Clayton Shonkwiler PRO
Mathematician and artist
Symplectic geometry is a powerful set of tools which is useful for the study of frames
A frame in \(\mathbb{C}^d\) is a collection \(\{\phi_1,\dots ,\phi_N\}\subset \mathbb{C}^d\) so that
for some \(b\geq a > 0\) and for all \(v\in\mathbb{C}^d\).
The frame is tight if \(a=b\); equivalently, if \(\Phi\) is the matrix with columns \(\phi_i\), then the frame operator \(\Phi\Phi^*=\frac{1}{a}I_d\).
If \(\|\phi_i\|=1\) for all \(i\), the frame is unit norm. We abbreviate (Finite) Unit Norm Tight Frame as FUNTF.
Examples.
\(\mathcal{F}^{N,d}_S\) is the collection of length-\(N\) frames in \(\mathbb{C}^d\) with frame operator \(S\); i.e., \(k \times N\) matrices \(\Phi\) with \(\Phi\Phi^*=S\).
Note that \(S\) is necessarily invertible and positive-definite.
\(\mathcal{F}^{N,d}_S(\vec{r})\) is the subset of \(\mathcal{F}^{N,d}_S\) of frames with \(\|\phi_i\| = r_i\).
In particular, the length-\(N\) FUNTFs in \(\mathbb{C}^d\) are precisely \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d}(\vec{1})\) since
Definition. A symplectic manifold is a smooth manifold \(M\) together with a closed, non-degenerate 2-form \({\omega \in \Omega^2(M)}\).
Example: \((S^2,d\theta\wedge dz)\)
Example. \((\mathbb{R}^2,dx \wedge dy) = (\mathbb{C},\frac{i}{2}dz \wedge d\bar{z})\)
Example. \((T^*\mathbb{R}^n, \sum_{i=1}^n dq_i \wedge dp_i)\)
Example. \((S^2,\omega)\), where \(\omega_p(u,v) = (u \times v) \cdot p\)
Example. \((\mathbb{R}^2,\omega)\) where \(\omega(u,v) = \langle i u, v \rangle \)
Example. \((\mathbb{C}^n, \frac{i}{2} \sum dz_k \wedge d\overline{z}_k)\)
If \((M,\omega)\) is symplectic and \(g\) is a Riemannian metric on \(M\), then \(g(\cdot, \cdot) = \omega(\cdot , J \cdot)\) defines a compatible almost complex structure \(J\) on \(M\).
If this compatible almost complex structure is integrable, then \((M,J,g,\omega)\) is Kähler.
Since the Fubini–Study form \(\omega_{\text{FS}}\) on \(\mathbb{CP}^n\) is Kähler, all smooth projective varieties are Kähler, and in particular symplectic.
The name ‘complex group’ formerly proposed by me..has become more and more embarrassing through collision with the word ‘complex’ in the connotation of complex number. I therefore propose to replace it by the corresponding Greek adjective ‘symplectic’.
— Hermann Weyl
A symplectic manifold must be even-dimensional (over \(\mathbb{R}\)).
\(\omega^{\wedge n} = \omega \wedge \dots \wedge \omega\) is a volume form on \(M\), and induces a measure
called Liouville measure on \(M\).
If \(H: M \to \mathbb{R}\) is smooth, then there exists a unique vector field \(X_H\) (the Hamiltonian vector field for \(H\)) so that \({dH = \iota_{X_H}\omega}\), i.e.,
(\(X_H\) is sometimes called the symplectic gradient of \(H\))
\(H: (S^2, d\theta\wedge dz) \to \mathbb{R}\) given by \(H(\theta,z) = z\).
\(dH = dz = \iota_{\frac{\partial}{\partial \theta}}\omega\), so \(X_H = \frac{\partial}{\partial \theta}\).
Integrating \(X_H\) produces the one-parameter family of diffeomorphisms \(\psi_t(\theta, z) = (\theta+t,z)\).
Let \(G\) be a Lie group, and let \(\mathfrak{g}\) be its Lie algebra. If \(G\) acts on \((M,\omega)\), then each \(V \in \mathfrak{g}\) determines a vector field \(X_V\) on \(M\) by
\(S^1=U(1)\) acts on \((S^2,d\theta \wedge dz)\) by
For \(r \in \mathbb{R} \simeq \mathfrak{u}(1)\), \(X_r = r \frac{\partial}{\partial \theta}\).
