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- <title>Research by E. Blåsten</title>
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- <li><a href="index.html">Home</a></li>
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- <li><a href="research.html" class="active">Research</a></li>
- <ul class="navsubbar">
- <li><a href="#topics">Topics</a></li>
- <li><a href="#publications">Publications</a></li>
- <li><a href="#descriptions">Descriptions</a></li>
- <li><a href="#talks">Talks</a></li>
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- <div class="paper">
- <section class="content">
- <h1>Mathematics</h1>
- <section>
- <a class="anchor" id="topics"></a>
- <h2>Research topics</h2>
- <p>
- I deal with
- mathematical <a href="https://en.wikipedia.org/wiki/Inverse_problem">inverse
- problems</a>
- and <a href="https://en.wikipedia.org/wiki/Scattering_theory">scattering
- theory</a>. The latter is a subfield of partial
- differential equations where the purpose is to study
- phenomena which occur <em>after a wave hits an
- obstacle</em>. The field of inverse problems is related to
- mathematical modeling. In traditional or <em>direct</em>
- modeling, the goal is to predict the effects from the
- causes. On the other hand, in <em>inverse problems</em>,
- one is interested in finding the model, or causes, when a
- set of observations is given. Famous examples include
-
- <ul>
- <li><em>electrical impedance tomography</em>: find the
- electrical conductivity at various points in the
- interior of an object by doing voltage–current
- measurements on its surface,</li>
-
- <li><em>travel-time tomography</em>: calculate the
- density of the Earth by observing the travel times of
- earthquakes,</li>
-
- <li><em>3D X-ray tomography</em>: determine the 3D
- structure of a body by taking X-ray pictures from
- various directions around it (CT-scan).</li>
- </ul>
-
- All of these examples involve tomography, which means
- imaging an object by sending waves into it. This is
- related to non-destructive testing by not having to cut
- the object open.
- </p>
- </section>
- <section>
- <a class="anchor" id="publications"></a>
- <h2>List of
- Publications <a href="pdf/blasten_publications.pdf">PDF</a></h2>
- <ul class="navsubbar">
- <li><a href="#pubs_submitted">Submitted</a></li>
- <li><a href="#pubs_accepted">Accepted</a></li>
- <li><a href="#pubs_proceedings">Proceedings</a></li>
- <li><a href="#pubs_theses">Theses</a></li>
- <li><a href="#pubs_others">Others</a></li>
- </ul>
- <p style="clear:left">
- <a class="anchor" id="pubs_submitted"></a>
- <h3>Submitted</h3>
- <ol reversed>
- <li>
- <a class="anchor" id="Blasten-Li-Liu-Wang-2018"></a>
- <span class="ref-authors">E. Blåsten, H. Li, H. Liu, Y. Wang,</span>
- <span class="ref-title">Localization and
- geometrization in plasmon resonances and geometric
- structures of Neumann-Poincaré
- eigenfunctions.</span>
- <div class="ref-links">
- <a href="#desc_Blasten-Li-Liu-Wang-2018">Description</a>
- <a href="https://arxiv.org/abs/1809.08533">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Blasten-Vesalainen-2018"></a>
- <span class="ref-authors">E. Blåsten, E. V. Vesalainen,</span>
- <span class="ref-title">Non-Scattering Energies and Transmission Eigenvalues in
- <i>H<sup>n</sup></i>.</span>
- <div class="ref-links">
- <a href="#desc_Blasten-Vesalainen-2018">Description</a>
- <a href="http://arxiv.org/abs/1809.04426/">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Blasten-Liu-2018"></a>
- <span class="ref-authors">E. Blåsten, H. Liu,</span>
- <span class="ref-title">Scattering by curvatures,
- radiationless sources, transmission eigenfunctions and
- inverse scattering problems.</span>
- <div class="ref-links">
- <a href="#desc_Blasten-Liu-2018">Description</a>
- <a href="https://arxiv.org/abs/1808.01425">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenLiu2017b"></a>
- <span class="ref-authors">E. Blåsten, H. Liu,</span>
- <span class="ref-title">Recovering piecewise constant
- refractive indices by a single far-field
- pattern.</span>
- <div class="ref-links">
- <a href="#desc_BlastenLiu2017b">Description</a>
- <a href="https://arxiv.org/abs/1705.00815">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenTzouWang2017"></a>
- <span class="ref-authors">E. Blåsten, L. Tzou,
- J. Wang,</span>
- <span class="ref-title">Uniqueness for the inverse
- boundary value problem with singular potentials in
- 2D.</span>
- <div class="ref-links">
- <a href="#desc_BlastenTzouWang2017">Description</a>
- <a href="https://arxiv.org/abs/1704.06397">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenLiu2016"></a>
- <span class="ref-authors">E. Blåsten, H. Liu,</span>
- <span class="ref-title"> On corners scattering stably
- and stable shape determination by a single far-field
- pattern.</span>
- <div class="ref-links">
- <a href="#desc_BlastenLiu2016">Description</a>
- <a href="https://arxiv.org/abs/1611.03647">Preprint</a>
- </div>
- </li>
- </ol>
- </p>
- <p>
- <a class="anchor" id="pubs_accepted"></a>
- <h3>Accepted in Peer-Reviewed Journals</h3>
- <ol reversed>
- <li>
- <a class="anchor" id="Blasten2018"></a>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">Nonradiating sources and
- transmission eigenfunctions vanish at corners and
- edges.</span>
- <div class="ref-links">
- <a href="#desc_Blasten2018">Description</a>
- <a href="https://doi.org/10.1137/18M1182048">Publication</a>
