{p,M}(X)Y)\]

where $q=L_{M,p}(X)$ denotes the generalized Lyapunov operator, i.e. it solves $pqM + Mqp = X$.

is_flat(::MetricManifold{ℝ,<:SymmetricPositiveDefinite,<:GeneralizedBuresWassersteinMetric})

Return false. SymmetricPositiveDefinite with GeneralizedBuresWassersteinMetric is not a flat manifold.

LogEuclideanMetric <: RiemannianMetric

The LogEuclidean Metric consists of the Euclidean metric applied to all elements after mapping them into the Lie Algebra, i.e. performing a matrix logarithm beforehand.

distance(M::MetricManifold{SymmetricPositiveDefinite{N},LogEuclideanMetric}, p, q)

Compute the distance on the SymmetricPositiveDefinite manifold between p and q as a MetricManifold with LogEuclideanMetric. The formula reads

\[ d_{\mathcal P(n)}(p,q) = \lVert \operatorname{Log} p - \operatorname{Log} q \rVert_{\mathrm{F}}\]

where $\operatorname{Log}$ denotes the matrix logarithm and $\lVert\cdot\rVert_{\mathrm{F}}$ denotes the matrix Frobenius norm.

is_flat(::MetricManifold{ℝ,<:SymmetricPositiveDefinite,LogEuclideanMetric})

Return false. SymmetricPositiveDefinite with LogEuclideanMetric is not a flat manifold.

LogCholeskyMetric <: RiemannianMetric

The Log-Cholesky metric imposes a metric based on the Cholesky decomposition as introduced by [Lin19].

exp(M::MetricManifold{SymmetricPositiveDefinite,LogCholeskyMetric}, p, X)

Compute the exponential map on the SymmetricPositiveDefinite M with LogCholeskyMetric from p into direction X. The formula reads

\[\exp_p X = (\exp_y W)(\exp_y W)^\mathrm{T}\]

where $\exp_xW$ is the exponential map on CholeskySpace, $y$ is the cholesky decomposition of $p$, $W = y(y^{-1}Xy^{-\mathrm{T}})_\frac{1}{2}$, and $(\cdot)_\frac{1}{2}$ denotes the lower triangular matrix with the diagonal multiplied by $\frac{1}{2}$.

log(M::MetricManifold{SymmetricPositiveDefinite,LogCholeskyMetric}, p, q)

Compute the logarithmic map on SymmetricPositiveDefinite M with respect to the LogCholeskyMetric emanating from p to q. The formula can be adapted from the CholeskySpace as

\[\log_p q = xW^{\mathrm{T}} + Wx^{\mathrm{T}},\]

where $x$ is the cholesky factor of $p$ and $W=\log_x y$ for $y$ the cholesky factor of $q$ and the just mentioned logarithmic map is the one on CholeskySpace.

distance(M::MetricManifold{SymmetricPositiveDefinite,LogCholeskyMetric}, p, q)

Compute the distance on the manifold of SymmetricPositiveDefinite nmatrices, i.e. between two symmetric positive definite matrices p and q with respect to the LogCholeskyMetric. The formula reads

\[d_{\mathcal P(n)}(p,q) = \sqrt{ \lVert ⌊ x ⌋ - ⌊ y ⌋ \rVert_{\mathrm{F}}^2 + \lVert \log(\operatorname{diag}(x)) - \log(\operatorname{diag}(y))\rVert_{\mathrm{F}}^2 }\ \ ,\]

where $x$ and $y$ are the cholesky factors of $p$ and $q$, respectively, $⌊\cdot⌋$ denbotes the strictly lower triangular matrix of its argument, and $\lVert\cdot\rVert_{\mathrm{F}}$ the Frobenius norm.

inner(M::MetricManifold{LogCholeskyMetric,ℝ,SymmetricPositiveDefinite}, p, X, Y)

Compute the inner product of two matrices X, Y in the tangent space of p on the SymmetricPositiveDefinite manifold M, as a MetricManifold with LogCholeskyMetric. The formula reads

\[ g_p(X,Y) = ⟨a_z(X),a_z(Y)⟩_z,\]

where $⟨\cdot,\cdot⟩_x$ denotes inner product on the CholeskySpace, $z$ is the cholesky factor of $p$, $a_z(W) = z (z^{-1}Wz^{-\mathrm{T}})_{\frac{1}{2}}$, and $(\cdot)_\frac{1}{2}$ denotes the lower triangular matrix with the diagonal multiplied by $\frac{1}{2}$

is_flat(::MetricManifold{ℝ,<:SymmetricPositiveDefinite,LogCholeskyMetric})

Return false. SymmetricPositiveDefinite with LogCholeskyMetric is not a flat manifold.

vector_transport_to(
    M::MetricManifold{SymmetricPositiveDefinite,LogCholeskyMetric},
    p,
    X,
    q,
    ::ParallelTransport,
)

Parallel transport the tangent vector X at p along the geodesic to q with respect to the SymmetricPositiveDefinite manifold M and LogCholeskyMetric. The parallel transport is based on the parallel transport on CholeskySpace: Let $x$ and $y$ denote the cholesky factors of p and q, respectively and $W = x(x^{-1}Xx^{-\mathrm{T}})_\frac{1}{2}$, where $(\cdot)_\frac{1}{2}$ denotes the lower triangular matrix with the diagonal multiplied by $\frac{1}{2}$. With $V$ the parallel transport on CholeskySpace from $x$ to $y$. The formula hear reads

\[\mathcal P_{q←p}X = yV^{\mathrm{T}} + Vy^{\mathrm{T}}.\]

mean(
    M::SymmetricPositiveDefinite,
    x::AbstractVector,
    [w::AbstractWeights,]
    method = GeodesicInterpolation();
    kwargs...,
)

Compute the Riemannian mean of x using GeodesicInterpolation.

SPDPoint <: AbstractManifoldsPoint

Store the result of eigen(p) of an SPD matrix and (optionally) $p^{1/2}$ and $p^{-1/2}$ to avoid their repeated computations.

This result only has the result of eigen as a mandatory storage, the other three can be stored. If they are not stored they are computed and returned (but then still not stored) when required.

Constructor

SPDPoint(p::AbstractMatrix; store_p=true, store_sqrt=true, store_sqrt_inv=true)

Create an SPD point using an symmetric positive defincite matrix p, where you can optionally store p, sqrt and sqrt_inv

spd_sqrt(p::AbstractMatrix)
spd_sqrt(p::SPDPoint)

return $p^{\frac{1}{2}}$ by either computing it (if it is missing or for the AbstractMatrix) or returning the stored value from within the SPDPoint.

This method assumes that p represents an spd matrix.

spd_sqrt_inv(p::SPDPoint)

return $p^{-\frac{1}{2}}$ by either computing it (if it is missing or for the AbstractMatrix) or returning the stored value from within the SPDPoint.

This method assumes that p represents an spd matrix.

spd_sqrt_and_sqrt_inv(p::AbstractMatrix)
spd_sqrt_and_sqrt_inv(p::SPDPoint)

return $p^{\frac{1}{2}}$ and $p^{-\frac{1}{2}}$ by either computing them (if they are missing or for the AbstractMatrix) or returning their stored value from within the SPDPoint.

Compared to calling single methods spd_sqrt and spd_sqrt_inv this method only computes the eigenvectors once for the case of the AbstractMatrix or if both are missing.

This method assumes that p represents an spd matrix.