Past exam of the mathematics course of the University of Cambridge / 2011 / ia / Paper 1 / 3F (Analysis I)

The Bolzano-Weierstrass theorem states that every bounded sequence in $\mathbb{R}$ has a convergent subsequence. More generally, a bounded sequence in ${\mathbb{R}}^d$ has a subsequence converging to a point of ${\mathbb{R}}^d$.

Take $a_n=n$. Every subsequence has the form $a_{n_k}=n_k$, with $n_k\ge k$, so it tends to positive infinity and cannot converge to a real number. Thus this sequence has no convergent subsequence.

Define $a_{2k}=0$ and $a_{2k-1}=k$. The odd-indexed terms are unbounded, while the even-indexed subsequence is identically zero and hence converges to zero. This gives an unbounded sequence with a convergent subsequence.

The floor function has even value on indices congruent to zero or one modulo four, and odd value on indices congruent to two or three. Accordingly the displayed sequence has terms $2+1/n$ in the first two residue classes and $-1/n$ in the other two. In particular,Every term is within $1/n$ of the set $\{0,2\}$. If a subsequence converged to $c\notin \{0,2\}$, its eventual distance from that set would be bounded below by a positive constant, contradicting this bound. The complete set of subsequential limits is $\{0,2\}$.