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The cones of nonnegative polynomials and sums of squares arise as central objects in convex algebraic geometry and have their origin in the seminal work of Hilbert ([Hil88]). Depending on the number of variables n and the degree d of the polynomials, Hilbert famously characterizes all cases of equality between the cone of nonnegative polynomials and the cone of sums of squares. This equality precisely holds for bivariate forms, quadratic forms and ternary quartics ([Hil88]). Since then, a lot of work has been done in understanding the difference between these two cones, which has major consequences for many practical applications such as for polynomial optimization problems. Roughly speaking, minimizing polynomial functions (constrained as well as unconstrained) can be done efficiently whenever certain nonnegative polynomials can be written as sums of squares (see Section 2.3 for the precise relationship). The underlying reason is the fundamental difference that checking nonnegativity of polynomials is an NP-hard problem whenever the degree is greater or equal than four ([BCSS98]), whereas checking whether a polynomial can be written as a sum of squares is a semidefinite feasibility problem (see Section 2.2). Although the complexity status of the semidefinite feasibility problem is still an open problem, it is polynomial for fixed number of variables. Hence, understanding the difference between nonnegative polynomials and sums of squares is highly desirable both from a theoretical and a practical viewpoint.

This work is concerned with two topics at the intersection of convex algebraic geometry and optimization.
We develop a new method for the optimization of polynomials over polytopes. From the point of view of convex algebraic geometry the most common method for the approximation of polynomial optimization problems is to solve semidefinite programming relaxations coming from the application of Positivstellensätze. In optimization, non-linear programming problems are often solved using branch and bound methods. We propose a fused method that uses Positivstellensatz-relaxations as lower bounding methods in a branch and bound scheme. By deriving a new error bound for Handelman's Positivstellensatz, we show convergence of the resulting branch and bound method. Through the application of Positivstellensätze, semidefinite programming has gained importance in polynomial optimization in recent years. While it arises to be a powerful tool, the underlying geometry of the feasibility regions (spectrahedra) is not yet well understood. In this work, we study polyhedral and spectrahedral containment problems, in particular we classify their complexity and introduce sufficient criteria to certify the containment of one spectrahedron in another one.

The results of this thesis lie in the area of convex algebraic geometry, which is the intersection of real algebraic geometry, convex geometry, and optimization.
We study sums of nonnegative circuit polynomials (SONC) and their related cone, both geometrically and in application to polynomial optimization. SONC polynomials are certain sparse polynomials having a special structure in terms of their Newton polytopes and supports, and serve as a certificate of nonnegativity for real polynomials, which is independent of sums of squares.
The first part of this thesis is dedicated to the convex geometric study of the SONC cone. As main results we show that the SONC cone is full-dimensional in the cone of nonnegative polynomials, we exactly determine the number of zeros of a nonnegative circuit polynomial, and we give a complete and explicit characterization of the number of zeros of SONC polynomials and forms. Moreover, we provide a first approach to the study of the exposed faces of the SONC cone and their dimensions.
In the second part of the thesis we use SONC polynomials to tackle constrained polynomial optimization problems (CPOPs).
As a first step, we derive a lower bound for the optimal value of CPOP based on SONC polynomials by using a single convex optimization program, which is a geometric program (GP) under certain assumptions. GPs are a special type of convex optimization problems and can be solved in polynomial time. We test the new method experimentally and provide examples comparing our new SONC/GP approach with Lasserre's relaxation, a common approach for tackling CPOPs, which approximates nonnegative polynomials via sums of squares and semidefinite programming (SDP). The new approach comes with the benefit that in practice GPs can be solved significantly faster than SDPs. Furthermore, increasing the degree of a given problem has almost no effect on the runtime of the new program, which is in sharp contrast to SDPs.
As a second step, we establish a hierarchy of efficiently computable lower bounds converging to the optimal value of CPOP based on SONC polynomials. For a given degree each bound is computable by a relative entropy program. This program is also a convex optimization program, which is more general than a geometric program, but still efficiently solvable via interior point methods.

