The Einstein Relation on Metric Measure Spaces

This note is based on F. Burghart's master thesis at Stuttgart university from July 2018, supervised by Prof. Freiberg. We review the Einstein relation, which connects the Hausdorff, local walk and spectral dimensions on a space, in the abstract setting of a metric measure space equipped with a suitable operator. This requires some twists compared to the usual definitions from fractal geometry. The main result establishes the invariance of the three involved notions of fractal dimension under bi-Lipschitz continuous isomorphisms between mm-spaces and explains, more generally, how the transport of the analytic and stochastic structure behind the Einstein relation works. While any homeomorphism suffices for this transport of structure, non-Lipschitz maps distort the Hausdorff and the local walk dimension in different ways. To illustrate this, we take a look at H\"older regular transformations and how they influence the local walk dimension and prove some partial results concerning the Einstein relation on graphs of fractional Brownian motions. We conclude by giving a short list of further questions that may help building a general theory of the Einstein relation.Comment: 28 pages, 3 figure

A Modification of the Random Cutting Model

We propose a modification to the random destruction of graphs: Given a finite network with a distinguished set of sources and targets, remove (cut) vertices at random, discarding components that do not contain a source node. We investigate the number of cuts required until all targets are removed, and the size of the remaining graph. This model interpolates between the random cutting model going back to Meir and Moon [Meir and Moon, 1970] and site percolation. We prove several general results, including that the size of the remaining graph is a tight family of random variables for compatible sequences of expander-type graphs, and determine limiting distributions complete binary trees

A Modification of the Random Cutting Model

We propose a modification to the random destruction of graphs: Given a finite network with a distinguished set of sources and targets, remove (cut) vertices at random, discarding components that do not contain a source node. We investigate the number of cuts required until all targets are removed, and the size of the remaining graph. This model interpolates between the random cutting model going back to Meir and Moon [Meir and Moon, 1970] and site percolation. We prove several general results, including that the size of the remaining graph is a tight family of random variables for compatible sequences of expander-type graphs, and determine limiting distributions complete binary trees

Building and Destroying Urns, Graphs, and Trees

In this thesis, consisting of an introduction and four papers, different models in the mathematical area of combinatorial probability are investigated. In Paper I, two operations for combining generalised Pólya urns, called disjoint union and product, are defined. This is then shown to turn the set of isomorphism classes of Pólya urns into a semiring, and we find that assigning to an urn its intensity matrix is a semiring homomorphism. In paper II, a modification and generalisation of the random cutting model is introduced. For a finite graph with given source and target vertices, we remove vertices at random and discard all resulting components without a source node. The results concern the number of cuts needed to remove all target vertices and the size of the remaining graph, and suggest that this model interpolates between the traditional cutting model and site percolation. In paper III, we define several polynomial invariants for rooted trees based on the modified cutting model in Paper II.We find recursive identities for these invariants and, using an approach via irreducibility of polynomials, prove that two specific invariants are complete, that is, they distinguish rooted trees up to isomorphism. In paper IV, joint with Paul Thévenin, we consider an operation of concatenating t random perfect matchings on 2n vertices. Our analysis of the resulting random graph as t tends to infinity shows that there is a giant component if and only if n is odd, and that the size of this giant component as well as the number of components is asymptotically normally distributed

Building and Destroying Urns, Graphs, and Trees

In this thesis, consisting of an introduction and four papers, different models in the mathematical area of combinatorial probability are investigated. In Paper I, two operations for combining generalised Pólya urns, called disjoint union and product, are defined. This is then shown to turn the set of isomorphism classes of Pólya urns into a semiring, and we find that assigning to an urn its intensity matrix is a semiring homomorphism. In paper II, a modification and generalisation of the random cutting model is introduced. For a finite graph with given source and target vertices, we remove vertices at random and discard all resulting components without a source node. The results concern the number of cuts needed to remove all target vertices and the size of the remaining graph, and suggest that this model interpolates between the traditional cutting model and site percolation. In paper III, we define several polynomial invariants for rooted trees based on the modified cutting model in Paper II.We find recursive identities for these invariants and, using an approach via irreducibility of polynomials, prove that two specific invariants are complete, that is, they distinguish rooted trees up to isomorphism. In paper IV, joint with Paul Thévenin, we consider an operation of concatenating t random perfect matchings on 2n vertices. Our analysis of the resulting random graph as t tends to infinity shows that there is a giant component if and only if n is odd, and that the size of this giant component as well as the number of components is asymptotically normally distributed