Radu Balan University of Maryland College Park, MD 208742 email: rvbalan@math.umd

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A Nonlinear Reconstruction Algorithm from Absolute Value of Frame Coefficients for Low Redundancy Frames. Radu Balan University of Maryland College Park, MD 208742 email: rvbalan@math.umd.edu SampTA 2009 - Marseille, France. Statement of the Problem . H=E n , where E= R or E= C

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### A Nonlinear Reconstruction Algorithm from Absolute Value of Frame Coefficients for Low Redundancy Frames

University of Maryland

College Park, MD 208742

email: rvbalan@math.umd.edu

SampTA 2009 - Marseille, France

Statement of the Problem
• H=En , where E=R or E=C
• F={f1,f2,...,fm} a spanning set of m>n vectors
• Assume the map:

is injective up to a constant phase factor ambiguity

• The Problem: Given c=N(x) construct a vector y equivalent to x (that is, invert N up to a constant phase factor)
Where is this problem relevant?

X-Ray Crystallography

Very thin layer, so that r is 2-D

Problem:

Given I(k) , estimate R(r).

What is known?

Theorem [R.B.,Casazza, Edidin, ACHA(2006)]

• For E = R , m  2n-1, and a generic frame set F, then N is injective.
• For E = C , m  4n-2, and a generic frame set F, then N is injective.
But how to invert?
• Observation [R.B.,Bodman, Casazza, Edidin, SPIE(2007)/JFAA(2009)]
• Algorithm: Assume {Kfk} is spanning in M(En)
• Compute the dual set { } to {Kfk}
• Compute
• Compute

Then y~x

The algorithm is quasi-linear, but has a drawback: it requires a high redundancy.
• Specifically: it requires m=O(n2)

whereas we know that m=O(n) should be sufficient.

This paper presents a novel algorithm that interpolates between O(n) and O(n2) keeping similar properties to the previous algorithm.

Nonlinear Embedding
• Generalize xKx to (d,d) tensors:
• and embed the frame set F into s.lin func.

(En)

P

x

P

Geometry

En

Key Observation:

Dimension Condition for Linear Reconstruction

Necessary Condition for Linear Reconstruction:

Real Case: E=R

The condition:

is satisfied for

Note the critical case

m=2n-1 !!

Complex Case: E=C

The condition:

is satisfied by the following (suboptimal) choice

Note m=O(n)

but larger than 4n-2

Is this enough?
• We obtained a necessary condition for the set to be spanning in Ed.
• However this is not a guarantee that this happens!
• In our experimental testing the set is spanning in Ed.
• Open Problem/Conjecture:

is generically spanning in Ed.