Let $S\subset \mathbb{R^2}$ be defined by $S=\{(m+\frac{1}{4^{|p|}}, n+\frac{1}{4^{|q|}}):m,n,p,q \in \mathbb{Z}\}$. . Then,

  1. S is discrete in $\mathbb R^2$ .
  2. The set of limit points of S is the set {(m,n);m,n $\in \mathbb Z$}.

i don't understand this

  • $\begingroup$ If A is the set of limit points of S, then S⊆A ?? in this link... $\endgroup$ Aug 3, 2017 at 16:41

1 Answer 1


Discrete set: Discrete set is a set of points of a topological space such that each point in the set is an isolated point, i.e. a point that has a neighborhood that contains no other points of the set.

Clearly, $(5/4,1)$ is a limit point of $S$ and $(5/4,1)\not\in \{(m,n);m,n ∈\mathbb Z\}$.
Also $(1,1)\in S$ and $(1,1)$ is limit point as $(1,1)=(0+1/4^0, 0+1/4^0)$.
Thus $S$ is not discrete set.
Hence both the two options are incorrect.


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