This is an overview of geometric algebras in dimensions 0-4 from the perspective of the concept of number itself. It is developed from a historic viewpoint and investigates and develops a pedagogic approach emphasizing the geometric aspects of the subject. There is a focus broadly on three main but interconnected areas: the relation between the discrete and the continuous, the centrality of complex numbers, and the hypothesis that the octonions may be expressed in the even subalgebra of four dimensions. This will not be proved, and the focus is on the overall perspective and presentation. One central result is a proposal for the identification of the Cayley-Dickson process with the gluing together of spheres, extending it to start from ℕ. This leads to a projective representation of a unified elliptic/parabolic/hyperbolic geometry of 4-dimensional space. Coupled with this is a discrete representation of it that can be classified with Geometric Algebra.

In this work we introduce the Voronoi fundamental zone octonion (VFZO) interpolation framework for grain boundary (GB) structure-property models and surrogates. The VFZO framework offers an advantage over other five degree-of-freedom (5DOF) based property interpolation methods because it is constructed as a point set in a Riemannian manifold. This means that directly computed Euclidean distances approximate the original octonion distance with significantly reduced computation runtime (∼7 CPU minutes vs. 153 CPU days for a 50000×50000 pairwise-distance matrix). This increased efficiency facilitates lower interpolation error through the use of significantly more input data. We demonstrate grain boundary energy (GBE) interpolation results for a non-smooth validation function and simulated bi-crystal datasets for Fe and Ni using four interpolation methods: barycentric interpolation, Gaussian process regression (GPR) or Kriging, inverse-distance weighting (IDW), and nearest neighbor (NN)interpolation. These are evaluated for 50000 random input GBs and 10000 random prediction GBs. The best performance was achieved with GPR, which resulted in a reduction of the root mean square error(RMSE) by 83.0% relative to RMSE of a constant, average model. Likewise, interpolation on a large, noisy, molecular statics (MS) Fe simulation dataset improves performance by 34.4 % compared to 21.2 %in prior work. Interpolation on a small, low-noise MS Ni simulation dataset is similar to interpolation results for the original octonion metric (57.6 % vs. 56.4 %). A vectorized, parallelized, MATLAB interpolation function (interp5DOF.m) and related routines are available in our VFZO repository (github.com/sgbaird-5dof/interp) which can be applied to any of the 32 crystallographic point groups1. The VFZO framework offers advantages for computing distances between GBs, estimating property values for arbitrary GBs, and modeling surrogates of computationally expensive 5DOF functions and simulations.

In this work we introduce the Voronoi fundamental zone octonion (VFZO) interpolation framework for grain boundary (GB) structure-property models and surrogates. The VFZO framework offers an advantage over other five degree-of-freedom (5DOF) based property interpolation methods because it is constructed as a point set in a Riemannian manifold. This means that directly computed Euclidean distances approximate the original octonion distance with significantly reduced computation runtime (∼7 CPU minutes vs. 153 CPU days for a 50000×50000 pairwise-distance matrix). This increased efficiency facilitates lower interpolation error through the use of significantly more input data. We demonstrate grain boundary energy (GBE) interpolation results for a non-smooth validation function and simulated bi-crystal datasets for Fe and Ni using four interpolation methods: barycentric interpolation, Gaussian process regression (GPR) or Kriging, inverse-distance weighting (IDW), and nearest neighbor (NN)interpolation. These are evaluated for 50000 random input GBs and 10000 random prediction GBs. The best performance was achieved with GPR, which resulted in a reduction of the root mean square error(RMSE) by 83.0% relative to RMSE of a constant, average model. Likewise, interpolation on a large, noisy, molecular statics (MS) Fe simulation dataset improves performance by 34.4 % compared to 21.2 %in prior work. Interpolation on a small, low-noise MS Ni simulation dataset is similar to interpolation results for the original octonion metric (57.6 % vs. 56.4 %). A vectorized, parallelized, MATLAB interpolation function (interp5DOF.m) and related routines are available in our VFZO repository (github.com/sgbaird-5dof/interp) which can be applied to any of the 32 crystallographic point groups1. The VFZO framework offers advantages for computing distances between GBs, estimating property values for arbitrary GBs, and modeling surrogates of computationally expensive 5DOF functions and simulations.

Octonion algebras are certain algebras with a multiplicative quadratic form. In 2019, Alsaody and Gille showed that, for octonion algebras over unital commutative rings, there is an equivalence between isotopes and isometric quadratic forms. The contravariant equivalence from unital commutative rings to affine schemes, sending a ring to its spectrum, leads us to a question: can the equivalence of isometry and isotopy be generalized to octonion algebras over a (not necessarily affine) scheme? We present the basic definitions and properties of octonion algebras, both over rings and over schemes. Then we show that an isotope of an octonion algebra C over a scheme is isomorphic to a twist by an Aut(C)–torsor. We conclude the thesis by giving an affirmative answer to our question.

We first introduce the octonions as an eight dimensional vector space over a field of characteristic zero with a multiplication defined using a table. We also show that the multiplication rules for octonions can be derived from a special graph with seven vertices call the Fano Plane. Next we explain the Cayley-Dickson construction, which exhibits the octonions as the set of ordered pairs of quaternions. This approach parallels the realization of the complex numbers as ordered pairs of real numbers. The rest of the thesis is devoted to following a paper by N. Jacobson written in 1939 entitled "Cayley Numbers and Normal Simple Lie Algebras of Type G". We prove that the algebra of derivations on the octonions is a Lie algebra of type G_2. The proof proceeds by showing the set of derivations on the octonions is a Lie algebra, has dimension fourteen, and is semisimple. Next, we complexify the algebra of derivations on the octonions and show the complexification is simple. This suffices to show the complexification of the algebra of derivations is isomorphic to g_2 since g_2 is the only semisimple complex Lie algebra of dimension fourteen. Finally, we conclude the algebra of derivations on the octonions is a simple Lie algebra of type G_2. / Master of Science

Wir behandeln nichtkommutative Algebren über Ringen und auf algebraischen Varietäten. Im ersten Teil beschreiben wir ein Kriterium, das angibt, ob und wie weit sich eine gegebene Azumaya-Algebra über dem Funktionenkörper einer algebraischen Varietät als Garbe von Azumaya-Algebren auf die Varietät ausdehnen lässt. Außerdem untersuchen wir die lokale Struktur von Azumaya-Algebren oder allgemeiner von Maximalordnungen, die mit Hilfe des Cyclic-Covering-Tricks von Chan konstruiert werden. Mit dieser Methode lassen sich Maximalordnungen auf algebraischen Flächen konstruieren, die zudem genau über einer gewählten Kurve verzweigen. Im zweiten Teil betrachten wir die nichtassoziativen Oktavenalgebren und allgemeiner auch Kompositionsalgebren über Ringen. Dabei übertragen wir die bekannten Aussagen von Kompositionsalgebren über Körpern auf die Situation von Algebren über Ringen. Wir untersuchen Oktavenalgebren und Maximalordnungen über diskreten Bewertungsringen und verallgemeinern ein Resultat von van der Blij und Springer über die lokale Natur von Maximalordnungen über den rationalen Zahlen und über algebraischen Zahlkörpern auf den Fall von beliebigen noetherschen, ganzabgeschlossenen Integritätsbereichen. Abschließend führen wir eine Definition von Garben von Oktavenalgebren und Garben von Maximalordnungen in Oktavenalgebren über algebraischen Varietäten ein.