- <a href="http://arxiv.org/abs/1803.10917/">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Pipes1"></a>
- <span class="ref-authors">F. Zouari, E. Blåsten,
- M. Louati, and M. S. Ghidaoui,</span>
- <span class="ref-title">Internal pipe area
- reconstruction as a tool for blockage
- detection.</span>
- <span class="ref-journal">Journal of Hydraulic
- Engineering, ASCE,</span>
- <span class="ref-year">accepted 2018.</span>
- <div class="ref-links">
- <a href="#desc_Pipes1">Description</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenLin2018"></a>
- <span class="ref-authors">E. Blåsten, Y.-H. Lin</span>
- <span class="ref-title">Radiating and non-radiating
- sources in elasticity.</span>
- <span class="ref-journal">Inverse Problems,</span>
- <span class="ref-year">accepted (2018).</span>
- <div class="ref-links">
- <a href="#desc_BlastenLin2018">Description</a>
- <a href="https://doi.org/10.1088/1361-6420/aae99e">Publication</a>
- <a href="http://arxiv.org/abs/1807.07225/">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Blasten2017"></a>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">Well-posedness of the Goursat
- problem and stability for point source inverse
- backscattering,</span>
- <span class="ref-journal">Inverse Problems,</span>
- <span class="ref-volume">33,</span>
- <span class="ref-issue">12,</span>
- <span class="ref-year">(2017)</span>
- <span class="ref-pages">125003.</span>
- <div class="ref-links">
- <a href="#desc_Blasten2017">Description</a>
- <a href="https://doi.org/10.1088/1361-6420/aa941f">Publication</a>
- <a href="https://arxiv.org/abs/1705.09442">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenLiu2017a"></a>
- <span class="ref-authors">E. Blåsten, H. Liu,</span>
- <span class="ref-title">On vanishing near corners of
- transmission eigenfunctions,</span>
- <span class="ref-journal">Journal of Functional
- Analysis,</span>
- <span class="ref-volume">273,</span>
- <span class="ref-issue">11</span>
- <span class="ref-year">(2017),</span>
- <span class="ref-pages">3616–3632.</span>
- <div class="ref-links">
- <a href="#desc_BlastenLiu2017a">Description</a>
- <a href="https://doi.org/10.1016/j.jfa.2017.08.023">Publication</a>
- <a href="https://arxiv.org/abs/1701.07957">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenLiLiuWang2017"></a>
- <span class="ref-authors">E. Blåsten, X. Li, H. Liu,
- Y. Wang,</span>
- <span class="ref-title"> On vanishing and localizing
- of transmission eigenfunctions near singular points:
- A numerical study,</span>
- <span class="ref-journal">Inverse Problems,</span>
- <span class="ref-volume">33,</span>
- <span class="ref-issue">10</span>
- <span class="ref-year">(2017),</span>
- <span class="ref-pages">105001.</span>
- <div class="ref-links">
- <a href="#desc_BlastenLiLiuWang2017">Description</a>
- <a href="https://doi.org/10.1088/1361-6420/aa8826">Publication</a>
- <a href="https://arxiv.org/abs/1704.01885">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenSylvester2016"></a>
- <span class="ref-authors">E. Blåsten, J. Sylvester,</span>
- <span class="ref-title">Translation-Invariant
- Estimates for Operators with Simple
- Characteristics,</span>
- <span class="ref-journal">Journal of Differential
- Equations,</span>
- <span class="ref-volume">263,</span>
- <span class="ref-issue">9</span>
- <span class="ref-year">(2017),</span>
- <span class="ref-pages">5656–5695.</span>
- <div class="ref-links">
- <a href="#desc_BlastenSylvester2016">Description</a>
- <a href="https://doi.org/10.1016/j.jde.2017.06.033">Publication</a>
- <a href="http://arxiv.org/abs/1607.06214">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenImanuvilovYamamoto2015"></a>
- <span class="ref-authors">E. Blåsten,
- O. Yu. Imanuvilov, M. Yamamoto,</span>
- <span class="ref-title">Stability and uniqueness for a
- two-dimensional inverse boundary value problem for
- less regular potentials,</span>
- <span class="ref-journal">Inverse Problems and
- Imaging,</span>
- <span class="ref-volume">9,</span>
- <span class="ref-issue">3</span>
- <span class="ref-year">(2015),</span>
- <span class="ref-pages">709–723.</span>
- <div class="ref-links">
- <a href="#desc_BlastenImanuvilovYamamoto2015">Description</a>
- <a href="http://dx.doi.org/10.3934/ipi.2015.9.709">Publication</a>
- <a href="http://arxiv.org/abs/1504.02207">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenPaivarintaSylvester2014"></a>
- <span class="ref-authors">E. Blåsten, L. Päivärinta,
- J. Sylvester,</span>
- <span class="ref-title">Corners Always Scatter,</span>
- <span class="ref-journal">Communications in
- Mathematical Physics,</span>
- <span class="ref-volume">331,</span>
- <span class="ref-issue">2</span>
- <span class="ref-year">(2014),</span>
- <span class="PUGpages">725–753.</span>
- <div class="ref-links">
- <a href="#desc_BlastenPaivarintaSylvester2014">Description</a>
- <a href="http://dx.doi.org/10.1007/s00220-014-2030-0">Publication</a>
- <a href="http://arxiv.org/abs/1211.1848">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="BlastenPaivarinta2013"></a>
- <span class="ref-authors">E. Blåsten,
- L. Päivärinta,</span>
- <span class="ref-title">Completeness of generalized
- transmission eigenstates,</span>
- <span class="ref-journal">Inverse Problems,</span>
- <span class="ref-volume">29,</span>
- <span class="ref-issue">10</span>
- <span class="ref-year">(2013),</span>
- <span class="ref-pages">104002.</span>
