Examples of $G_\delta$ sets Recall that a subset $A$ of a metric space $X$ is a $G_\delta$ subset if it can be written as a countable intersection of open sets. This notion is related to the Baire category theorem. Here are three examples of interesting sets that are 
$G_\delta$ sets.


*

* The set of continuity points of a function $f:X\rightarrow R$.


* The set of positively recurrent points of a continuous transformation $T: X \rightarrow X$. Recall that a point is recurrent if there exists some sequence $n_i\rightarrow \infty$ such that $T^{n_i}x \rightarrow x$.

* The set of transitive points of a continuous transformation $T: X \rightarrow X$. Recall that a point is transitive if its orbit $\{T^n(x)\}_{n\in Z}$ is dense in $X$.


Question :
Can you provide more examples of interesting $G_\delta$ sets ?
 A: Let $f\colon\mathbb{R}\to\mathbb{R}$ be differentiable. The set of points where $f'$ vanishes is a $G_\delta$.
A: Most mathematicians consider connected spaces that are not path-connected to be a pathology (and perhaps they are right). So, it may come as a surprise that most continua are actually not path-connected. The 
pseudo-arc is a non-degenerate (i.e. at least $2$ points), hereditarily indecomposable, chainable continuum. Chainable means that 
if $P$ is a pseudo-arc then, for every $\varepsilon>0$, $P$ is $\varepsilon$-close to an arc (i.e. a path). On the other hand being hereditarily indecomposable implies that the pseudo-arc is very much not path-connected: Each of its path-components is a singleton! 
Bing has proved in Concerning hereditarily indecomposable continua, Pacific J. Math. V.1,N.1(1951), 43-51 http://projecteuclid.org/euclid.pjm/1102613150 that 
(the pseudo-arc is unique up to a homeomorphism, and) 
for any Euclidean space or Hilbert space $E$ of dimension greater
than $1$ the collection of subcontinua of $E$ which are pseudo-arcs is 
a dense $G_\delta$ in the hyperspace of subcontinua of $E$. 
W. Lewis later proved in Most maps of the pseudo-arc are homeomorphisms, 
Proc. Amer. Math. Soc. 91 (1984), no. 1, 147–154 http://www.ams.org/journals/proc/1984-091-01/S0002-9939-1984-0735582-4/ that: 
If $M(P)$ is the space of maps of the pseudo-arc into itself with the sup metric,
then the subset $H(P)$ of maps of the pseudo-arc into itself which are homeomorphisms
onto their images is a dense $G_\delta$ in $M(P)$. 
A: Here is an example by Roman Sikorski:
$$(H\setminus\{\mathbf 0\})\,^\mathbb N$$
in $\ H=_{top} H\,^\mathbb N,\ $where $\ H\ $ is the countably-dimensional Hilbert space (you can have the Hilbert cube instead). Of course any metric space would do, but this one is universal for $G_\delta$-sets in the Sikorski's sense. This is just one of the double transfinite sequence of universal sets for all Borel classes. Sikorski called Borel classes additive and productive, so that--for instance--$G_\delta$ would be productive of class 1.
A: The set $\mathbb R\setminus\mathbb Q$ of all irrational numbers.
A: Let $E\subset\mathbb{R}^n$ be a Lebesgue measurable  set: then there exists a $G_\delta$ set $G\supset E$ such that $m(G\setminus E)=0.$
A: This is related to the example of @AndreasBlass.
If $A$ is some finite set, then consider the space of all infinite sequences $A^{\mathbb N}$ with the product topology. Then certain sets of infinite sequences, called recognizable, could be described by Büchi automata. A recognizable set is a $G_{\delta}$-set if and only if it could be described by a deterministic Büchi automaton.
A: Here are a few $G_\delta$-sets related to measure theory.
Let $S$ be a $\sigma$-compact metric space
and denote by ${\cal M}(S)$ the set of Borel positive measures on $S$ endowed
with the weak topology. The following sets are $G_\delta$ sets:


*

*the set of non-atomic measures in ${\cal M}(S)$, 

*the set of measures in ${\cal M}(S)$
singular with respect to a given measure,

*the set of measures in ${\cal M}(S)$ whose support 
contains a given closed set.
This leads to the following surprising theorem.
Let $U$ be an open subset of ${\bf R}^d$, 
then the set of singular continuous measures is a dense $G_\delta$ subset
of the set of ${\cal M}(U)$. (Lenz, Stollmann 2004).
There are related results in the theory of self-adjoint operators on a separable Hilbert space endowed with the strong resolvent topology. These are $G_\delta$ subsets:


*

*operators with no eigenvalues in a given closed subset of ${\bf R}$,

*operators with spectrum containing a given closed subset of ${\bf R}$,

*operators such that the spectral measure has no absolutely continuous component on a given open subset of ${\bf R}$.
This leads to the following result.
The set of self-adjoint operators with spectrum $[-1,1]$ 
and with purely singular continuous spectrum is a dense $G_\delta$  subset of the set of self-adjoint operators of norm $\leq 1$ on a separable Hilbert space, endowed with the strong topology.
This is a consequence of the Wonderland theorem of B. Simon, 1993.
A: The space of nowhere differentiable continuous functions $f: [0, 1] \to \mathbb{R}$, as a subspace of the space of all continuous functions under the sup-norm topology.  
A: The set of oracles A (as elements of the product space $2^\omega$) such that $\mathrm P^A\ne\mathrm{NP}^A$. (This also works for nonequality of other oracle complexity classes.)
A: In the space of all subsets of $\mathbb{N}$ (identified via characteristic functions with $2^{\mathbb{N}}$ and topologized as the product of copies of the discrete 2-point space), the set of infinite subsets of $\mathbb{N}$ is a $G_\delta$ set.  So is the set of those $A\subseteq\mathbb{N}$ for which $\sum_{n\in A}1/n$ diverges.  (The same goes with other sequences in place of $1/n$.)  
A: The set of $C^1$ diffeomorphisms of a compact manifold for which all periodic points are hyperbolic and their stable and unstable manifolds intersect transversely.
A: Let $X$ be a metric space, $A \subset X$, and $f:A \to Y$ be continuous, where $Y$ is a complete metric space.  The $f$ extends to  $f^* : A^* \to Y$ for a $G_\delta$ subset 
$A^* \subset X$  s.t.
$A \subset A^* \subset \overline{A}$.  (Willard, Lemma 25.8)
Also, completely metrizable spaces are those which are $G_\delta$-sets in any metric space in which they are embedded.  More exhaustively, 
$X$ is completely metrizable $\iff$ $X$ is a $G_\delta$ in its completion $\iff$ $X$ is a $G_\delta$ in every metric embedding $\iff$ $X$ is a $G_\delta$ in $\beta X$ $\iff$ $X$ is a $G_\delta$ whenever embedded densely in a Tychonoff space.  (Willard, Thm 24.13)
References are to the very best textbook I have ever seen on point set topology, 
Willard's General Topology, 1970.
(I was looking up the universal properties of completions for my number theory class, in order to avoid reinventing the house, when I came across these nice characterizations.)
