The rigid body has been one of the most noteworthy applications of Newtonian mechanics. Applying the principles of classical mechanics to the rigid body is by no means routine. The equations of motion, though discovered two hundred and fifty years ago by Euler, have remained quite elusive since their introduction. Understanding the rigid body has required the applications of concepts from integrable systems, algebraic geometry, Lie groups, representation theory, and symplectic geometry to name a few. Moreover, several important developments in these fields have in fact originated with the study of the rigid body and subsequently have grown into general theories with much wider applications.In this work, we study the stability of equilibria of non-degenerate orbits of the generalized rigid body. The energy-Casimir method introduced by V.I. Arnold in 1966 allows us to prove stability of certain non-degenerate equilibria of systems on Lie groups. Applied to the three dimensional rigid body, it recovers the classical Euler stability theorem [12]: rotations around the longest and shortest principal moments of inertia are stable equilibria. This method has not been applied to the analysis of rigid body dynamics beyond dimension n = 3. Furthermore, no conditions for the stability of equilibria are known at all beyond n = 4, in which case the conditions are not of the elegant longest/shortest type [10].Utilizing the rich geometric structures of the symmetry group G = SO(2n), we obtain stability results for generic equilibria of the even dimensional free rigid body. After obtaining a general expression for the generic equilibria, we apply the energy-Casimir method and find that indeed the classical longest/shortest conditions on the entries of the inertia matrix are suffcient to prove stability of generic equilibria for the generalized rigid body in even dimensions.

When the tensor product of two irreducible representations contains multiple copies of some of its irreducible constituents, there is a problem of choosing specific copies: resolving the multiplicity. This is typically accomplished by some ad hoc method chosen primarily for convenience in labelling and calculations. This thesis addresses the possibility of making choices according to other criteria. One possible criterion is to choose copies for which the Clebsch-Gordan coefficients have a simple form. A method fulfilling this is introduced for the tensor product of three irreps of $su(2)$. This method is then extended to the tensor product of two irreps of $su(3)$. In both cases the method is shown to construct a full nested sequence of basis independent highest weight subspaces. Another possible criterion is to make choices which are intrinsic, independent of all choices of bases. This is investigated in the final part of the thesis with a basis independent method that applies to the tensor product of finite dimensional irreps of any semisimple Lie algebra over $\mathbb{C}$.

When solving problems involving quantum mechanical systems, it is frequently desirable to find the matrix elements of a unitary representation $T$ of a real algebraic Lie group $G$. This requires defining an inner product on the Hilbert space $\mathbb{H}$ that carries the representation $T$. In the case where the representation is determined by a representation of a subgroup containing the lowest weight vector of $T$, this can be achieved through the coherent state construction. In both the scalar and vector coherent state methods, the process of finding the overlaps can be simplified by introducing the coherent state triplet ($\mathfrak{F}_{\mathbb{H}_D}$, $\mathbb{H}_D$, $\mathfrak{F}^{\mathfrak{H}_D}$) of Bargmann spaces. Coherent state wave functions -- the elements of $\mathfrak{F}_{\mathbb{H}_D}$ and of $\mathfrak{F}^{\mathbb{H}_D}$ -- are used to define the inner product on $\mathbb{H}_D$ in a way that simplifies the calculation of the overlaps. This inner product and the group action $\Gamma$ of $G$ on $\mathfrak{F}^{\mathbb{H}_D}$ are used to formulate expressions for the matrix elements of $T$ with coefficients from the given subrepresentation. The process of finding an explicit expression for $\Gamma$ relies on matrix factorizations in the complexification of $G$ even though the representation $T$ does not extend to the complexification. It will be shown that these factorizations are, in fact, justified, that the overlaps and $\Gamma$ action can be expressed in terms of the given subrepresentation, and that it is possible to find numerical values for the inner product in $\mathbb{H}$. The scalar and vector coherent state methods will both be applied to Sp($n$) and Sp($n,\mathbb{R}$).