- <div class="ref-links">
- <a href="#desc_BlastenPaivarinta2013">Description</a>
- <a href="http://dx.doi.org/10.1088/0266-5611/29/10/104002">Publication</a>
- <a href="http://arxiv.org/abs/1307.4863">Preprint</a>
- </div>
- </li>
- </ol>
- </p>
- <p>
- <a class="anchor" id="pubs_proceedings"></a>
- <h3>Conference Proceedings</h3>
- <ol reversed>
- <li>
- <a class="anchor" id="PipesProceedings1"></a>
- <span class="ref-authors">F. Zouari, M. Louati,
- E. Blåsten, and M. S. Ghidaoui,</span>
- <span class="ref-title">Multiple defects detection and
- characterization in pipes.</span>
- <span class="ref-journal">13<sup>th</sup> International Conference on
- <span style="font-variant: small-caps">Pressure
- Surges</span>,</span>
- <span class="ref-year">Bordeaux, France,
- 14<sup>th</sup>–16<sup>th</sup> November
- 2018.</span>
- <div class="ref-links">
- <a href="#desc_PipesProceedings1">Description</a>
- </div>
- </li>
- </ol>
- </p>
- <p>
- <a class="anchor" id="pubs_theses"></a>
- <h3>Theses</h3>
- <ol reversed>
- <li>
- <a class="anchor" id="Blasten2013"></a>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">On the Gel’Fand-Calderón
- inverse problem in two dimensions,</span>
- <span class="ref-journal">Doctoral thesis, University
- of Helsinki, Faculty of Science, Department of
- Mathematics and Statistics,</span>
- <span class="ref-year">2013.</span>
- <div class="ref-links">
- <a href="#desc_Blasten2013">Description</a>
- <a href="http://urn.fi/URN:ISBN:978-952-10-8699-1">Publication</a>
- <a href="http://arxiv.org/abs/1307.4870">Preprint</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Blasten2010"></a>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">The Inverse Problem of the
- Schrödinger Equation in the Plane: A Dissection of
- Bukhgeim's Result,</span>
- <span class="ref-journal">Licentiate thesis,
- University of Helsinki, Faculty of Science,
- Department of Mathematics and Statistics,</span>
- <span class="ref-year">2010.</span>
- <div class="ref-links">
- <a href="#desc_Blasten2010">Description</a>
- <a href="http://arxiv.org/abs/1103.6200">arXiv version</a>
- </div>
- </li>
- <li>
- <a class="anchor" id="Blasten2008"></a>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">Distribuutioteorian alkeet ja
- sovellutuksia,</span>
- <span class="ref-journal">Master’s thesis, University
- of Helsinki, Faculty of Science, Department of
- Mathematics and Statistics,</span>
- <span class="ref-year">2008.</span>
- <div class="ref-links">
- <a href="#desc_Blasten2008">Description</a>
- <a href="pdf/blasten_master.pdf">PDF (in Finnish)</a>
- </div>
- </li>
- </ol>
- </p>
- <p>
- <a class="anchor" id="pubs_others"></a>
- <h3>Other publications</h3>
- <ol reversed>
- <li>
- <span class="ref-authors">E. Blåsten, H. Liu,</span>
- <span class="ref-title">Addendum to: "On vanishing
- near corners of transmission eigenfunctions",</span>
- <span class="ref-journal">arXiv:</span>
- <span class="ref-pages">1710.08089.</span>
- <div class="ref-links">
- <a href="https://arxiv.org/abs/1710.08089">Addendum</a>
- </div>
- </li>
- <li>
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">Matematiikkaa
- Venäjällä,</span>
- <span class="ref-journal">Solmu:
- matematiikkalehti,</span>
- <span class="ref-issue">3</span>
- <span class="ref-year">(2007).</span>
- <div class="ref-links">
- <a href="http://matematiikkalehtisolmu.fi/2007/3/Dubna.pdf">Article</a>
- </div>
- </li>
- </ol>
- </p>
- </section>
- <section>
- <a class="anchor" id="descriptions"></a>
- <h2>Description of projects</h2>
- <dl>
- <dt>
- <a class="anchor" id="desc_PipesProceedings1"></a>
- <a href="#PipesProceedings1">
- <span class="ref-authors">F. Zouari, M. Louati,
- E. Blåsten, and M. S. Ghidaoui,</span>
- <span class="ref-title">Multiple defects detection and
- characterization in pipes.</span>
- </a>
- </dt>
- <dd>
- <p>
- This is a continuation and generalization of the work
- done for pipe <a href="#Pipes1">area
- reconstruction</a>. We extend that method to the
- detection of leaks and discrete blockages. Because of
- a mathematical non-uniqueness to this one-dimensional
- inverse problem, certain configurations of defects
- cannot be uniquely identified from certain others. We
- propose a solution to this problem: measure the
- impulse-response function from both ends of the pipe.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Blasten-Li-Liu-Wang-2018"></a>
- <a href="#Blasten-Li-Liu-Wang-2018">
- <span class="ref-authors">E. Blåsten, H. Li, H. Liu, Y. Wang</span>
- <span class="ref-title">Localization and
- geometrization in plasmon resonances and geometric
- structures of Neumann-Poincaré eigenfunctions</span>
- </a>
- </dt>
- <dd>
- <p>
- This article provides a new point of view to plasmon
- resonance. The motivation comes from high curvature
- scattering by myself and
- Liu <a href="#Blasten-Liu-2018">[2018]</a>. Usually
- plasmon resonance occurs around particles that are
- smaller than the wavelength of the incident
- light. Previously we showed that small scatterers
- always scatter, but also that the same phenomenon
- occurs near certain high curvature points of large
- scatterers. In this manuscript we study several
- theoretical and numerical examples, and conclude that
- the same is true of plasmon resonance. In other words,
- plasmon resonance occurs not only for small particles,
- but also near high curvature points on the boundary of
- large particles!
