Excluding subdivisions of bounded degree graphs

Let $H$ be a fixed graph. What can be said about graphs $G$ that have no subgraph isomorphic to a subdivision of $H$? Grohe and Marx proved that such graphs $G$ satisfy a certain structure theorem that is not satisfied by graphs that contain a subdivision of a (larger) graph $H_1$. Dvo\v{r}\'ak found a clever strengthening---his structure is not satisfied by graphs that contain a subdivision of a graph $H_2$, where $H_2$ has "similar embedding properties" as $H$. Building upon Dvo\v{r}\'ak's theorem, we prove that said graphs $G$ satisfy a similar structure theorem. Our structure is not satisfied by graphs that contain a subdivision of a graph $H_3$ that has similar embedding properties as $H$ and has the same maximum degree as $H$. This will be important in a forthcoming application to well-quasi-ordering

The Euclidean distance degree of an algebraic variety

The nearest point map of a real algebraic variety with respect to Euclidean distance is an algebraic function. For instance, for varieties of low rank matrices, the Eckart-Young Theorem states that this map is given by the singular value decomposition. This article develops a theory of such nearest point maps from the perspective of computational algebraic geometry. The Euclidean distance degree of a variety is the number of critical points of the squared distance to a generic point outside the variety. Focusing on varieties seen in applications, we present numerous tools for exact computations.Comment: to appear in Foundations of Computational Mathematic

Packing Topological Minors Half-Integrally

The packing problem and the covering problem are two of the most general questions in graph theory. The Erd\H{o}s-P\'{o}sa property characterizes the cases when the optimal solutions of these two problems are bounded by functions of each other. Robertson and Seymour proved that when packing and covering $H$-minors for any fixed graph $H$, the planarity of $H$ is equivalent with the Erd\H{o}s-P\'{o}sa property. Thomas conjectured that the planarity is no longer required if the solution of the packing problem is allowed to be half-integral. In this paper, we prove that this half-integral version of Erd\H{o}s-P\'{o}sa property holds with respect to the topological minor containment, which easily implies Thomas' conjecture. Indeed, we prove an even stronger statement in which those subdivisions are rooted at any choice of prescribed subsets of vertices. Precisely, we prove that for every graph $H$, there exists a function $f$ such that for every graph $G$, every sequence $(R_v: v \in V(H))$ of subsets of $V(G)$ and every integer $k$, either there exist $k$ subgraphs $G_1,G_2,...,G_k$ of $G$ such that every vertex of $G$ belongs to at most two of $G_1,...,G_k$ and each $G_i$ is isomorphic to a subdivision of $H$ whose branch vertex corresponding to $v$ belongs to $R_v$ for each $v \in V(H)$, or there exists a set $Z \subseteq V(G)$ with size at most $f(k)$ intersecting all subgraphs of $G$ isomorphic to a subdivision of $H$ whose branch vertex corresponding to $v$ belongs to $R_v$ for each $v \in V(H)$. Applications of this theorem include generalizations of algorithmic meta-theorems and structure theorems for $H$-topological minor free (or $H$-minor free) graphs to graphs that do not half-integrally pack many $H$-topological minors (or $H$-minors)

Dynamic programming for graphs on surfaces

We provide a framework for the design and analysis of dynamic programming algorithms for surface-embedded graphs on n vertices and branchwidth at most k. Our technique applies to general families of problems where standard dynamic programming runs in 2O(k·log k). Our approach combines tools from topological graph theory and analytic combinatorics.Postprint (updated version

Dynamic Programming for Graphs on Surfaces

We provide a framework for the design and analysis of dynamic programming algorithms for surface-embedded graphs on n vertices and branchwidth at most k. Our technique applies to general families of problems where standard dynamic programming runs in 2^{O(k log k)} n steps. Our approach combines tools from topological graph theory and analytic combinatorics. In particular, we introduce a new type of branch decomposition called "surface cut decomposition", generalizing sphere cut decompositions of planar graphs introduced by Seymour and Thomas, which has nice combinatorial properties. Namely, the number of partial solutions that can be arranged on a surface cut decomposition can be upper-bounded by the number of non-crossing partitions on surfaces with boundary. It follows that partial solutions can be represented by a single-exponential (in the branchwidth k) number of configurations. This proves that, when applied on surface cut decompositions, dynamic programming runs in 2^{O(k)} n steps. That way, we considerably extend the class of problems that can be solved in running times with a single-exponential dependence on branchwidth and unify/improve most previous results in this direction.Comment: 28 pages, 3 figure