A dual certificate analysis of compressive off-the-grid recoveryA dual certificate analysis of compressive off-the-grid recovery Nicolas Keriven Ecole Normale Supérieure (Paris) CFM-ENS

A dual certificate analysis of compressive off-the-grid recovery

Nicolas KerivenEcole Normale Supérieure (Paris)

CFM-ENS chair in Data Science

Joint work with Clarice Poon (Cambridge Uni.), Gabriel Peyré (ENS)

Journée GdR MIA, May 3rd 2018

Discrete compressive sensing

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• Signal: vector


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• Signal: vector• Sparsity: few non-zeros coefficients


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• Signal: vector• Sparsity: few non-zeros coefficients• Dimensionality reduction (often random matrix)


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• Signal: vector• Sparsity: few non-zeros coefficients• Dimensionality reduction (often random matrix)• Recovery: convex relaxation LASSO


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Off-the-grid recovery: « super-resolution »

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- Fourier

- Convolution


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- Fourier

- Convolution


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- Fourier

- Convolution


• Signal: Radon measure

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- Fourier

- Convolution


• Signal: Radon measure• Sparsity:

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- Fourier

- Convolution


• Signal: Radon measure• Sparsity:• Dimensionality reduction (e.g. first Fourier coefficients)

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- Fourier

- Convolution


• Signal: Radon measure• Sparsity:• Dimensionality reduction (e.g. first Fourier coefficients)• Recovery: convex relaxation?

See Keriven 2017, Gribonval 2017

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- Fourier

- Convolution


• Signal: Radon measure• Sparsity:• Dimensionality reduction (e.g. first Fourier coefficients)• Recovery: convex relaxation? BLASSO [De Castro, Gamboa 2012]


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- Fourier

- Convolution


• Signal: Radon measure• Sparsity:• Dimensionality reduction (e.g. first Fourier coefficients)• Recovery: convex relaxation? BLASSO [De Castro, Gamboa 2012]

Other approaches: « Prony-like » ESPRIT, MUSIC… (but only 1d noiseless Fourier)


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Example of applications

Fluorescence microscopy[Betzig 2006]

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Astronomy[Puschmann 2017]

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Compressive k-means(GMM…) [Keriven 2017]


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Compressive k-means(GMM…) [Keriven 2017]


• Neuro-imaging with EEG [Gramfort 2013]

• 1-layer neural network [Bach 2017]

• Radar• Geophysics• …

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A seminal result [Candès, Fernandez-Granda 2012]

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First Fourier coefficients

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Inverse Fourier

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Inverse Fourier

BLASSO

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• Minimal separation

Inverse Fourier

BLASSO

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• Regular Fourier on the Torus

Inverse Fourier

BLASSO

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• Reconstruction: formulated as SDP (other: Frank-Wolfe,

greedy, Prony-like…)

Inverse Fourier

BLASSO

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• Noise: weak convergence (mass of concentrated

around true Diracs)

Inverse Fourier

BLASSO

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• Noise: weak convergence (mass of concentrated

around true Diracs)

Inverse Fourier

BLASSO

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Randomness ? Numberof measurements ?

Compressed sensing off-the-grid [Tang, Recht 2013]

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Random Fourier coefficients

random

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BLASSO

random

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• As in compressive sensing, randomFourier sampling is possible

But:• Limited to 1d Regular Fourier (relies

heavily on previous work by Candès)

• Random signs assumption

BLASSO

random

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• As in compressive sensing, randomFourier sampling is possible

But:• Limited to 1d Regular Fourier (relies

heavily on previous work by Candès)

• Random signs assumption

BLASSO

Questions:

• More general sampling scheme ?

• Multi-dimensional result ?

• Get rid of random signs ?

random

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Outline

Background on dual certificates

Compressive off-the-grid recovery

Conclusion, outlooks

Dual certificates

Measurements

Hilbert space

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Dual certificates

Measurements

Hilbert space

BLASSO

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Dual certificates

Measurements

Hilbert space

BLASSO

First-order conditions

solution ofBLASSO

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solution of

Dual certificates

Measurements

Hilbert space

BLASSO

First-order conditions

Dual certificate (noiseless case)

solution ofBLASSO

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What is a dual certificate ?

