#### DOI

10.17077/etd.nuavskz5

#### Document Type

Dissertation

#### Date of Degree

Spring 2011

#### Degree Name

PhD (Doctor of Philosophy)

#### Degree In

Mathematics

#### First Advisor

Raul Curto

#### First Committee Member

Muhly Paul

#### Second Committee Member

Richard Baker

#### Third Committee Member

Palle Jorgensen

#### Fourth Committee Member

Victor Camillo

#### Abstract

Inverse problems naturally occur in many branches of science and mathematics. An inverse problem entails finding the values of one or more parameters using the values obtained from observed data. A typical example of an inverse problem is the inversion of the Radon transform. Here a function (for example of two variables) is deduced from its integrals along all possible lines. This problem is intimately connected with image reconstruction for X-ray computerized tomography.

Moment problems are a special class of inverse problems. While the classical theory of moments dates back to the beginning of the 20th century, the systematic study of *truncated* moment problems began only a few years ago. In this dissertation we will first survey the elementary theory of truncated moment problems, and then focus on those problems with cubic column relations.

For a degree *2n* real *d*-dimensional multisequence β ≡ β * ^{(2n)}* ={β

*}*

_{i}*to have a representing measure μ, it is necessary for the*

_{i∈Zd+,|i|≤2n}*associated moment matrix Μ(n)*to be positive semidefinite, and for the

*algebraic variety*associated to β,

*V*, to satisfy rank

_{β}*Μ(n)*≤ card

*V*as well as the following

_{β}*consistency condition*: if a polynomial

*p(x*)≡

*∑*vanishes on

_{|i|≤2n}a_{i}x^{i}*V*, then Λ

_{β}*(p)*:=∑

*=0. In 2005, Professor Raúl Curto collaborated with L. Fialkow and M. Möller to prove that for the*

_{|i|≤2n}a_{i}β_{i}*extremal*case (

*Μ(n)= V*), positivity and consistency are sufficient for the existence of a (unique, rank

_{β}*Μ(n)*-atomic) representing measure.

In joint work with Professor Raúl Curto we have considered cubic column relations in *M*(3) of the form (in complex notation) *Z*^{3}=*itZ*+u*bar Z*, where *u* and *t* are real numbers. For *(u,t)* in the interior of a real cone, we prove that the algebraic variety *V _{β}* consists of exactly

*7*points, and we then apply the above mentioned solution of the extremal moment problem to obtain a necessary and sufficient condition for the existence of a representing measure. This requires a new representation theorem for sextic polynomials in

*Z*and

*bar Z*which vanish in the

*7*-point set

*V*. Our proof of this representation theorem relies on two successive applications of the Fundamental Theorem of Linear Algebra. Finally, we use the Division Algorithm from algebraic geometry to extend this result to other situations involving cubic column relations.

_{β}#### Keywords

Algebraic Variety, Extremal Truncated Moment Problem, Moment Matrix Extension, Positive Semidefinite, Riesz Functional

#### Pages

ix, 136 pages

#### Bibliography

Includes bibliographical references (pages 134-136).

#### Copyright

Copyright 2011 Seonguk Yoo

#### Recommended Citation

Yoo, Seonguk. "Extremal sextic truncated moment problems." PhD (Doctor of Philosophy) thesis, University of Iowa, 2011.

https://doi.org/10.17077/etd.nuavskz5