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Blasten-Vesalainen-2018"></a>
- <a href="#Blasten-Vesalainen-2018">
- <span class="ref-authors">E. Blåsten, E. V. Vesalainen</span>
- <span class="ref-title">Non-Scattering Energies and
- Transmission Eigenvalues
- in <i>H<sup>n</sup></i></span>
- </a>
- </dt>
- <dd>
- <p>
- The article concerns scattering theory for the
- Helmholtz and Schrödinger equations in hyperbolic
- spaces. We show that corners always scatter there too
- despite there being so-called transmission
- eigenvalues, which we show also. This brings the
- results of myself, Päivärinta and
- Sylvester <a href="#BlastenPaivarintaSylvester2014">[2014]</a>,
- and Hu, Salo,
- Vesalainen <a href="http://dx.doi.org/10.1137/15M1032958">[2015]</a>
- into the setting of hyperbolic manifolds.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Blasten-Liu-2018"></a>
- <a href="#Blasten-Liu-2018">
- <span class="ref-authors">E. Blåsten, H. Liu</span>
- <span class="ref-title">Scattering by curvatures,
- radiationless sources, transmission eigenfunctions and
- inverse scattering problems</span>
- </a>
- </dt>
- <dd>
- <p>
- We study how high curvature affects wave
- scattering. Our previous work focused on having a
- corner point in the boundary of a penetrable
- scatterer, and this had various consequences: corners
- always scatter, the shape of polyhedral scatterers can
- be determined by a single measurement, and smooth
- enough transmission eigenfunctions vanish at convex
- corners. In essence these properties stay true with
- slight modifications if the corner is replaced by high
- curvature point.
- </p>
- <p>
- Our research shows a somewhat surprising result: an
- active source or inactive scatterer cannot produce a
- zero far-field if it is small (as measured by the
- wavenumber), or if its boundary has an admissible high
- curvature point. This fact is
- counter-intuitive. Surely small scatterers are barely
- seen, however the mathematical model implies that it
- cannot be completely invisible.
- </p>
- <p>
- An additional consequence of this study is that a
- single measurement of the complete far-field of a
- scattered wave determines the rough location and exact
- number of small well-separated scatterers. This gives
- a theoretical proof for the results by Griesmaier,
- Hanke,
- Raasch <a href="https://doi.org/10.1137/110855880">2013a</a>
- and <a href="https://doi.org/10.1137/130908658">2013b</a>.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_BlastenLin2018"></a>
- <a href="#BlastenLin2018">
- <span class="ref-authors">E. Blåsten, Y.-H. Lin</span>
- <span class="ref-title">Radiating and non-radiating
- sources in elasticity</span>
- </a>
- </dt>
- <dd>
- <p>
- We consider the inverse source problem for the Navier
- system, in other words linear isotropic elasticity. We
- consider a homogeneous background, and waves created
- by a force applied to part of the domain. The force
- can be inhomogeneous and anisotropic.
- </p>
- <p>
- We show two things. First of all the literature seemed
- to lack an example of a <em>non-radiating source</em>
- for elasticity. We show that a constant force applied
- on a disc causes no scattered elastic waves beyond the
- disc at a fixed frequency, if the radius of the disc,
- frequency and elastic parameters are in a given
- algebraic relation.
- </p>
- <p>
- Secondly, we show that if an inhomogeneous force is
- applied on a region which has a corner on its exterior
- boundary, and if the magnitude of the force is nonzero
- there, then no matter how the rest of region, or the
- rest of the force looks like, this force would always
- cause scattered waves that can be detected in the
- far-field.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Blasten2018"></a>
- <a href="#Blasten2018">
- <span class="ref-authors">E. Blåsten,</span>
- <span class="ref-title">Nonradiating sources and
- transmission eigenfunctions vanish at corners and
- edges</span>
- </a>
- </dt>
- <dd>
- <p>
- This paper considers the inverse source problem,
- namely we do not send an incident wave, and are
- instead using data provided by an unknown source
- itself. We show that so called <em>nonradiating
- sources</em> cannot have sharp corner or edge jumps on
- their boundary. This also implies a simpler proof for
- the result of <a href="#BlastenLiu2017a">Blåsten, Liu
- 2017a</a> assuming that the transmission
- eigenfunctions are smooth enough. Furthermore it gives
- uniqueness for the shape determination of convex
- polyhedral sources.
- </p>
- <p>
- The method has some similarities with Ikehata's
- enclosure method. A major difference is that we use a
- more complicated probing function which allows us to
- consider non-convex geometry. This function is the
- exponential of a complex square root. The branch is
- chosen such that the modulus of this function decays
- in all directions except the branch cut.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Pipes1"></a>
- <a href="#Pipes1">
- <span class="ref-authors">F. Zouari, E. Blåsten,
- M. Louati, and M. S. Ghidaoui,</span>
- <span class="ref-title">Internal pipe area
- reconstruction as a tool for blockage
- detection</span>
- </a>
- </dt>
- <dd>
- <p>
- Water supply systems around the cities of the world
- suffer from various defects occasionally: the pipes
- can deteriorate, start leaking or become blocked by
- mineral deposits. We introduce a new method for
- detecting the location and shape of such blockages
- from an impulse-response measurement at the pipe
- end. Such <em>non-invasive</em> methods avoid the
- expenses of having to dig out and open the pipe just
- to inspect it. The method is based on determining the
- internal cross-sectional area of the pipe at any
- location inside it.
