An order is a commutative ring that as an abelian group is finitely generated and free. A commutative ring is reduced if it has no non-zero nilpotent elements. In this paper we use a new tool, namely, the fact that every reduced order has a universal grading, to answer questions about realizing orders as group rings. In particular, we address the Isomorphism Problem for group rings in the case where the ring is a reduced order. We prove that any non-zero reduced order R can be written as a group ring in a unique “maximal” way, up to isomorphism. More precisely, there exist a ring A and a finite abelian group G, both uniquely determined up to isomorphism, such that R≅A[G] as rings, and such that if B is a ring and H is a group, then R≅B[H] as rings if and only if there is a finite abelian group J such that B≅A[J] as rings and J×H≅G as groups. Computing A and G for given R can be done by means of an algorithm that is not quite polynomial-time. We also give a description of the automorphism group of R in terms of A and G.

In this work we consider a generalization of graph flows. A graph flow is, in its simplest formulation, a labeling of the directed edges with real numbers subject to various constraints. A common constraint is conservation in a vertex, meaning that the sum of the labels on the incoming edges of this vertex equals the sum of those on the outgoing edges. One easy fact is that if a flow is conserving in all but one vertex, then it is also conserving in the remaining one. In our generalization we do not label the edges with real numbers, but with elements from an arbitrary group, where this fact becomes false in general. As we will show, graphs with the property that conservation of a flow in all but one vertex implies conservation in all vertices are precisely the planar graphs.

In algebraic number theory, the finiteness of the Picard group of an order in a number field is generally proved via a lattice argument: the order forms a lattice and every ideal class contains an integral ideal with a small enough non-zero element. In this work we will consider $\overline{\mathbb{Z}}$, the ring of algebraic integers, which is a lattice in a similar sense, and we will treat this lattice as intrinsically interesting. We will prove several properties of $\overline{\mathbb{Z}}$ and state some open problems. Many of these properties have connections to the indecomposable elements of the lattice, and our main result regards the decidability of the question whether an algebraic integer is indecomposable.

The positional game of Order versus Chaos can be considered a maker-breaker variant. The players Order and Chaos take turns placing circles or crosses on a board, in which the goal of Order is to create a consecutive line of identical symbols of a certain length, while Chaos aims to prevent this. In this paper, we provide some theoretical results on winning strategies for both players on finite boards of varying sizes, as well as on infinite boards. The composition of these strategies was aided by the use of Monte-Carlo Tree Search (MCTS) players, as well as a SAT solver. In addition to these theoretical results, we provide some more experimental results obtained using MCTS.

This work is a Master thesis supervised by Prof. Dr. H.W. Lenstra. Lenstra and Silverberg showed that each reduced order has a universal grading, which can be viewed as the `largest possible grading'. We present an algorithm to compute the universal grading for a given order $R$, which has runtime $n^{O(m)}$, where n is the length of the input and m is the size of the minimal spectrum of $R$. We do this by computing all gradings of the corresponding reduced $\mathbb{Q}$-algebra with cyclic abelian groups of prime-power order. We additionally generalize the result of Lenstra and Silverberg that reduced orders have a universal grading to a broader class of rings.