In this thesis we introduce the imaginary projection of (multivariate) polynomials as the projection of their variety onto its imaginary part, I(f) = { Im(z_1, ... , z_n) : f(z_1, ... , z_n) = 0 }. This induces a geometric viewpoint to stability, since a polynomial f is stable if and only if its imaginary projection does not intersect the positive orthant. Accordingly, the thesis is mainly motivated by the theory of stable polynomials.
Interested in the number and structure of components of the complement of imaginary projections, we show as a key result that there are only finitely many components which are all convex. This offers a connection to the theory of amoebas and coamoebas as well as to the theory of hyperbolic polynomials.
For hyperbolic polynomials, we show that hyperbolicity cones coincide with components of the complement of imaginary projections, which provides a strong structural relationship between these two sets. Based on this, we prove a tight upper bound for the number of hyperbolicity cones and, respectively, for the number of components of the complement in the case of homogeneous polynomials. Beside this, we investigate various aspects of imaginary projections and compute imaginary projections of several classes explicitly.
Finally, we initiate the study of a conic generalization of stability by considering polynomials whose roots have no imaginary part in the interior of a given real, n-dimensional, proper cone K. This appears to be very natural, since many statements known for univariate and multivariate stable polynomials can be transferred to the conic situation, like the Hermite-Biehler Theorem and the Hermite-Kakeya-Obreschkoff Theorem. When considering K to be the cone of positive semidefinite matrices, we prove a criterion for conic stability of determinantal polynomials.

In recent years using symmetry has proven to be a very useful tool to simplify computations in semidefinite programming. This dissertation examines the possibilities of exploiting discrete symmetries in three contexts: In SDP-based relaxations for polynomial optimization, in testing positivity of symmetric polynomials, and combinatorial optimization. In these contexts the thesis provides new ways for exploiting symmetries and thus deeper insight in the paradigms behind the techniques and studies a concrete combinatorial optimization question.

Mixed volumes, mixed Ehrhart theory and applications to tropical geometry and linkage configurations
(2009)

The aim of this thesis is the discussion of mixed volumes, their interplay with algebraic geometry, discrete geometry and tropical geometry and their use in applications such as linkage configuration problems. Namely we present new technical tools for mixed volume computation, a novel approach to Ehrhart theory that links mixed volumes with counting integer points in Minkowski sums, new expressions in terms of mixed volumes of combinatorial quantities in tropical geometry and furthermore we employ mixed volume techniques to obtain bounds in certain graph embedding problems.