When the tensor product of two irreducible representations contains multiple copies of some of its irreducible constituents, there is a problem of choosing specific copies: resolving the multiplicity. This is typically accomplished by some ad hoc method chosen primarily for convenience in labelling and calculations. This thesis addresses the possibility of making choices according to other criteria. One possible criterion is to choose copies for which the Clebsch-Gordan coefficients have a simple form. A method fulfilling this is introduced for the tensor product of three irreps of $su(2)$. This method is then extended to the tensor product of two irreps of $su(3)$. In both cases the method is shown to construct a full nested sequence of basis independent highest weight subspaces. Another possible criterion is to make choices which are intrinsic, independent of all choices of bases. This is investigated in the final part of the thesis with a basis independent method that applies to the tensor product of finite dimensional irreps of any semisimple Lie algebra over $\mathbb{C}$.

When solving problems involving quantum mechanical systems, it is frequently desirable to find the matrix elements of a unitary representation $T$ of a real algebraic Lie group $G$. This requires defining an inner product on the Hilbert space $\mathbb{H}$ that carries the representation $T$. In the case where the representation is determined by a representation of a subgroup containing the lowest weight vector of $T$, this can be achieved through the coherent state construction. In both the scalar and vector coherent state methods, the process of finding the overlaps can be simplified by introducing the coherent state triplet ($\mathfrak{F}_{\mathbb{H}_D}$, $\mathbb{H}_D$, $\mathfrak{F}^{\mathfrak{H}_D}$) of Bargmann spaces. Coherent state wave functions -- the elements of $\mathfrak{F}_{\mathbb{H}_D}$ and of $\mathfrak{F}^{\mathbb{H}_D}$ -- are used to define the inner product on $\mathbb{H}_D$ in a way that simplifies the calculation of the overlaps. This inner product and the group action $\Gamma$ of $G$ on $\mathfrak{F}^{\mathbb{H}_D}$ are used to formulate expressions for the matrix elements of $T$ with coefficients from the given subrepresentation. The process of finding an explicit expression for $\Gamma$ relies on matrix factorizations in the complexification of $G$ even though the representation $T$ does not extend to the complexification. It will be shown that these factorizations are, in fact, justified, that the overlaps and $\Gamma$ action can be expressed in terms of the given subrepresentation, and that it is possible to find numerical values for the inner product in $\mathbb{H}$. The scalar and vector coherent state methods will both be applied to Sp($n$) and Sp($n,\mathbb{R}$).

Universiẗat, Diss., 2004--Saarbrücken.

Η θεωρία των συμμετρικών χώρων αποτελεί μια σπουδαία κλάση των ομογενών χώρων, με εφαρμογές σε πολλούς κλάδους των μαθηματικών όπως στην αλγεβρική και την διαφορική γεωμετρία. Σε αυτήν την εργασία θα δώσουμμε τον ορισμό των συμμετρικών χώρων, τα βασικά τους χαρακτηριστικά και την ταξινόμησή τους. Θα περιγράψουμε τους χώρους αυτούς κυρίως αλγεβρικά, οπότε δεν θεωρείται απαραίτητο από τον αναγνώστη να γνωρίζει εκτενώς την θεωρία της διαφορικής γεωμετρίας για να κατανοήσει πλήρως την εργασία. / The theory of symmetric spaces is an important class of homogeneous spaces, with applications in many branches of mathematics such as algebraic and differential geometry. In this work we will define the symmetric spaces, their key features and sort them. We will describe these spaces mainly algebraic, so it is not considered necessary by the reader to know in detail the theory of differential geometry to understand the work.

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas. Programa de Pós-graduação em Física / Made available in DSpace on 2012-10-24T18:37:40Z (GMT). No. of bitstreams: 1 263132.pdf: 526017 bytes, checksum: 5840fcf1dc0e49a55cf108092da716f2 (MD5) / Monopolos magnéticos têm sido objetos de grande interesse nos últimos anos, principalmente por serem previstos em algumas teorias de grande unificação e por, possivelmente, serem relevantes no fenômeno do confinamento em QCD. Consideramos uma teoria de Yang-Mills-Higgs com simetria de gauge SU(n) quebrada espontaneamente em SO(n) que apresenta condições topológicas necessárias para a existência de monopolos Z2. Construímos as formas assintóticas desses monopolos, considerando duas quebras distintas do SU(n) em SO(n), e verificamos que os monopolos fundamentais estão associados aos pesos da representação definidora da álgebra so(n)v.