- </p>
- <p>
- The area reconstruction method is introduced, both
- from an engineering point of view and a mathematical
- one. It is furthermore compared numerically to other
- state of the art blockage detection algorithms. We
- conclude that in noiseless numerical experiments this
- method is superior to others, especially when the
- number of blockages is unknown, or the blockages have
- irregular shape.
- </p>
- </dd>
- <dt>
- <a class="anchor" id="desc_Blasten2017"></a>
- <a href="#Blasten2017">
- <span class="ref-authors">Blåsten,</span>
- <span class="ref-title">Well-posedness of the Goursat
- problem and stability for point source inverse
- backscattering</span>
- </a>
- </dt>
- <dd>
- <p>
- The topic of this paper is on the inverse
- backscattering method by Rakesh and Uhlmann, which
- they use both for
- the <a href="https://doi.org/10.1088/0266-5611/30/6/065005">ordinary</a>
- and <a href="http://www.ams.org/books/conm/644/">point-source</a>
- backscattering. Their method lives in the time-domain
- and works for potentials that are <em>angularly
- controlled</em>. Our primary goal is to prove
- stability for their uniqueness result of point-source
- inverse backscattering.
- </p>
- <p>
- A difficulty arising in the proof relates to required
- norm-estimates. It is very difficult to find suitable
- estimates for the direct point-source problem, or a
- related problem which is called the Goursat problem.
- </p>
- <p>
- The Goursat problem is the wave equation with boundary
- data given on a characteristic cone. Some call it
- the <em>characteristic initial value problem</em>. We
- prove well-posedness for it. That is, we show that
- given the initial data and coefficients, the solution
- exists, is unique (in a chosen function space), and
- depends continuously on the inputs. This then leads to
- the well-posedness of the point-source problem for the
- wave equation.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenLiu2017b"></a>
- <a href="#BlastenLiu2017b">
- <span class="ref-authors">Blåsten, Liu,</span>
- <span class="ref-title">Recovering piecewise constant
- refractive indices by a single far-field
- pattern</span>
- </a>
- </dt>
- <dd>
- <p>
- We improve past corner scattering results in two
- ways. Compared to the earlier
- papers <a href="http://dx.doi.org/10.1137/15M1032958">Hu,
- Salo, Vesalainen 2015</a>
- and <a href="#BlastenLiu2016">Blåsten, Liu 2016</a>
- the results now apply to arbritrary three-dimensional
- convex polyhedra instead of just rectangular boxes. A
- second improvement is more useful: we show that a
- single far-field pattern not only determines the shape
- of a convex polyhedral scatterer, as known before, but
- also the values of the potential function at the
- vertices.
- </p>
- <p>
- Using the corner value recovery and Holmgren's
- uniqueness theorem we are able to show the following
- under certain geometric conditions: given a single
- incident wave, two polyhedral piecewise constant
- potentials produce the same far-field pattern only if
- the potentials are the same.
- </p>
- <p>
- Depending on application, the geometric a-priori
- conditions might be bothersome. However if the
- potentials are assumed to be piecewise constant on a
- square lattice, no matter how fine the mesh is, then
- our results apply. We hope that this would give ideas
- for novel reconstruction methods in engineering.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenTzouWang2017"></a>
- <a href="#BlastenTzouWang2017">
- <span class="ref-authors">Blåsten, Tzou, Wang,</span>
- <span class="ref-title">Uniqueness for the inverse
- boundary value problem with singular potentials in
- 2D</span>
- </a>
- </dt>
- <dd>
- <p>
- In this paper we improve the integrability condition
- used
- in <a href="#BlastenImanuvilovYamamoto2015">Blåsten,
- Imanuvilov, Yamamoto 2015</a>. We show that the
- Dirichlet-Neumann map of a potential defined on a
- two-dimensional bounded domain determines uniquely the
- potential. Previously this could be done for almost
- square-integrable potentials. In this paper we improve
- that bound up to almost 4/3-integrable potentials.
- </p>
- <p>
- Our improvement is based on two techniques. Firstly we
- use the fact that if two potentials have the same
- boundary data, then their difference is smoother (and
- hence more integrable) than either of the original
- potentials. Secondly we prove a more refined estimate
- for the Cauchy operators conjugated by a complex
- Gaussian function.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenLiLiuWang2017"></a>
- <a href="#BlastenLiLiuWang2017">
- <span class="ref-authors">Blåsten, Li, Liu, Wang,</span>
- <span class="ref-title"> On vanishing and localizing
- of transmission eigenfunctions near singular points:
- A numerical study</span>
- </a>
- </dt>
- <dd>
- <p>
- We study numerically the observations proved
- in <a href="#BlastenLiu2017a">Blåsten, Liu 2017</a> in
- a more general setting. Namely, we study where the
- energy of interior transmission eigenfunctions are
- located in domains with corners, edges or cusps.
- </p>
- <p>
- We find the following: transmission eigenfunctions
- indeed do vanish on corners and edges of various
- domain in two and three dimensions. The order of
- vanishing also seems to depend inversely on the
- magnitude of the angle. If the angle is less than π
- then the eigenfunction decays when approacing the
- corner. If the angle is larger than π then the
- eigenfunction blows up at the corner. This is a
- completely new finding in the study of the interior
- transmission problem.
- </p>
- <p>
- The method used for calculating the transmission
- eigenfunctions is by first transforming the interior
- transmission problem into an integral formulation, and
- then using a finite element approximation. The
- resulting sparse non-Hermitean discrete eigenvalue
- problem is solved by the <code>sptarn</code>-function
- in MATLAB.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenLiu2017a"></a>
- <a href="#BlastenLiu2017a">
- <span class="ref-authors">Blåsten, Liu,</span>
- <span class="ref-title"> On vanishing near corners of
- transmission eigenfunctions</span>
- </a>
- </dt>
- <dd>
- <p>
- We prove that transmission eigenfunctions carry
- geometrical information about their domain. Namely,
- that under some conditions the eigenfunctions vanish
- at corner points. To achieve this we also show a lower
- bound for the energy of the far-field pattern of a
- Herglotz wave scattered by penetrable polygonal
- scatterer. We have written an
- addendum <a href="https://arxiv.org/abs/1710.08089">arXiv:1710.08089</a>
- that relaxes the assumptions of the theorem.