Tropical geometry is the geometry of the tropical semiring \[\mathbb{T}:=(\mathbb{R}\cup\{\infty\},\min,+).\] Classical algebraic structures correspond to tropical structures. If $I\lhd K[x_1,\ldots,x_n]$ is an ideal in a polynomial ring over a field $K$ with valuation $v$, then the classical algebraic variety correspond to the tropical variety $T(I)$. It is the set of all points $w$, such that the minimum $\min\{v(c_\alpha)+w\cdot\alpha\}$ is achieved twice for all $f=\sum_\alpha c_\alpha x^\alpha\in I$. So tropical geometry relates algebraic geometric problems with discrete geometric problems. In this thesis we obtain a tropical version of the Eisenbud-Evans Theorem which states that every algebraic variety in $\mathbb{R}^n$ is the intersection of $n$ hypersurfaces. We find out that in the tropical setting every tropical variety $T(I)$ can be written as an intersection of only $(n+1)$ tropical hypersurfaces. So we get a finite generating system of $I$ such that the corresponding tropical hypersurfaces intersect to the tropical variety, a so-called tropical basis. Let $I \lhd K[x_1,\ldots,x_n]$ be a prime ideal generated by the polynomials $f_1, \ldots, f_r$. Then there exist $g_0,\ldots,g_{n} \in I$ such that \[ T(I) \ = \ \bigcap_{i=0}^{n}T(g_i)\] and thus $\mathcal{G} := \{f_1, \ldots, f_r, g_0, \ldots, g_{n}\}$ is a tropical basis for $I$ of cardinality $r+n+1$. Tropical bases are discussed by Bogart, Jensen, Speyer, Sturmfels and Thomas where it is shown that tropical bases of linear polynomials of a linear ideal have to be very large. We do not restrict the tropical basis to consist of linear polynomials and therefore we get a shorter tropical basis. But the degrees of our polynomials can be very large. The main ingredient to get a short tropical basis is the use of projections, in particular geometrically regular projections. Together with the fact that preimages of projections of tropical varieties are themselves tropical varieties of a certain elimination ideal we get the desired result. Let $I \lhd K[x_1, \ldots, x_n]$ be an $m$-dimensional prime ideal and $\pi : \mathbb{R}^n \to \mathbb{R}^{m+1}$ be a rational projection. Then $\pi^{-1}(\pi(T(I)))$ is a tropical variety, namely \[ \pi^{-1}(\pi(T(I))) \ = \ T(J \cap K[x_1, \ldots, x_n]) \,\] Here $J$ is an ideal in $K[x_1,\ldots,x_n,\lambda_1,\ldots,\lambda_{n-m-1}]$ derived from the ideal $I$. We show that this elimination ideal is a principal ideal which yields a polynomial in our tropical basis. The advantage of our method is that we find our polynomials by projections and therefore we can use the results of Gelfand, Kapranov and Zelevinsky , of Esterov and Khovanskii , and of Sturmfels, Tevelev and Yu. With mixed fiber polytopes we get the structure and combinatorics of the image of a tropical variety and therefore the structure of the polynomials in our tropical basis. Let $I=\lhd K[x_1,\ldots,x_n]$ an $m$-dimensional ideal, generated by generic polynomials $f_1,\ldots, f_{n-m}$, $\pi:\mathbb{R}^n\to\mathbb{R}^{m+1}$ a projection and $\psi$ a projection presented by a matrix with a rowspace equal to the kernel of $\pi$. Then up to affine isomorphisms, the cells of the dual subdivision of $\pi^{-1} \pi T(I)$ are of the form \[ \sum_{i=1}^p \Sigma_{\psi} (C_{i1}^{\vee}, \ldots, C_{i{k}}^{\vee}) \] for some $p\in\mathbb{N}$ and faces $F_1, \ldots, F_p$ of $T(f_1)\cap\ldots\cap T(f_k)$ and the dual cell of $F_i\subseteq U = T(f_1)\cup\ldots\cup T(f_k)$ is given by $F_i^\vee=C_{i1}^{\vee}+ \ldots+ C_{ik}^{\vee}$ with faces $C_{i1}, \ldots, C_{i k}$ of $T(f_1), \ldots, T(f_{k})$. In case that we project a tropical curve we want to find the number of $(n-1)$-cells of the above form with $p>1$, i.e. the cells which are dual to vertices of $\pi(T(I))$ which are the intersection of the images of two non-adjacent $1$-cells of $T(I)$. Vertices of this type are called selfintersection points. We show that there exist a tropcal line $L_n\subset\mathbb{R}^n$ and a projection $\pi:\mathbb{R}^n\to\mathbb{R}^2$, such that $L_n$ has $\sum_{i=1}^{n-2}i$ selfintersection points. Furthermore we find tropical curves $\mathcal{C}\subset\mathbb{R}^n$, which are transversal intersections of $n-1$ tropical hypersurfaces of degrees $d_1,\ldots,d_{n-1}$ and a projection $\pi:\mathbb{R}^n\to\mathbb{R}^2$, such that $\mathcal{C}$ has at least $(d_1\cdot\ldots\cdot d_{n-1})^2\cdot \sum_{i=1}^{n-2}i) $ selfintersection points. A caterpillar is a certain simple type of a tropical line and for this type we show that it can have at most $\sum_{i=1}^{n-2}i$ selfintersection points.