What does it look like ?

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Case :



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Case :



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Case :



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Ensures uniqueness and robustness…

Non-degenerate dual certif.

Recovery results

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Recovery results

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Recovery results

Theorem (refinement of [Azaïs et al. 2015])

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Recovery results


Hyp: there exists a ND dual certif.

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Recovery results


Hyp: there exists a ND dual certif.Result: (wrt Bregman divergence)

- not necessarily sparse, but-

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Recovery results



- not necessarily sparse, but- Mass of concentrated around

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Recovery results



- not necessarily sparse, but- Mass of concentrated around- Concentration increases when:

noise

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Recovery results




noise

Theorem ([Duval Peyré 2015])

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Recovery results




noise


Hyp: the minimal norm certificate is non-degenerate

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Recovery results




noise


Hyp: the minimal norm certificate is non-degenerateResult: (in the small noise regime)

- is sparse, with the right number of components

-

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Outline




Goal

Goal: random sampling

Low-dim, random

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Goal

Strategy: Start with « high »-dimensional problemGoal: random sampling

Low-dim, random Ex: full Fourier (high-dim), full convolution (infinite-dim)

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Goal


Step 1: Build ND certificate with full kernel


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Goal




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Goal




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Goal



Step 2: Use Random Features on full kernel to define sampling


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Goal




Assuming

Define


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Goal




Assuming

Define

Then


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Goal




Assuming

Define

Then


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Goal




m=10

Assuming

Define

Then


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Goal




m=10 m=50

Assuming

Define

Then


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Goal




m=500m=10 m=50

Assuming

Define

Then


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Step 1: Acceptable full kernels

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Min. Separation

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Min. Separation

1: kernel at each saturation point

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Min. Separation

2: Small adjustements: minimal separation


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Min. Separation

2: Small adjustements: minimal separation

- Multi-d square Féjer kernel (regular Fourier on Torus)

- Multi-d Gaussian kernel


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Step 2: sampling

Thm: Ideal scaling in sparsity: infinite-dim. golfing scheme

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Step 2: sampling


Depend on kernel

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Step 2: sampling


• There exists a ND dual certificate (however not the minimal norm certificate)

Depend on kernel

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Step 2: sampling



• No need for random signs

Depend on kernel

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Step 2: sampling




• Proof: golfing scheme [Gross 2009, Candès Plan 2011]

Depend on kernel

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Step 2: sampling





Thm: Minimal norm certificate (adaptation of [Tang, Recht 2013])

Depend on kernel

With random signs Without random signs

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Step 2: sampling





Thm: Minimal norm certificate (adaptation of [Tang, Recht 2013])

Depend on kernel

With random signs Without random signs

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Fun application: convex approach for automatic estimation of number of components in a GMM

Number of measurements in practice ?

Nicolas Keriven

Compressive k-means [Keriven 2017]

Relative number of measurements m/(sd)

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Outline




Summary, outlooks

• Summary: generalization of existing results on super-resolutionwith random measurements (and minimal separation)

• Beyond Fourier on the Torus (« acceptable » kernels)• Multi-d• No need for random signs for basic recovery result• Support recovery when random signs (or quadratic number of

measurements)

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Summary, outlooks

• Summary: generalization of existing results on super-resolutionwith random measurements (and minimal separation)

• Beyond Fourier on the Torus (« acceptable » kernels)• Multi-d• No need for random signs for basic recovery result• Support recovery when random signs (or quadratic number of

measurements)

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• Outlooks• Other kernels, very different from translation-invariant• More quantified treatment of dimension• Other practical applications (eg 1-layer neural networks with continuum of

neurons [Bach 2017])

Poon, Keriven, Peyré. A Dual Certificates Analysis of Compressive Off-the-Grid Recovery. Preprint arxiv:1802.08464

Code: sketchml.gforge.inria.fr,github: nkeriven

Thank you !

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data-ens.github.ioEnter the data challenges!Come to the colloquium!Come to the Laplace seminars!

Documents

A dual certificate analysis of compressive off-the-grid recoveryA dual certificate analysis of compressive off-the-grid recovery Nicolas Keriven Ecole Normale Supérieure (Paris) CFM-ENS