- </p>
- <p>
- This is a first result showing intrinsic properties of
- the transmission eigenfunctions. Just as in spectral
- theory, the transmission eigenvalues have been studied
- in great detail previously. Big challenges have
- prevented touching the eigenfunctions until now.
- </p>
- <p>
- A related numerical work, which is still in progress,
- raises many interesting theoretical questions: what's
- the effect on the angle of the corner? what about
- complex transmission eigenvalues? what about
- localization?
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenLiu2016"></a>
- <a href="#BlastenLiu2016">
- <span class="ref-authors">Blåsten, Liu,</span>
- <span class="ref-title">On corners scattering stably
- and stable shape determination by a single far-field
- pattern</span>
- </a>
- </dt>
- <dd>
- <p>
- In this work we prove stability estimates for
- penetrable potential support recovery and also corner
- scattering by incident plane-waves. The corresponding
- uniqueness results are
- in <a href="#BlastenPaivarintaSylvester2014">[BPS2014]</a>,
- <a href="https://arxiv.org/abs/1404.2513">[PSV]</a>
- and <a href="http://dx.doi.org/10.1137/15M1032958">[HSV2016]</a>.
- </p>
- <p>
- A new key tool we show is a quantitative Rellich's
- theorem for penetrable scatterers. This means that if
- the scattered waves created by two potentials are
- close to each other at infinity, then the values of
- the these waves are close to each other on the
- boundary of the obstacle.
- </p>
- <p>
- The stability of corner scattering means that given
- any incident plane-wave of unit modulus, and a
- penetrable potential having a sharp corner, the energy
- of the far-field pattern has a uniform lower
- bound. This raises questions on whether interior
- transmission eigenfunctions could in fact be
- approximated by the more general Herglotz incident
- waves as is commonly believed.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenSylvester2016"></a>
- <a href="#BlastenSylvester2016">
- <span class="ref-authors">Blåsten, Sylvester,</span>
- <span class="ref-title">Translation-Invariant
- Estimates for Operators with Simple
- Characteristics</span>
- </a>
- </dt>
- <dd>
- <p>
- The goal of this work is to prove new estimates for
- constant coefficient partial differential
- equations. The estimates have a simple formulation
- (they are seminorm estimates) and are invariant under
- geometric transformations of the Euclidean space. Once
- a solution to an inhomogeneous PDE satisfies our
- estimate the Agmon-Hörmander estimates from scattering
- theory are a trivial corollary. We prove the estimates
- for a wide class of PDE's, including all second order
- operators with real coefficients, and give some
- examples showing that the method can still be
- generalized, for example to systems of PDE's.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenImanuvilovYamamoto2015"></a>
- <a href="#BlastenImanuvilovYamamoto2015">
- <span class="ref-authors">Blåsten, Imanuvilov, Yamamoto,</span>
- <span class="ref-title">Stability and uniqueness for a
- two-dimensional inverse boundary value problem for
- less regular potentials</span>
- </a>
- </dt>
- <dd>
- <p>
- This article combines methods from my PhD
- thesis <a class="referenceNbr"
- href="#Blasten2013">[2013]</a> and a manuscript by
- Imanuvilov and Yamamoto <a class="referenceNbr"
- href="http://arxiv.org/abs/1208.3775">[2012]</a> to
- solve the Gel'Fand-Calderón problem. The new unified
- method allowed for a simpler proof of uniqueness for
- potentials which are just integrable and stability for
- potentials having a fractional derivative of any
- positive order.
- </p>
- <p>
- The change from Bukhgeim's
- method <a class="referenceNbr"
- href="http://dx.doi.org/10.1515/jiip.2008.002">[2008]</a>
- is a smarter choice for the first two terms in the
- Neumann series of the complex geometric optics
- solutions.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenPaivarintaSylvester2014"></a>
- <a href="#BlastenPaivarintaSylvester2014">
- <span class="ref-authors">Blåsten, Päivärinta,
- Sylvester,</span>
- <span class="ref-title">Corners Always Scatter</span>
- </a>
- </dt>
- <dd>
- <p>
- In this paper we proved that in single-frequency
- Helmholtz scattering, no matter which incident wave
- (Herglotz wave) and no matter at which frequency, a
- potential with a jump shaped like a 90° sharp
- corner will always produce a non-trivial scattered
- wave. This implies that transmission eigenvalues are
- not the same as <em>non-scattering energies</em>,
- i.e. energies at which the the relative scattering
- operator (which maps the asymptotics of the incident
- wave to the asymptotics of the scattered wave) has a
- non-trivial kernel. This is in contrast to the
- radially symmetric case where these two concepts were
- known to be the same.
