Let $(E,\tau)$ be a topological space and $A\subseteq B\subseteq E$. Consider the following claims
- $A$ is compact in $E$
- $A$ is compact in $B$
- $B$ is compact in $E$
Since $\left.\left.\tau\right|_B\right|_A=\left.\tau\right|_A$ it should hold (1.) $\Rightarrow$ (2.). On the other hand, if $A$ is closed and $E$ is Hausdorff, it should hold (3.) $\Rightarrow$ (1.). Are the any other implications which are true?
Remark: And just to be sure: The terminology (1.) does mean nothing else than $(A,\left.\tau\right|_A$) is compact and (2.) does mean that $(A,\left.\left.\tau\right|_B\right|_A)$ is compact, right?
$(1)\implies (2)$.
$(2)\not\implies (3)$.
Let $E=\Bbb{R}$, $B=(0,1)$ and take $A$ any finite subset of $B$.
$(3)\not\implies (1)$.
Let $E=\Bbb{R}$, $B=[0,2]$ and $A=(0,1)$.
$(1)\not\implies (3)$.