A subset of ℝ is [meagre](http://en.wikipedia.org/wiki/Meagre_set) if it is a countable union of nowhere dense subsets (a set is nowhere dense if every open interval contains an open subinterval that misses the set).


Any countable set is meagre. The Cantor set is nowhere dense, so it's meagre. A countable union of meagre sets is meagre (e.g. all rational translates of the Cantor set).


There can also be meagre sets of positive measure, like "fat Cantor sets". To form a fat Cantor set, you start with a closed interval, then remove some open interval from the middle of it, then remove some open intervals from the remaining intervals, and so on. The result is nowhere dense because you removed open intervals all over the place. If the sizes of the intervals you remove get small fast, then the result has positive measure.


So does meagreness have any connection at all to measure? Specifically, are all measure zero sets meagre?



Is there a measure zero set which isn't meagre?
On the relation between null sets and meagre sets, you can also look at
[this article](http://www.artsci.kyushu-u.ac.jp/~ssaito/eng/maths/duality.pdf).
Two theorems mentioned in this note (both classical and not due to the author):


1. (As already mentioned above) There exist a meagre $F\_\sigma$ subset $A$ and a null $G\_\delta$ subset $B$ of $\mathbb R$ that satisfy $A\cap B=\emptyset$ and $A\cup B=\mathbb R$.
2. (The Erdős-Sierpiński Duality Theorem) Assume that the Continuum Hypothesis
holds. Then there exists an involution (bijection of order two) $f:\mathbb R\to\mathbb R$ such that $f[A]$
is meagre if and only if $A$ is null, and $f[A]$ is null if and only if $A$ is meagre for every subset $A$ of $\mathbb R$.


While (1) says that the ideals of null, respectively meager sets are "orthogonal", (2) says that assuming CH they behave identically. But it is well known that this duality between measure and category fails dramatically once we take a more abstract point of view: Shelah proved that you need large cardinals to construct a model of set theory (ZF, no axiom of choice) where every set of reals is Lebesgue measurable, but no large cardinals are necessary to construct a model where every set of reals has the Baire property (the corresponding notion to measurability for category).