- </p>
- <p>
- A key-ingredient for the proof is a new resolvent-type
- estimate that uses norms from the Fourier transforms
- of Besov spaces. These give a better local
- integrability than past estimates by Agmon and
- Hörmander <a class="referenceNbr"
- href="http://dx.doi.org/10.1007/BF02786703">[1976]</a>
- or Sylvester and Uhlmann <a class="referenceNbr"
- href="http://dx.doi.org/10.2307/1971291">[1987]</a>.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_BlastenPaivarinta2013"></a>
- <a href="#BlastenPaivarinta2013">
- <span class="ref-authors">Blåsten, Päivärinta,</span>
- <span class="ref-title">Completeness of generalized
- transmission eigenstates</span>
- </a>
- </dt>
- <dd>
- <p>
- This article answers a question posed by Cakoni,
- Gintides and
- Haddar <a href="http://dx.doi.org/10.1137/090769338"
- class="referenceNbr">[2010]</a>:
- <blockquote>
- Although the results of this paper provide an
- important step forward in understanding the spectral
- properties of the interior transmission problem,
- many questions still remain. We think that some
- interesting open problems in this direction are ...
- and the completeness of the eigensystem of the
- interior transmission problem.
- </blockquote>
- </p>
- <p>
- We proved that the eigensystems associated with all
- the transmission eigenvalues form a complete set. We
- used the consept of <em>parameter-ellipticity</em> by
- Agranovich and Vishik, analytic Fredholm theory, and
- Nevanlinna theory for operator-valued meromorphic
- functions. These imply that a fourth-order operator
- related to the transmission eigenvalue problem
- satisfies three assumptions that are stated in the
- language of meromorphic functions. These then imply
- the completeness of the eigensystem.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_Blasten2013"></a>
- <a href="#Blasten2013">
- <span class="ref-authors">Blåsten,</span>
- <span class="ref-title">On the Gel’Fand-Calderón
- inverse problem in two dimensions</span>
- </a>
- </dt>
- <dd>
- <p>
- This is my PhD thesis, succesfully defended at the
- University of Helsinki in 2013. In it I prove
- stability for the Gel'Fand-Calderón problem with
- rough, or non-smooth potential, using the methods of
- Bukhgeim <a class="referenceNbr"
- href="http://dx.doi.org/10.1515/jiip.2008.002">[2008]</a>.
- </p>
- <p>
- The problem of uniqueness for the Gel'Fand-Calderón
- problem had been open in two dimensions for at least
- twenty years and Bukhgeim was the first one to manage
- to solve it for a general potential. His arguments
- work when the potential has one derivative. Using
- function theory and interpolation theory, I managed to
- prove stability in addition to uniqueness, and for
- potentials that have any positive fractional
- derivative.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_Blasten2010"></a>
- <a href="#Blasten2010">
- <span class="ref-authors">Blåsten,</span>
- <span class="ref-title">The Inverse Problem of the
- Schrödinger Equation in the Plane: A Dissection of
- Bukhgeim's Result</span>
- </a>
- </dt>
- <dd>
- <p>
- In this licentiate thesis I studied and explained
- Bukhgeim's solution to the Gel'Fand-Calderón problem
- in two dimensions <a class="referenceNbr"
- href="http://dx.doi.org/10.1515/jiip.2008.002">[2008]</a>.
- </p>
- <p>
- The Finnish licentiate degree is an elective
- intermediate degree between master's degree, which is
- five years of university-level study, and the doctor's
- degree, which totals nine to ten years of study.
- </p>
- </dd>
-
- <dt>
- <a class="anchor" id="desc_Blasten2008"></a>
- <a href="#Blasten2008">
- <span class="ref-authors">Blåsten,</span>
- <span class="ref-title">Distribuutioteorian alkeet ja
- sovellutuksia</span>
- </a>
- </dt>
- <dd>
- <p>
- An English translation of the title would be <q>An
- introduction to the theory of distributions and its
- applications</q>. This is my master's thesis, written
- in Finnish. In it I state and prove elementary results
- from distribution theory. The more advanced topics
- include the Fourier transform, how to define the
- division of arbitrary polynomials in one dimension and
- the division by homogeneous polynomials in arbitrary
- dimensions.
- </p>
- </dd>
- </dl>
- </section>
- <section>
- <a class="anchor" id="talks"></a>
- <p>
- <h2>Conference and Seminar Talks</h2>
- <ol reversed>
- <li>
- <span class="ref-title">Inverse problems with one
- measurement,</span>
- <span class="ref-journal">Inverse Days 2018; Aalto
- University, Finland,</span>
- <span class="ref-year">12 December 2018.</span>
- <div class="ref-links">
- <a href="https://math.aalto.fi/en/research/applied/inverseproblems/inverse_days-2018/">Event</a>
- <a href="pdf/blasten_talk_2018_helsinki.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Inverse problems with one
- measurement,</span>
- <span class="ref-journal">Inverse problems, PDE and
- geometry; University of Jyväskylä, Finland,</span>
- <span class="ref-year">22 August 2018.</span>
- <div class="ref-links">
- <a href="https://www.jyu.fi/science/en/maths/research/inverse-problems/inverse-problems-pde-and-geometry-2018">Event</a>
- <a href="pdf/blasten_talk_2018_jyvaskyla.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Inverse problems with one
- measurement,</span>
- <span class="ref-journal">The 9th International
- Conference on Inverse Problems and Related Topics;
- National University of Singapore, Singapore,</span>
- <span class="ref-year">15 August 2018.</span>
- <div class="ref-links">
- <a href="https://www.ece.nus.edu.sg/stfpage/elechenx/Conference/index.htm">Event</a>
- <a href="pdf/blasten_talk_2018_singapore.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Applications of corner
- scattering: intrinsic properties of transmission
- eigenfunctions and single wave probing,</span>
