Definition:Accumulation Point/Sequence

Definition
Let $\left({X, \tau}\right)$ be a topological space.

Let $A \subseteq X$.

Let $\left \langle {x_n} \right \rangle_{n \in \N}$ be a sequence in $A$.

Let $\alpha \in X$.

Suppose that:
 * $\forall U \in \tau: \alpha \in U \implies \left\{{n \in \N: x_n \in U}\right\}$ is infinite

Then $\alpha$ is an accumulation point of $\left \langle {x_n} \right \rangle$.