Orientador: Geraldo Nunes Silva / Banca: Antonio Carlos Gardel Leitão / Banca: Fernando Manuel Ferreira Lobo Pereira / Resumo: Neste trabalho é feito um estudo aprofundado da estabilidade de sistemas alternantes, principalmente via teoria de Lie. Inicialmente são apresentados os principais conceitos básicos da álgebra de Lie, necessários para o estudo dos critérios de estabilidade dos sistemas alternantes. Depois são discutidos critérios de estabilidade para sistemas alternantes. É feita a exposição da demonstração de que para todo sistema linear da forma ? x = Apx p = 1, 2, ...,N, com as matrizes Ap assintóticamente estáveis e comutativas duas a duas, existe uma função de Lyapunov quadrática comum. Uma condição suficiente para estabilidade assintótica de um sistema linear alternante é apresentada em termos da álgebra de Lie gerada por uma família infinita de matrizes. A saber, se esta álgebra de Lie é solúvel, então o sistema alternante é estável para uma mudança arbitrária de sinal. Em seguida são estudadas condições mais fracas. Supondo que a álgebra de Lie não é solúvel, mas é decomponível na soma de um ideal solúvel e uma subálgebra com grupo de Lie compacto, então o sistema alternante é globalmente exponencialmente uniformemente estável. Entretanto, se o grupo de Lie não for compacto, verifica-se que é possível gerar uma família finita de matrizes estáveis tais que o correspondente sistema linear alternante não é estável. Finalmente, os resultados correspondentes de estabilidade local para sistemas alternantes não lineares são apresentados. / Abstract: In this work it is undertaken a deep study of stability for switched systems, mainly via Lie algebraic Theory. At first, the basic concepts and results from Lie algebra necessary for the study of stability of switched systems are presented. Criteria for stability are discussed. It is also done an exposition of the proof that all linear systems ? x = Apx, p = 1, 2, ...,N, with stable and pairwisely commutative matrices Ap, have common quadratic Lyapounov functions. A sufficient condition for asymptotic stability of switched linear systems is presented in term of the Lie algebra generated by a family infinite matrices. That is, if this Lie algebra is solvable, then the switched systems are stable for an arbitrary change of sinal. Next weaker conditions are studied. If the Lie algebra is decomposable into two subalgebras in which one is a solvable ideal and the other has a compact Lie group, then the switched systems are globally exponentially uniformly stable. However, if the Lie group is not compact, it is also possible to generate a finite family of stable matrices such that the corresponding switched linear systems are not stable. Finally, corresponding local stability results are presented for nonlinear systems. / Mestre

Vi ger en överskådlig introduktion till mångfalder, Lie-grupper och deras associerade Lie-algebror. En karaktärisering av Lie-parentesen som naturligt kopplar ihop de vanligast förekommande karaktäriseringarna presenteras (sats 4.4.1). Vi använder idéer från Riemanngeometrin för att inleda en undersökning av vad det betyder för vektorfält på Lie-grupper att vara _mer eller mindre kommutativa_. Vi presenterar ett mått av kommutativitet, diskuterar dess egenskaper och avslutar med några förslag på framtida undersökningar. / We give a comprehensive introduction to manifolds, Lie groups, and their associated Lie algebras. A characterization of the Lie bracket which connects the most commonly seen characterizations in a canonical fashion is presented (thm. 4.4.1). We make use of ideas from Riemannian geometry to begin an investigation of what it means for vector elds on Lie groups to be more or less commutative. We present a measure of commutativity, discuss its properties, and close with a few suggestions for future work.