- <span class="ref-journal">School of Mathematical
- Sciences, Fudan University, China,</span>
- <span class="ref-year">5 December 2017.</span>
- <div class="ref-links">
- <a href="http://math.fudan.edu.cn/show.aspx?info_lb=765&flag=527&info_id=5565">Event</a>
- <a href="pdf/blasten_talk_2017_shanghai.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Planar inverse boundary value
- problem for <i>L<sup>p</sup></i> potentials
- with <i>p>4/3</i>,</span>
- <span class="ref-journal">Analysis Seminar, Department
- of Mathematics and Statistics; University of
- Jyväskylä, Finland,</span>
- <span class="ref-year">23 August 2017.</span>
- </li>
- <li>
- <span class="ref-title">Inverse backscattering with
- point-source waves,</span>
- <span class="ref-journal">Inverse Problems Seminar,
- Department of Mathematics and Statistics; University
- of Helsinki, Finland,</span>
- <span class="ref-year">17 August 2017.</span>
- <div class="ref-links">
- <a href="pdf/blasten_talk_2017_UH.pdf">Notes</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Corners always scatter —
- quantitative results,</span>
- <span class="ref-journal">Applied Inverse Problems
- Conference 2017; Zhejiang University, Hangzhou,
- China,</span>
- <span class="ref-year">31 May 2017.</span>
- <div class="ref-links">
- <a href="http://aip2017.csp.escience.cn/dct/page/70004">Programme</a>
- <a href="http://aip2017.csp.escience.cn/dct/page/1">Event</a>
- <a href="pdf/blasten_talk_2017_AIP.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Transmission eigenfunction
- localization,</span>
- <span class="ref-journal">Annual meeting of the Hong
- Kong Mathematical Society; The Hong Kong University
- of Science and Technology, Hong Kong,</span>
- <span class="ref-year">20 May 2017.</span>
- <div class="ref-links">
- <a href="pdf/blasten_talk_2017_HKMS.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Topics in Corner Scattering:
- Non-Scattering Waves, Potential Probing with a
- Single Incident Wave, and the Interior Transmission
- Problem,</span>
- <span class="ref-journal">NCTS PDE and Analysis
- Seminar; National Center for Theoretical Sciences,
- National Taiwan University, Taipei, Taiwan,</span>
- <span class="ref-year">9 Marh 2017.</span>
- <div class="ref-links">
- <a href="pdf/blasten_talk_2017_taipei.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Inverse scattering using a
- single incident wave,</span>
- <span class="ref-journal">2<sup>nd</sup> East Asia
- Section of IPIA, Young Scholars Symposium; National
- Center for Theoretical Sciences, National Taiwan
- University, Taipei, Taiwan,</span>
- <span class="ref-year">5 November 2016.</span>
- <div class="ref-links">
- <a href="pdf/taipei2016_programme.pdf">Programme</a>
- <a href="pdf/blasten_talk_2016_taipei.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Non-scattering energies, new
- resolvent estimates and other projects,</span>
- <span class="ref-journal">1<sup>st</sup> East Asia
- Symposium of IPIA; South University of Science and
- Technology, Shenzhen, China,</span>
- <span class="ref-year">29 February 2016.</span>
- <div class="ref-links">
- <a href="pdf/shenzhen2016_programme.pdf">Programme</a>
- <a href="pdf/blasten_talk_2016_shenzhen.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Non-scattering energies and
- interior transmission eigenvalues,</span>
- <span class="ref-journal">Workshop on Inverse Problems
- and Related Topics; Zhejiang University, Hangzhou,
- China,</span>
- <span class="ref-year">9 December 2015.</span>
- <div class="ref-links">
- <a href="pdf/hangzhou2015_programme.pdf">Programme</a>
- <a href="pdf/blasten_talk_2015_hangzhou.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">A new viewpoint to scattering
- theory à la Hörmander,</span>
- <span class="ref-journal">Spectral and Analytic
- Inverse Problems, Thematic Programme on Inverse
- Problems; Institut Henri Poincaré, Paris,
- France,</span>
- <span class="ref-year">4 May 2015.</span>
- <div class="ref-links">
- <a href="http://www.ihp.fr/en/CEB/T2-2015">Programme</a>
- <a href="http://www.ihp.fr/en/CEB/T2-2015/workshop1">Event</a>
- <a href="pdf/blasten_talk_2015_paris.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Solving the Inverse Problem
- for the 2D Schrödinger Equation with
- Lp-potential,</span>
- <span class="ref-journal">17<sup>th</sup> Annual
- Workshop on Applications and generalizations of
- complex analysis; University of Aveiro, Aveiro,
- Portugal,</span>
- <span class="ref-year">21 March 2015.</span>
- <div class="ref-links">
- <a href="http://sweet.ua.pt/pceres/complex2015/Webpage/Workshop.html">Event</a>
- <a href="pdf/blasten_talk_2015_aveiro.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Solving the Inverse Problem
- for the 2D Schrödinger Equation with
- Lp-potential,</span>
- <span class="ref-journal">The 10<sup>th</sup> AIMS
- Conference on Dynamical Systems, Differential
- Equations and Applications; Instituto de Ciencias
- Matemáticas (ICMAT) and the Universidad Autónoma de
- Madrid (UAM), Madrid, Spain,</span>
- <span class="ref-year">9 July 2014.</span>
- <div class="ref-links">
- <a href="http://aimsciences.org/conferences/2014/">Event</a>
- <a href="pdf/blasten_talk_2014_madrid.pdf">Talk</a>
- </div>
- </li>
- <li>
- <span class="ref-title">Completeness of the
- generalized transmission eigenstates,</span>
- <span class="ref-journal">International Conference on
- Novel Directions in Inverse Scattering; University
- of Delaware, Delaware, USA,</span>
- <span class="ref-year">29 July 2013.</span>
- <div class="ref-links">
- <a href="http://www.cmap.polytechnique.fr/~colton/">Event</a>
- <a href="pdf/blasten_talk_2013_delaware.pdf">Talk</a>
- </div>
- </li>
- </ol>
- </p>
- </section>
- </section>
- </div>
- </boby>
- </html>
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