Let \(G\) be a Lie group, and let \(\mathfrak{g}\) be its Lie algebra. If \(G\) acts on \((M,\omega)\), then each \(V \in \mathfrak{g}\) determines a vector field \(X_V\) on \(M\) by
\(SO(3)\) acts on \(S^2\) by rotations.
\(X_{V_{(a,b,c)}}((x,y,z)) = (bz-cy)\frac{\partial}{\partial x} + (cx - az) \frac{\partial}{\partial y} + (ay - bx) \frac{\partial}{\partial z}\)
For \(V_{(a,b,c)} = \begin{bmatrix} 0 & -c & b \\ c & 0 & -a \\ -b & a & 0 \end{bmatrix} \in \mathfrak{so}(3)\),
\(= (a,b,c) \times (x,y,z)\)
Definition. An action of \(U(1)\) on \((M,\omega)\) is Hamiltonian if there exists a map
so that \(d\mu = \iota_{X}\omega\), where \(X\) is the vector field generated by the circle action.
\(X = \frac{\partial}{\partial \theta}\)
\(\iota_X\omega = \iota_{\frac{\partial}{\partial \theta}} d\theta \wedge dz = dz \)
\(\mu(\theta,z) = z\)
Definition. An action of \(G\) on \((M,\omega)\) is Hamiltonian if each one-parameter subgroup action is Hamiltonian. Equivalently, there exists a map
so that \(\omega_p(X_V, X) = D_p \mu(X)(V)\) for each \(p \in M\), \(X \in T_pM\), and \(V \in \mathfrak{g}\).
\(X_{V_{(a,b,c)}}(x,y,z) = (a,b,c) \times (x,y,z)\)
\((\iota_{X_{V_{(a,b,c)}}}\omega)_{(x,y,z)} = a dx + b dy + c dz \)
\(\mu(x,y,z)(V_{(a,b,c)}) = (x,y,z)\cdot(a,b,c)\)
Let \(U(1)\) act on \((\mathbb{C}^d,\omega_{\text{std}})\) by \(e^{it}\cdot \vec{v} = \vec{v} e^{-it}\).
The vector field generated by the circle action is
so \((\iota_X\omega_{\text{std}})_{\vec{v}}(Y) = (\omega_{\text{std}})_{\vec{v}}(X,Y) = \langle iX,Y\rangle_{\vec{v}} = \langle \vec{v},Y\rangle\).
The moment map is
since \(d\mu_{\vec{v}}(Y) = \left.\frac{d}{dt}\right|_{t=0} \left(\frac{1}{2} \langle \vec{v} + tY, \vec{v} + tY\rangle\right) = \langle \vec{v},Y\rangle\).
Theorem (Atiyah, Guillemin–Sternberg).
Let \((M^{2n},\omega)\) be a compact connected symplectic manifold with a Hamiltonian \(k\)-torus action with momentum map \(\mu: M \to \mathbb{R}^k\). Then
Theorem (Mayer, Marsden–Weinstein)
Let \(\mu: M \to \mathfrak{g}^*\) be the moment map for a Hamiltonian action of \(G\) on \((M,\omega)\). If \(\xi \in \mathfrak{g}^*\) is a regular value of \(\mu\) and \(\mathcal{O}_\xi\) is its coadjoint orbit, then
is a symplectic manifold with symplectic form \(\omega_{\text{red}}\) such that
\(\mu^{-1}(\mathcal{O}_\xi)\)
\(M /\!/\!_\xi\, G = \mu^{-1}(\mathcal{O}_\xi)/G\)
\(M\)
\(\pi\)
\(\iota\)
\(U(1)^N\) acts on \(\mathbb{C}^{d \times N}\) by right-multiplication by diagonal unitary matrices.
Columnwise, this is the \(U(1)\) action on \(\mathbb{C}^d\) with moment map \(\vec{v} \mapsto \frac{1}{2}|\vec{v}|^2\).
