bio  website  regularize.wordpress.com 

location  Braunschweig, Germany  
age  36  
visits  member for  4 years 
seen  9 hours ago  
stats  profile views  1,695 
Professor at TU Braunschweig.
Area: Inverse problems, regularization theory, applied functional analysis, mathematical image processing.
2d

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A fixed point problem about the iterated mappings
As the question appeared in the review queue, I started editing. Halfway through, I realized that the question does not make sense as stated… For what it's worth: The answers to your questions is: 1. $\lim f^m(x)$ does not exist (hence asking if it is in $\Omega$ or not does not make sense). 2. No. 3. See 1. 
2d

reviewed  Edit suggested edit on A fixed point problem about the iterated mappings 
2d

revised 
A fixed point problem about the iterated mappings
Texify the question 
Oct 15 
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Inverse Function Theorem on Zygmund Spaces, is the inverse in the same Zygmund Space?
What I mean is that you should post the same question here and at math.stackexchange simultaneously. This produces unrelated threads and wasted time of the community, see here. 
Oct 15 
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Inverse Function Theorem on Zygmund Spaces, is the inverse in the same Zygmund Space?
Please do not crosspost the same question. 
Oct 14 
answered  Selecting Rays for Simulated Radon Transform 
Oct 14 
reviewed  Approve suggested edit on Two Equal Series? 
Oct 13 
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Is this series well known?
Oops, totally missed that $!$… 
Oct 13 
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Is this series well known?
Well, its only loosely related. Your f is almost the integral of $\sum t^{n^2}$… 
Oct 13 
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Is this series well known?
Related to (integral of) the theta function $\theta(0,\tau)$, de.wikipedia.org/wiki/Thetafunktion. 
Oct 9 
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Proofs without words
I find the underlying argument extremely nice but the picture (especially the animated one) does not really work for me without any words. The animation makes me think that is has some meaning that the yellow dots are visited in a certain order. 
Oct 9 
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Why are the angular differences of these random complex polynomial coefficients almost constant?
Yeah sorry, I still had the question mathoverflow.net/questions/182412/… in mind. For roots in the unit square the effect I see is much smaller but still can be seen (still working with double precision floating points). I still tend to blame the random number generators; but this is far from my field and so this suspicion is not backed up… 
Oct 9 
answered  Why are the angular differences of these random complex polynomial coefficients almost constant? 
Oct 8 
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Can one always find sparse solutions to an $\ell^1$minimization problem?
Question reformulated in terms of the optimality system: $x^*$ is an $\ell^1$minimal solution if $Ax^*=b$ and there exists $w$ such that $A^T w \in\partial\x\_1$. This says: there is no $m$sparse solution if the range of $A^T$ (which is $m$dimensional) does not intersect the $m$dimensional faces of the unit cube. Answers to this question show that for $m\geq n/2$ this can not happen. (Unfortunately, $m$ and $n$ are swapped in the linked question.) 
Oct 8 
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Why do roots of polynomials tend to have absolute value close to 1?
This is really strange. My experiments with MATLAB (just with double precision floating points) show strange effects even when I calculate the coefficients of a polynomial which has $N$ roots precisely (up to eps) at the roots of unity when $N$ is as small as 100. I obtain a polynomial with very large coefficients for the middle exponents, and comparably small coefficients close $x^0$ and $x^N$. 
Oct 8 
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Why do roots of polynomials tend to have absolute value close to 1?
I notice that this answer is close in spirit to the one by @tros443. 
Oct 8 
answered  Why do roots of polynomials tend to have absolute value close to 1? 
Oct 1 
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ellipsoids have spherical section
Capitalization and punctuation as well as consistent TeX would be helpful. 
Sep 29 
awarded  Yearling 
Sep 29 
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What is the most useful nonexisting object of your field?
I use the nonexistence of a number that is both positive and negative much more often then the nonexistence of a number that is less than and greater than $1$. However, the nonexistence of a number that is both positive and negative follows easily: Let $x$ fulfill i) $x<1$ and ii) $x>1$. Then for $y = x1$ we have by i) that $y<0$ and by ii) that $y>0$ and hence, $y$ has the desired properties. Since $x$ does not exist, $y$ also does not exist either. 