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Waring-type problems for polynomials: Algebra meets GeometryPrimeFaces.cw("AccordionPanel","widget_formSmash_some",{id:"formSmash:some",widgetVar:"widget_formSmash_some",multiple:true}); PrimeFaces.cw("AccordionPanel","widget_formSmash_all",{id:"formSmash:all",widgetVar:"widget_formSmash_all",multiple:true});
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PrimeFaces.cw("AccordionPanel","widget_formSmash_responsibleOrgs",{id:"formSmash:responsibleOrgs",widgetVar:"widget_formSmash_responsibleOrgs",multiple:true}); 2016 (English)Doctoral thesis, monograph (Other academic)
##### Abstract [en]

##### Place, publisher, year, edition, pages

Stockholm: Department of Mathematics, Stockholm University , 2016. , 120 p.
##### Keyword [en]

polynomials, Waring problems, Hilbert series, fat points, power ideals, secant varieties
##### National Category

Algebra and Logic Geometry
##### Research subject

Mathematics
##### Identifiers

URN: urn:nbn:se:su:diva-129019ISBN: 978-91-7649-424-0OAI: oai:DiVA.org:su-129019DiVA: diva2:919009
##### Public defence

2016-06-03, Sal 14, hus 5, Kräftriket, Roslasvägen 101, Stockholm, 13:00 (English)
##### Opponent

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Available from: 2016-05-11 Created: 2016-04-12 Last updated: 2016-05-12Bibliographically approved

In the present thesis we analyze different types of additive decompositions of homogeneous polynomials. These problems are usually called *Waring-type problems* and their story go back to the mid-19th century and, recently, they received the attention of a large community of mathematicians and engineers due to several applications. At the same time, they are related to branches of Commutative Algebra and Algebraic Geometry.

The classical Waring problem investigates decompositions of homogeneous polynomials as sums of powers of linear forms. Via Apolarity Theory, the study of these decompositions for a given polynomial *F* is related to the study of configuration of points *apolar* to *F*, namely, configurations of points whose defining ideal is contained in the *``perp'' ideal* associated to *F*. In particular, we analyze which kind of minimal set of points can be apolar to some given polynomial in cases with small degrees and small number of variables. This let us introduce the concept of *Waring loci* of homogeneous polynomials.

From a geometric point of view, questions about additive decompositions of polynomials can be described in terms of *secant varieties* of projective varieties. In particular, we are interested in the dimensions of such varieties. By using an old result due to Terracini, we can compute these dimensions by looking at the *Hilbert series* of homogeneous ideal.

Hilbert series are very important algebraic invariants associated to homogeneous ideals. In the case of classical Waring problem, we have to look at *power ideals*, i.e., ideals generated by powers of linear forms. Via Apolarity Theory, their Hilbert series are related to Hilbert series of ideals of *fat points*, i.e., ideals of configurations of points with some multiplicity. In this thesis, we consider some special configuration of fat points. In general, Hilbert series of ideals of fat points is a very active field of research. We explain how it is related to the famous Fröberg's conjecture about Hilbert series of generic ideals.

Moreover, we use Fröberg's conjecture to deduce the dimensions of several secant varieties of particular projective varieties and, then, to deduce results regarding some particular Waring-type problems for polynomials.

In this thesis, we mostly work over the complex numbers. However, we also analyze the case of classical Waring decompositions for monomials over the real numbers. In particular, we classify for which monomials the minimal length of a decomposition in sum of powers of linear forms is independent from choosing the ground field as the field of complex or real numbers.

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