The moment map for the big torus action is
If \(\vec{\frac{1}{2}} = \left(\frac{1}{2},\dots , \frac{1}{2}\right)\), then \(\mu_{U(1)^N}^{-1}\!\left(\vec{\frac{1}{2}}\right)\simeq \left(S^{2k-1}\right)^N\) consists of all matrices with unit-norm columns and
If \(\vec{w} \in \operatorname{image}(\mu_{U(1)^N})\), then \(\mu_{U(1)^N}^{-1}\!\left(\vec{w}\right)\simeq \prod_{i=1}^NS^{2k-1}(\sqrt{2w_i})\) consists of matrices with column norms given by the \(\sqrt{2w_i}\) and
\(U(d)\) acts on \(\mathbb{C}^{d \times N}\) by left multiplication with moment map
given by
Hermitian \(d \times d\) matrices
For invertible, positive-definite \(S \in \mathcal{H}(d)\), \(\mu_{U(d)}^{-1}(S) = \mathcal{F}^{N,d}_S\), the set of frames with frame operator \(S\).
E.g., \(\mathcal{F}^{N,d}_{I_d}= \mu_{U(d)}^{-1}(I_d)\) is (some of) the tight frames.
E.g., \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d} = \mu_{U(d)}^{-1}\left(\frac{N}{d}I_d\right)\) is (some of) the tight frames.
Recall \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d} = \left\{\Phi : \Phi\Phi^* = \frac{N}{d}I_d\right\}\) is the space of tight frames which contains the FUNTFs.
is symplectic.
Observation.
If \(S \in \mathcal{H}(d) \simeq \mathfrak{u}(d)^*\) is invertible and positive-definite, with eigenvalues \(\lambda_1 > \lambda_2 > \dots > \lambda_\ell > 0\) of multiplicities \(k_1, \dots , k_\ell\), then
where \(d_i = \sum_{j=1}^i k_j\).
is symplectic. \(\mu_{U(d)}^{-1}(\mathcal{O}_S)\) is the space of all frames whose frame operator is conjugate to \(S\); call it \(\widetilde{\mathcal{F}^{N,d}_S}\).
Notice that FUNTF space \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d}(\vec{1})\) is exactly
Since \(\mu:=\mu_{U(d)} \times \mu_{U(1)^N}\) is the moment map of the \(U(d) \times U(1)^N\) action on \(\mathbb{C}^{d \times N}\),
Since \(U(d) \times U(1)^N\) is connected, \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d}(\vec{1})\) is connected if and only if
is connected.
By Atiyah’s connectedness theorem, this is connected!
Theorem (Cahill–Mixon–Strawn ’17)
The space \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d}(\vec{1})\) of length-\(N\) FUNTFs in \(\mathbb{C}^d\) is path-connected for all \(N \geq d\geq 1\).
Proof. By the foregoing, \(\mathcal{F}^{N,d}_{\frac{N}{d}I_d}(\vec{1})\) is connected.
Since this is a real algebraic set in \(\mathbb{C}^{d \times N} \simeq \mathbb{R}^{2dN}\), it is locally path-connected and therefore path-connected.
Theorem (with Needham ’18)
For any frame operator \(S\) and any admissible vector \(\vec{r}\) of frame norms, the space \(\mathcal{F}^{N,d}_S (\vec{r})\) is path-connected.
Proof idea.
is connected by the same argument, so \(\widetilde{\mathcal{F}^{N,d}_S}(\vec{r})\) is connected and path-connected.
If \(V_t\) is a path in \(\widetilde{\mathcal{F}^{N,d}_S}(\vec{r})\) connecting two points \({V_0,V_1 \in \mathcal{F}^{N,d}_S(\vec{r})}\), then \(V_tV_t^* = U_t^*SU_t\) and \(U_tV_t\) also connects \(V_0\) to \(V_1\) and stays in \(\mathcal{F}^{N,d}_S(\vec{r})\).
Paulsen Problem: Given a frame \(\Phi \in \mathbb{C}^{d \times N}\) which is \(\epsilon\)-close to being a FUNTF, find a nearby FUNTF.
Current state of the art [Hamilton–Moitra]: can construct one within \(40\epsilon d^2\).
Sampling Problem: Find an efficient algorithm for sampling random FUNTFs according to the uniform distribution.
RIP Problem: What is the probability that a random FUNTF satisfies a Restricted Isometry Property?
Symplectic geometry and connectivity of space of frames
Tom Needham and Clayton Shonkwiler
Funding: Simons Foundation
The geometry of constrained random walks and an application to frame theory
Clayton Shonkwiler
2018 IEEE Statistical Signal Processing Workshop (SSP), 343–347
By Clayton Shonkwiler