Disjoint Borel Functions

For each $a \in \mathbb{R}$, we define a Borel function $f_a : \mathbb{R} \to \mathbb{R}$ which encodes $a$ in a certain sense. We show that for each Borel $g : \mathbb{R} \to \mathbb{R}$, $f_a \cap g = \emptyset$ implies $a \in \Delta^1_1(c)$ where $c$ is any code for $g$. We generalize this theorem for $g$ in larger pointclasses $\Gamma$. Specifically, if $\Gamma = \mathbf{\Delta}^1_2$, then $a \in L[c]$. Also for all $n \in \omega$, if $\Gamma = \mathbf{\Delta}^1_{3 + n}$, then $a \in \mathcal{M}_{1 + n}(c)$.Comment: 15 page

Disjoint Infinity-Borel Functions

This is a followup to a paper by the author where the disjointness relation for definable functions from ${^\omega \omega}$ to ${^\omega \omega}$ is analyzed. In that paper, for each $a \in {^\omega \omega}$ we defined a Baire class one function $f_a : {^\omega \omega} \to {^\omega \omega}$ which encoded $a$ in a certain sense. Given $g : {^\omega \omega} \to {^\omega \omega}$, let $\Psi(g)$ be the statement that $g$ is disjoint from at most countably many of the functions $f_a$. We show the consistency strength of $(\forall g)\, \Psi(g)$ is that of an inaccessible cardinal. We show that $\textrm{AD}^+$ implies $(\forall g)\, \Psi(g)$. Finally, we show that assuming large cardinals, $(\forall g)\, \Psi(g)$ holds in models of the form $L(\mathbb{R})[\mathcal{U}]$ where $\mathcal{U}$ is a selective ultrafilter on $\omega$.Comment: 16 page

Disjoint Infinity Borel Functions

Consider the statement that every uncountable set of reals can be surjected onto R by a Borel function. This is implied by the statement that every uncountable set of reals has a perfect subset. It is also implied by a new statement D which we will discuss: for each real a there is a Borel function fa : RtoR and for each function g : RtoR there is a countable set G(g) of reals such that the following is true: for each a in R and for each function g : R to R, if fa is disjoint from g, then a is in G(g). We will show that D follows from ZF +AD+ whereas the negation of D follows from ZFC

Menger curvature and rectifiability

For a Borel set E in R^n, the total Menger curvature of E, or c(E), is the integral over E^3 (with respect to 1-dimensional Hausdorff measure in each factor of E) of c(x,y,z)^2, where 1/c(x,y,z) is the radius of the circle passing through three points x, y, and z in E. Let H^1(X) denote the 1-dimensional Hausdorff measure of a set X. A Borel set E in R^n is purely unrectifiable if for any Lipschitz function gamma from R to R^n, H^1(E cap gamma(R)) = 0. It is said to be rectifiable if there exists a countable family of Lipschitz functions gamma_i from R to R^n such that H^1(E - union gamma_i(R)) = 0. It may be seen from this definition that any 1-set E (that is, E Borel and 0<H^1(E)<\infty) can be decomposed into two disjoint subsets E_irr and E_rect, where E_irr is purely unrectifiable and E_rect is rectifiable. Theorem. If E is a 1-set in R^n and c(E)^2 is finite, then E is rectifiable.Comment: 39 pages, 3 figures, published version, abstract added in migratio

On disjoint Borel uniformizations

Larman showed that any closed subset of the plane with uncountable vertical cross-sections has aleph_1 disjoint Borel uniformizing sets. Here we show that Larman's result is best possible: there exist closed sets with uncountable cross-sections which do not have more than aleph_1 disjoint Borel uniformizations, even if the continuum is much larger than aleph_1. This negatively answers some questions of Mauldin. The proof is based on a result of Stern, stating that certain Borel sets cannot be written as a small union of low-level Borel sets. The proof of the latter result uses Steel's method of forcing with tagged trees; a full presentation of this method, written in terms of Baire category rather than forcing, is given here

Baire measurable paradoxical decompositions via matchings

We show that every locally finite bipartite Borel graph satisfying a strengthening of Hall's condition has a Borel perfect matching on some comeager invariant Borel set. We apply this to show that if a group acting by Borel automorphisms on a Polish space has a paradoxical decomposition, then it admits a paradoxical decomposition using pieces having the Baire property. This strengthens a theorem of Dougherty and Foreman who showed that there is a paradoxical decomposition of the unit ball in $\mathbb{R}^3$ using Baire measurable pieces. We also obtain a Baire category solution to the dynamical von Neumann-Day problem: if $a$ is a nonamenable action of a group on a Polish space $X$ by Borel automorphisms, then there is a free Baire measurable action of $\mathbb{F}_2$ on $X$ which is Lipschitz with respect to $a$.Comment: Minor revision