## Dulac’s Problem

Let $\xi = (\xi_1,\xi_2):\RR^2 \into \RR^2$ be a vector field of class $C^1$. Recall that the singular set of $\xi$ is the set $$Z(\xi):= \set{z \in \RR^2:\ \xi(z) = 0}.$$ The general theory of ordinary differential equations shows that, for every connected open $U \subseteq \RR^2 \setminus Z(\xi)$ and every $z \in U$, there exists…

## Definable closure

Let ${\cal M}$ be an o-minimal expansion of a dense linear order $(M,\lt)$. For $A \subseteq M$, the definable closure of $A$ is defined by $$\dcl(A):= \set{b \in M:\ \{b\} \text{ is } A\text{-definable}}.$$ Exercise Let $A \subseteq M$. Prove that $\dcl(A) = \acl(A)$. Let $\phi(x)$ be a formula with parameters in $A$. Prove that…

## An “ordered Ramsey” theorem

Let ${\cal M}$ be an o-minimal expansion of a dense linear order $(M,\lt)$, and let $I$ be an open interval. Ordered Ramsey Theorem (Peterzil and Starchenko) Let $S_1, \dots, S_k \subseteq M^2$ be definable, and assume that $I^2 \subseteq S_1 \cup \cdots \cup S_k$. Then there exist $l \in \{1, \dots, k\}$ and an open…