# Math Help - Limit set question

1. ## Limit set question

Suppose you have an infinite tower of subsets of the integers: $A_1 \subsetneq A_2 \subsetneq \cdots \subset \mathbb{Z}$ such that for any $x \in \mathbb{Z}$, there exists $k \in A_n$ such that $x \equiv k \pmod{2^n}$. Can I determine whether or not $x \in \bigcup_{n\ge 1}A_n$?

2. ## Re: Limit set question

I figured out the answer is no. For example, if $A_n = [2n] = \{1,\ldots, 2n\}$, then this satisfies the claim that for any $x \in \mathbb{Z}$, there exists $k \in A_n$ with $x \equiv k \pmod {2^n}$. However, $\bigcup_{n\ge 1} A_n = \mathbb{N}$, so it would not contain any non-positive integers.