Events

Wed, 31.01.2018, 17:15
Stable Phase Retrieval and Spectral Clustering
Mathematisches Kolloquium

Speaker: Prof. Dr. Philipp Grohs, Universität Wien
Room: S2|14 /24

Zuvor findet um 16:45 Uhr die Teerunde in Raum 244 des Mathematikgebäudes (S2/15), Schlossgartenstaße 7, statt.

We consider the Gabor phase retrieval problem, i.e., the problem of reconstructing a signal $f$ from the magnitudes $|V_\varphi f|$ of its Gabor transform
$$V_\varphi f (x,y):=\int_{\mathbb{R}}f(t)e^{-\pi (t-x)^2}e^{-2\pi \i y t}dt, \quad x,y\in \mathbb{R}.$$
Such problems occur in a wide range of applications, from optical imaging of nanoscale structures to audio processing and classification.

While it is well-known that the solution of the Gabor phase retrieval problem is unique up to natural identifications, the stability of the reconstruction has remained wide open. The present paper discovers a surprising connection between phase retrieval, spectral clustering and spectral geometry. We show that the stability of the Gabor phase reconstruction is bounded by the Cheeger constant of the flat metric on $\mathbb{R}^2$, conformally multiplied with $|V_\varphi f|$. The Cheeger constant, in turn, plays a prominent role in the field of spectral clustering, and it precisely quantifies the `disconnectedness' of the measurements $V_\varphi f$.

It has long been known that a disconnected support of the measurements results in an instability -- our result for the first time provides a converse result in the sense that there are no other sources of instabilities.

Due to the fundamental importance of Gabor phase retrieval in coherent diffraction imaging, we also provide a new understanding of the stability properties of these imaging techniques: Contrary to most classical problems in imaging science whose regularization requires the promotion of smoothness or sparsity, the correct regularization of the phase retrieval problem promotes the `connectedness' of the measurements in terms of bounding the Cheeger constant from below. Our work thus, for the first time, opens the door to the development of efficient regularization strategies.

This is joint work with Rima Al-Aifari, Ingrid Daubechies, Martin Rathmair and Rachel Yin.

References:
[1] R. Al-Aifari, P. Grohs, I. Daubechies and R. Yin. Stable Infinite Dimensional Phase Retrieval. Foundations of Computational Mathematics, (2018), to appear.
[2] P. Grohs and M. Rathmair. Stable Gabor Phase Retrieval and Spectral Clustering. Communications on Pure and Applied Mathematics, (2018), to appear.


Contact

Fachbereich Mathematik
Technische Universität Darmstadt

Schlossgartenstraße 7
64289 Darmstadt

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