We investigate multivariate Laurent polynomials f \in \C[\mathbf{z}^{\pm 1}] = \C[z_1^{\pm 1},\ldots,z_n^{\pm 1}] with varieties \mathcal{V}(f) restricted to the algebraic torus (\C^*)^n = (\C \setminus \{0\})^n. For such Laurent polynomials f one defines the amoeba \mathcal{A}(f) of f as the image of the variety \mathcal{V}(f) under the \Log-map \Log : (\C^*)^n \to \R^n, (z_1,\ldots,z_n) \mapsto (\log|z_1|, \ldots, \log|z_n|). I.e., the amoeba \mathcal{A}(f) is the projection of the variety \mathcal{V}(f) on its (componentwise logarithmized) absolute values. Amoebas were first defined in 1994 by Gelfand, Kapranov and Zelevinksy. Amoeba theory has been strongly developed since the beginning of the new century. It is related to various mathematical subjects, e.g., complex analysis or real algebraic curves. In particular, amoeba theory can be understood as a natural connection between algebraic and tropical geometry.
In this thesis we investigate the geometry, topology and methods for the approximation of amoebas.
Let \C^A denote the space of all Laurent polynomials with a given, finite support set A \subset \Z^n and coefficients in \C^*. It is well known that, in general, the existence of specific complement components of the amoebas \mathcal{A}(f) for f \in \C^A depends on the choice of coefficients of f. One prominent key problem is to provide bounds on the coefficients in order to guarantee the existence of certain complement components. A second key problem is the question whether the set U_\alpha^A \subseteq \C^A of all polynomials whose amoeba has a complement component of order \alpha \in \conv(A) \cap \Z^n is always connected.
We prove such (upper and lower) bounds for multivariate Laurent polynomials supported on a circuit. If the support set A \subset \Z^n satisfies some additional barycentric condition, we can even give an exact description of the particular sets U_\alpha^A and, especially, prove that they are path-connected.
For the univariate case of polynomials supported on a circuit, i.e., trinomials f = z^{s+t} + p z^t + q (with p,q \in \C^*), we show that a couple of classical questions from the late 19th / early 20th century regarding the connection between the coefficients and the roots of trinomials can be traced back to questions in amoeba theory. This yields nice geometrical and topological counterparts for classical algebraic results. We show for example that a trinomial has a root of a certain, given modulus if and only if the coefficient p is located on a particular hypotrochoid curve. Furthermore, there exist two roots with the same modulus if and only if the coefficient p is located on a particular 1-fan. This local description of the configuration space \C^A yields in particular that all sets U_\alpha^A for \alpha \in \{0,1,\ldots,s+t\} \setminus \{t\} are connected but not simply connected.
We show that for a given lattice polytope P the set of all configuration spaces \C^A of amoebas with \conv(A) = P is a boolean lattice with respect to some order relation \sqsubseteq induced by the set theoretic order relation \subseteq. This boolean lattice turns out to have some nice structural properties and gives in particular an independent motivation for Passare's and Rullgard's conjecture about solidness of amoebas of maximally sparse polynomials. We prove this conjecture for special instances of support sets.
A further key problem in the theory of amoebas is the description of their boundaries. Obviously, every boundary point \mathbf{w} \in \partial \mathcal{A}(f) is the image of a critical point under the \Log-map (where \mathcal{V}(f) is supposed to be non-singular here). Mikhalkin showed that this is equivalent to the fact that there exists a point in the intersection of the variety \mathcal{V}(f) and the fiber \F_{\mathbf{w}} of \mathbf{w} (w.r.t. the \Log-map), which has a (projective) real image under the logarithmic Gauss map. We strengthen this result by showing that a point \mathbf{w} may only be contained in the boundary of \mathcal{A}(f), if every point in the intersection of \mathcal{V}(f) and \F_{\mathbf{w}} has a (projective) real image under the logarithmic Gauss map.
With respect to the approximation of amoebas one is in particular interested in deciding membership, i.e., whether a given point \mathbf{w} \in \R^n is contained in a given amoeba \mathcal{A}(f). We show that this problem can be traced back to a semidefinite optimization problem (SDP), basically via usage of the Real Nullstellensatz. This SDP can be implemented and solved with standard software (we use SOSTools and SeDuMi here). As main theoretic result we show that, from the complexity point of view, our approach is at least as good as Purbhoo's approximation process (which is state of the art).

We thoroughly study the properties of conically stable polynomials and imaginary projections. A multivariate complex polynomial is called stable if its nonzero whenever all coordinates of the respective argument have a positive imaginary part. In this dissertation we consider the generalized notion of K-stability. A multivariate complex polynomial is called K-stable if its non-zero whenever the imaginary part of the respective argument lies in the relative interior of the cone K. We study connections to various other objects, including imaginary projections as well as preservers and combinatorial criteria for conically stable polynomials.

The problem of unconstrained or constrained optimization occurs in many branches of mathematics and various fields of application. It is, however, an NP-hard problem in general. In this thesis, we examine an approximation approach based on the class of SAGE exponentials, which are nonnegative exponential sums. We examine this SAGE-cone, its geometry, and generalizations. The thesis consists of three main parts:
1. In the first part, we focus purely on the cone of sums of globally nonnegative exponential sums with at most one negative term, the SAGE-cone. We ex- amine the duality theory, extreme rays of the cone, and provide two efficient optimization approaches over the SAGE-cone and its dual.
2. In the second part, we introduce and study the so-called S-cone, which pro- vides a uniform framework for SAGE exponentials and SONC polynomials. In particular, we focus on second-order representations of the S-cone and its dual using extremality results from the first part.
3. In the third and last part of this thesis, we turn towards examining the con- ditional SAGE-cone. We develop a notion of sublinear circuits leading to new duality results and a partial characterization of extremality. In the case of poly- hedral constraint sets, this examination is simplified and allows us to classify sublinear circuits and extremality for some cases completely. For constraint sets with certain conditions such as sets with symmetries, conic, or polyhedral sets, various optimization and representation results from the unconstrained setting can be applied to the constrained case.